Knowledge Bank

4th August 2020

Understanding PAS 2035 and PAS 2030:2019

It’s one thing to acknowledge climate change; it is quite another to take practical steps that will make a difference. The introduction of reasonable levels is a particular challenge for the UK for historical reasons: over 50% of the housing stock was built before the 1965 Building Regulations brought in introductory regulations governing insulation.

Home dwellings account for 14% of greenhouse gas emissions. Meeting the ambitious net-zero target for emissions by 2050 (UK), 2045 (Scotland) is going to require shrinkage of heat emissions by 95 per cent. This figure looks even more daunting when considered in terms of CO2 tonnage. Because that’s a reduction from 2,345 Kg (current) to 138 Kg, that’s a challenge.

The size of this challenge is truly overwhelming. There will need to be a concerted effort to increase the energy efficiency of all homes. However, this is small potatoes compared to the vast retrofit programme that will be required—something that has never been attempted before. Given that such a programme will involve every single dwelling in the country, it isn’t surprising. Contractors have their work cut out to plan how this work could be carried out.

Of course, standards must be defined to guide the retrofit work, which is why the UK Government brought in a Retrofit Standards Framework, specifically to prevent piecemeal implementation of so significant a change. In effect, the government have set out three logical steps for the industry:

  • Carry out a property by property assessment of each property
  • Produce a medium-term plan of action
  • Introduce approved measures

What is PAS 2035 and what does it mean?

PAS 2035 is the key document. It sets out clear guidelines for contractors by offering a procedure for building assessment, a guide to selecting the most appropriate EEMs and instructions for long-term monitoring. The document also lays down minimum standards of qualifications, roles and responsibilities, for anyone carrying out retrofitting. The updated PAS 2030 can be seen as a companion to PAS 2035 since it focuses exclusively on commissioning, installation and hand over of EEMs.

Before 2015, unease grew concerning a perceived gap between energy efficiency promised and energy efficiency delivered and at the end of 2016, the Bonfield review was published. This report laid down 27 primary recommendations and suggested a brand new approach to retrofit projects which was to be underpinned by the introduction of a Home Quality Mark in addition to new codes of conduct and practice. The review also proposed a cradle-to-grave framework for the delivery of retrofit energy efficiency.
Efforts have been made by the UK government in tandem with an appointed implementation board, to put the review’s recommendations into effect.
In terms of quality assurance, an extension to the existing quality mark to encompass repair, maintenance and improvement, retrofits and energy efficiency sectors. This opens up a future in which retrofit schemes may be entered into only by TrustMark-registered companies.
In common with other such schemes, companies seeking accreditation will need to be formally assessed by a qualified certification body. Recently the government adopted three new requirements:

  • PAS 2030:2019 – Specification for the installation of energy efficiency measures in existing dwellings and insulation in residential park homes
  • PAS 2031:2019 – Certification of energy efficiency measure installation in existing buildings and insulation in residential park homes
  • PAS 2035:2019 – Retrofitting dwellings for improved energy efficiency. Specification and guidance.

Collectively, these specifications offer a thorough framework for energy efficiency retrofits on domestic properties. Companies should start now, to familiarise themselves with these specs—by 30 June 2021, it will be mandatory for certification bodies and companies registered under the TrustMark scheme to comply with them. As is the case with any robust system, compliance will need to be evidenced.

PAS 2035 – A whole-house approach

The third document listed above—PAS 2035:2019—offers the most comprehensive understanding of the entire Retrofit Standards Framework. Everything from assessment, design and monitoring is explained. PAS 2035 takes a pragmatic, so-called ‘whole-house approach,’ in recognition of the fact that there is no solution set that will fit all circumstances. This way, all aspects of the property require careful, holistic evaluation in advance of any EEMs being put in place. The ‘whole-house’ approach further recognises that insisting that all features are raised to an identical high standard is not only impractical but ill-advised.
PAS 2035 covers risk assessment and whole-dwelling assessment. This process encompasses:

Considering how retrofitting may impact on each. PAS2035 adopts a philosophy that raises considerations such as the protection of occupant health, wellbeing and comfort above those of merely upgrading the energy efficiency of the property.
20–30-year planning

  • Medium-term planning aims to determine a suitable improvement level, given any constraints. At the end of this period of consideration, the idea is to deploy a collection of EEMs.
  • Plans should include a review of unintended consequences arising from potential interactions between EEMs.
  • A precise order in which EEMs are to be deployed must also be included, with consideration given to any knock-on effects.
  • Plans must be formatted in a future-proof way, to facilitate inevitable changes.

Fabric first

A cost-effective method of improvement, emphasised in PAS2035, is the fabric first approach, which is both robust and cost-effective—an approach PAS2035 suggest should be taken into account in the planning process. Essentially, this means taking the following sequential steps:

  • Upgrade the building to a reasonable state of repair concerning pointing, brickwork integrity and damp
  • Replace older light fittings with low-energy bulbs and lighting units.
  • Improve temperature controls
  • Tackle more significant, possibly more costly upgrades to the fabric of the building, e.g. by installing insulation, thermal bridge/air leakage reduction, installing ventilation.

Actual scopes of work evolve from issues noted in the first building assessment. For example, contractors may be restricted in the range of work that can be carried out on listed buildings. In the same way, a risk-based approach is preferred for older properties.
Poor decisions taken in the design phase can significantly reduce effectiveness. For this reason, PAS2035 singles out the need to ‘concentrate on the interfaces’. This ensures that junctions are watched closely to make sure insulation layers and air barriers are continuous and thermal bridges dealt with effectively.
When this has been achieved, contractors can concentrate on residual heat and energy requirements and dealing with them as effectively as possible. This includes which renewable technologies are appropriate.

PAS 2035 facilitates best practice by offering guidance on methods for monitoring and evaluating properties and by considering ways that feedback from one project can be made available to others, for continual improvement.

Raising awareness

An essential facet of PAS 2035 is that it seeks to give a voice to occupants, putting their views front and centre of the refurbishment process. This certainly inspires consumer confidence, but it also acknowledges that occupant use is capable of undermining the effectiveness of EEMs, and even exacerbate the performance gap between the designed and actual performance of the building. An instantly recognisable example of this phenomenon is when occupants living in highly insulated and airtight buildings leave windows open while running heating systems.
PAS 2035 goes further with consultation by ensuring that occupants are kept informed of all developments in improvement plans relating to their properties. Updates are reasonably comprehensive, providing as they do, details about how interventions operate and explanations why they were scheduled in a specific order. Data is also provided to ensure that post works, energy efficiencies achieved during the retrofit are maintained over the longer term.

Data warehouse

In support of the Retrofit Standards Framework, an online data warehouse has been created. This enables the logging of work at one location and makes it available to other sites in real-time, facilitating an integrated approach. More value has been created with the introduction of a property hub that provides a window to view useful information for homeowners.

Roles and responsibilities

As a means to validate competency, several pivotal positions have been created within the Retrofit Standards Framework. PAS 2030 and PAS 2035 set out clear and mandatory vocational or professional qualification requirements. In reality, individuals may hold any number of posts simultaneously. As a check on this flexible arrangement, post holders must be sufficiently qualified, and any potential conflicts of interest must be made visible from the outset.
Of all roles, that of Retrofit Coordinator has primacy. The person in this role has responsibility for end-to-end project oversight. The Retrofit Co-ordinator:

• Commissions measure design
• Contracts installers
• Acts as the homeowner’s advocate

It does not matter who employs the Retrofit Coordinator. The client can do this, or any other party for that matter, so long as he or she is responsible for protecting the client and the public’s interest, and for ensuring compliance with PAS 2035.
PAS 2030 and PAS 2035 set out a series of well-defined steps through which all retrofit projects should proceed:

Step 1.
A conversation takes place between the Retrofit Adviser and the householder. The plan includes reducing consumption, through property improvements and adjustments to occupant behaviour.

Step 2.
The householder has the option to explore a range of improvements. If this exploration is taken up by the householder, the retrofit coordinator conducts a risk assessment informed by the contents of various key documents:

• Current Energy Performance Certificates
• Surveys
• Interviews with occupants and or owners
• Site visit observations

From these inputs, each property is graded A (lowest) to C (highest), and this grade will determine the future trajectory of the project. The risk assessment is repeated and updated as new measures are implemented.

Step 3.
Retrofit assessors carry out a whole-dwelling survey, to include a comprehensive review of:

• The property’s heritage, construction and dimensions
• Current services
• Defects and improvement opportunities
• Existing planning constraints
• U-values
• Moisture properties and suitability for improvement.

For risk paths B and C, further considerations are required, including:

• Occupant appraisal. Have they special requirements?
Ventilation review
Air permeability assessment
• Yearly fuel consumption and estimated emissions
• A significance estimate following BS 7913:2013 which is a guide to the conservation of historic buildings.

Step 4.
The retrofit designer now considers all the data collected and sets about designing a bespoke package of EEMs, paying close attention to fabric EEMs and to measures that might potentially interact. If dealing with risk paths B or C, the retrofit coordinator must include an improvement option evaluation before completing the retrofit design, calculating payback period and carbon cost-effectiveness.
Step 5.
At this stage, the retrofit coordinator sets out a twenty to thirty-year improvement plan, in which improvements are sequenced to achieve the optimal long term benefit of the owner. Close attention to sequencing has a further advantage: designer needs to ensure that one measure does not hamper another.
Step 6.
Improvement plans are discussed with the customer, including any required statutory approvals. Following from this retrofit installers receive briefings on design, sequencing new technologies.
Step 7.
PAS 2030 guidance is used to inform the installation of EEMs by retrofit installers. Responsibility for compliance lies with the installer, who must also supply evidence.
Step 8.
The retrofit installer monitors testing and commissioning of EEMs.
Step 9.
The retrofit coordinator assures an effective handover to the occupant or owner of the property. The transfer will include an on-site inspection and clear safety information, care and maintenance. The coordinator retains testing certificates, commissioning records and other paperwork, giving the client access. They will also suggest creating a new EPC and undertaking any further work arising.
Step 10.
At this stage, the role of retrofit evaluator comes to the fore. PAS 2035 requires the evaluation of retrofit projects, and the retrofit evaluator may have to monitor the site over time to confirm if expected outcomes have been met. And also, of course, to provide feedback to the whole supply chain.
Step 11.
The retrofit evaluator visits homeowners, no later than three months from handover, to carry out a brief questionnaire evaluation. This seeks to confirm whether expected outcomes from the previous work have been achieved. At this point, homeowners may register complaints or dissatisfaction.
If any interested parties conclude that there has been a significant shortfall in performance, two possibilities open up: intermediate and advanced. Such additional steps may include:

• Airtightness testing
• Fuel use metering
• Thermographic survey

Intermediate measures must be carried out within six months of handover; advanced measures within two years. The retrofit evaluator circulates recommended remedial actions.

TrustMark-registered business deadline

As referred to above, a transitional period—valid until 30 June 2021—has been introduced to allow firms time to reach the required standard and gain certification. Companies intending to trade under the TrustMark scheme must be certified to and compliant with PAS 2030 by the deadline.
As the Retrofit Standards Framework transitions responsibilities out of PAS 2030:2019, registered companies must also be able to prove compliance with PAS 2035, following certification.

The big picture

Upgrading homes to improved energy efficiency standards presents a massive task—and opportunity to today’s construction industry.
The Retrofit Standards Framework offers a clear and systematic approach to this work. In adopting this approach, industry professionals can be sure they are ready for the challenges of delivering retrofits that meet expectations and provide value to homeowners in the long-term.

Chris Nicholls
Commercial Director

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1st August 2020

Ambient loop technologies and carbon zero


Though global meetings aimed at galvanising world governments into climate change action, make steady but sometimes laboured progress, there can be no doubt that general awareness of climate change has never been higher. New laws and regulations resulting from governmental actions intended to reduce energy use and achieve zero carbon emissions have created a new market for emerging technologies that offer even greater energy efficiencies. Especially technologies for the next generation of highly efficient heating systems powered by electricity.
At an industry level, few industries have yet to consider practical steps to meet the effects of climate change on their sectors. This includes the construction sector.

What the data says

Current data helps us to understand carbon emission trends and how energy demand is changing. Specifically, the evidence points to a pronounced decrease in carbon emissions from grid electricity. This is pretty much the result of the systematic decarbonisation of power is the UK grid.

  • Since 1990, power station emissions (of carbon) have plunged by over 60%.
  • Extrapolations by The Department for Business, Energy, and Industrial Strategy (BEIS) predict an 80% plunge in carbon emissions from the most significant power companies, between 2010-2020.
  • Over the same period, UK electricity rose from 22% to 65%.
    On a slightly more technical level, estimations for emissions intensities relating to grid electricity by the year 2035 are calculated to be 41gCO2e/kWh. This is well below BEIS’s 2017 projections of 55gCO2e/kW. Great news. But what caused this abrupt downturn? Answers can be found in the data.
  • In 2010, the amount of power derived from fossil fuels was equivalent to 288TWh—75% of the UK total.
  • Renewables in the same timeframe was equivalent to 26TWh—a tenth of the fossil figure. (Source: Carbon Brief, 2019.)
  • United Kingdom electricity generation measured in 2018 showed the lowest levels in a quarter of a century–335TWh.
  • Renewable source data demonstrated that another record had been broken, with an estimated 33% of the UK total in 2018.

From a climate perspective, it is definite that in the United Kingdom, continued prosperity no longer requires an increase in electricity use to power economic growth. If we measure the per capita use of electricity since 2005, we find a 25% drop. This is the lowest level in over three decades. It is encouraging indeed.
In other words, the UK economy has doubled since 1980, without a concomitant growth in electricity demand.
Why is this? Although it’s supposition, the cost of power and austerity are likely factors. What’s sure is that there has been a steady, large-scale decline in UK manufacturing as service industries gained a foothold and expanded.
On a positive note, on the regulatory front, more stringent regulations governing consumer products’ efficiency, the wide-scale introduction of highly efficient lighting, and the change to more effective consumer appliances to replace older, less environmentally friendly devices as consumers become far more ecologically aware.

Where to now?

In a domestic and industrial near-future featuring declining emissions, technologies focused on renewables, and the continued development of nuclear power will increase market share as fossil fuels are displaced.
The future is indeed bright. Projections indicate that carbon emissions relating to electric power generation will trend steadily downwards over the next decade and a half. Of those countries on the vanguard of tackling climate change, Scotland offers a shining example of what might be achieved, with three-quarters of electricity coming from renewable sources (2018 figures). By 2020 it is predicted to reach 100%.

Opportunity knocks

These factors have driven revisions of planning strategies and building regulations to take account of tumbling emission predictions. Importantly, these changes offer huge commercial prospects for modern electric heating systems, precisely because of their increased capacity for lower emissions.
What will electrical heating systems look like in the future?
The acronym, SAP, stands for Standard Assessment Procedure for Energy Rating of Dwellings. Hardwired into the SAP calculations (2012 version) is an assumption that electricity produces just under 2.5 times the amount of carbon emission as that of mains gas. Such an assumption is inaccurate, based on historical carbon factors that fail to consider a critical fact: that the current grid energy mix contains a growing proportion of clean, renewable energy.
Viewed from an SAP 10 perspective, based on both contemporary and projected data, electricity-related emissions have fallen from 0.519kg CO2/kWh to 0.233kgCO2/kWh. This represents a considerable reduction of over fifty per cent. From this point of view, electricity emissions are slightly above those of mains gas. Nor does this downward trend stop there. SAP 10.1 sets its sights even more ambitiously on a reduction to 0.136kgCO2/kWh. This would make electricity significantly more effective in emission reduction than gas.

Distribution loss

Yet lower emissions from electric powered forms of heating aren’t the only reason; new power initiatives outflank gas boiler and CHP systems. Gas and CHP energy systems exhibit comparatively higher distribution losses. This is a scientific fact, demonstrated whenever hot water is used as a means to distribute heat.
BRE’s 2016 report took as its subject matter, distribution loss factors in heat networks for homes. The study suggested significantly higher distribution losses than SAP 2012’s figure of 1.05—a figure traditionally used to derive distribution losses between a building’s plant room and the point of use.
Once 12 months of data had been amassed, it became clear that the lower an apartment’s heat load, the higher were distribution losses expressed as a % of the overall load. Consequentially, the current base value of 1.05 was raised to 1.5. This is equivalent to a 33% loss—close to a seven-fold increase. Systems designed in line with CIBSE/ADE Heat Networks: Code of Practice for the UK, display a default value of 2—equivalent to a 50% loss.
Distribution loss factors matter because of efficiency. But there is a ‘comfort’ dimension too. Heat loss from distribution conduits in thermally efficient buildings can increase air temperatures, unhelpfully overheating risers, passageways, and rooms. In some cases, this heightens the need to ventilate. And if this doesn’t do the trick, mechanical cooling maybe needs to achieve optimum conditions. Such mitigation inevitably equates to higher bills and a less than effective building.
Although the longer-term aim is to connect to a district heating system, the preferred choice is for communal infrastructure based on heat pumps when choosing a common heating source. This type of system can be readily hooked up to a district heating system in the future. This strategy offers many opportunities to develop heat pump technology.
Heat pumps work on electricity—a key differentiator in this market because electricity is clean at the point when consumers use it. By comparison, burning gas generates toxic waste products such as nitrous oxide, necessitating flues, which simply adds to costs. And given that new regulations call for stricter regulation of air quality in new developments, it will become mandatory for companies applying for local authority permits for gas CHP, or boiler-based systems, to determine how they intend to minimise any impacts their technologies may have on air quality.

Planning policies

The Greater London Authority’s (GLA) 2017 plan (draft) adopted a different approach whose goals include:

  • Less up-front energy consumption
  • More effective home insulation
  • The ‘clean and efficient supply of energy,’ whatever the system used
  • Renewable energy in all new housing developments. This would achieve carbon emission reductions of 35% above Building Regulations requirements.

As a follow up to the 2017 draft plan, the GLA published an Energy Assessment Guidance document, urging energy assessors to employ SAP 10 carbon emission data for all new developments. Due to heat pump efficiency, gas boilers are likely to be phased out.
A tariff of £60 per tonne of carbon emissions per year (for 30 years) will be levied as a commercial incentive. The latest draft London Plan is likely to recommend an increase to £95 per tonne. These guidelines apply to London only, though it is thought possible that regional authorities will soon follow suit.

Heat pump solutions

It would be useful to now review how heat pumps work. In simple terms, heat pump solutions move heat around. The heat has to be moved from within the building to outside the building. The reverse is true when heating. Heat pumps achieve this through something called the vapour compression cycle, which employs a refrigerant.
Throughout the cycle, changes in pressure govern the refrigerant’s physical properties to either absorb or discard energy in the form of heat. Coolant transiting an expansion valve loses temperature and pressure.
Thus cooled, the refrigerant enters a heat exchanger. Within air-source heat pumps, the heat exchanger takes a coil housed in the unit outside. Air passing over the coil, transfers its heat to the very low-temperature refrigerant, causing the refrigerant to evaporate. Heated refrigerant vapour now moves to a compressor. Here, it rejects its heat across a 2nd heat exchanger, aka a condenser. As the refrigerant’s heat is expelled, the refrigerant condenses. The resultant liquid, now cooled, may now be used to run the cycle over again.
Heat pump efficiency is measured as an energy efficiency ratio (EER). This is calculated as the heat absorption ratio absorbed by the pump divided by the energy the heat pump uses. Discussions about heating efficiency use the term “coefficient of performance” (COP). This is the ratio of heat rejected divided by power input. This rejected heat is always the sum of the absorbed heat plus the power input. Basic maths help us demonstrate that heating outputs and efficiencies are higher than cooling.


Measures such as EER and COP are critical if we understand the energy efficiency of heat pumps. And also what this energy-efficiency tells us about carbon emissions and energy consumption. With the introduction of the Energy Related Products Regulations, the efficiency in cooling and heating of a heat pump system has changed to seasonal efficiency, which considers system performance throughout the year – as represented in SEER and SCOP.
This is not the place to carefully follow the derivation of such calculation. Suffice to say that the change’s primary effect is to gain a better insight into system performance throughout the year and the changes in external temperature that occur.
We can use this calculation to derive the carbon emissions from electricity consumed by the heat pump, which involves the multiplication of an energy consumption figure by a carbon emission factor (CEF). The primary energy factor (PEF) links primary to final energy by determining the amount of primary energy required to generate one electricity unit. Significantly, fundamental energy measurement is set to become the basis for compliance under new Part L guidance.

Performance and compliance

In the meantime, the yardstick for compliance remains carbon. Independent studies show the impact of the change in the carbon emission factor. If we use the former carbon emission factor of 0.519kgCO2/kWh, CHP systems just meet carbon reduction targets, but only when used in residential blocks consisting of over seventy units, or in hotels with around one hundred rooms. On the other hand, heat pump systems fail to meet the standards required.
However, considering how heat pumps work and using an SAP 10 carbon emissions factor of 0.233kgCO2/kWh, it can be demonstrated that heat pump systems are more than capable of achieving carbon reduction targets. CHP systems, communal boilers, and direct electrical systems need additional carbon reduction measures to make the 35% carbon reduction target.
Using up-to-date SAP 10.1 figure of 0.136kgCO2/kWh, the difference will be even higher. So while historically, planning sought to prefer combined heat and power systems in medium to large scale residential projects, the suggestion is that, in the future, new policies may wipe out any advantage CHP once had. Where carbon reductions of around 35% become the intended target, at least in residential developments, the default solution becomes the heat pump.

Ambient loops

And so, to ambient loop systems. These work by circulating low-grade heat to every dwelling in a development. Each apartment houses a water-to-water heat pump that draws water from the ambient loop, raising its temperature to a suitable heating or hot water level. Heat pumps in apartments function quite effectively when loop temperatures are maintained within quite broad parameters (between -10°C and +30°C.)
The ambient loop concept brings several heat sources within its scope, including:

  • ASHP (Air source heat pumps)
  • Boreholes with thermal piles
  • Surface water such as a sea, river or canal
  • A fourth or fifth generation district heating
  • Waste heat recovery.

We discussed above the recurring problem of overheating in high-temperature solutions. In an ambient loop system, moving low-temperature heat from site to site can be achieved with negligible heat loss. As a bonus, apartment heat pumps can generate cold water for cooling.
Another positive commercial aspect of ambient loop systems is that the same plant and pipework is used for both heating and cooling—no additional costs. Apartment heat pumps simply work in reverse, absorbing energy from the loop instead of discarding it. Considerable savings can be made by having a single, dual-purpose central plant heating system instead of two separate systems.
Heat pump efficiency varies according to the source temperature:

  • Shared borehole array: 10°C average to the heat pump = COP of 6 for heating; 35°C for underfloor heating, and 3.2 for domestic hot water (at 55°C). There are similar efficiencies for surface water.
  • Ambient loops pre-heated centrally by the heat pump to 25°C, a COP of 11.2 in heating (at 35°C for underfloor heating), or 5.7 for domestic hot water (at 55°C). This can achieve seasonal efficiencies of between 3 and 3.2. The point of optimal efficiency for a primary loop is 25°C.

Apartment heat pumps connect with the loop. For domestic or hot water heating, the heat pump draws water out of the loop before heating it between 35°C and 60°C (user’s preference). Economies of scale can be achieved in larger complexes through the use of centrally located heat pumps.

An example of typical carbon reduction

Results from an SAP-based calculation to compare an ambient loop system with a combi central boiler/CHP system are revealing. Assessments were made using SAP 2012 with adjustments to consider changes in carbon factor and distribution losses under SAP 10.


No discernible difference could be seen using Elmhurst’s Design SAP 2012—it didn’t appear to drive change. Switching to a SAP 10 perspective made a big difference. There was a marked reduction in the calculated dwelling emission ratio for the ambient loop system. Also, the dwelling emission ratio for the boiler/CHP system significantly increased. Over the whole block, comparing a current CHP/boiler system with a future ambient loop heat pump system generates CO2 savings of 143 tonnes.
Framing this reduction in carbon reduction in terms of reduced offset payments makes ambient loop heat system technology compelling.

Chris Nicholls
Commercial Director

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31st July 2020

How to pass a SAP Assessment

If you are looking to build any type of dwelling, then you will need SAP calculations. SAP calculations help you to work out the amount of energy your property is likely to use, and also the level of CO2 emissions it will produce. They are a requirement of the Building Regulations for all newly built dwellings in the UK. This has been a requirement for all new homes under Part L of the building regulations since 1995, so it is something that most developers are familiar with. 

What are SAP calculations and why are they important?

SAP stands for ‘Standard Assessment Procedure’ and is the only government-approved system for determining a new home’s energy rating. The scale for a SAP calculation runs from 1 to 100, with 100 representing no energy costs at all! The calculation works by showing the higher a dwelling’s SAP rating, the lower the fuel bills and the lower the CO2 emissions.

The role of SAP calculations for buildings is threefold:

  • To calculate the energy-related costs of a dwelling. The resulting figure is called a SAP Assessment Rating.
  • To evidence compliance with building regulations (Part L)
  • To generate data required to put together an Energy Performance Certificate (EPC)

No one is legally allowed to let or market a property for sale unless the relevant builder can demonstrate that they conform with SAP calculations. SAP calculations are an essential element in the design of any residential scheme, so if you are a first time builder it is important to get your head around the process and understand how it works. 

What do I need to pass a SAP assessment?

In order to ‘pass’ the SAP assessment, a builder must quantify the heat retention and solar gain properties of the dwellings. You must also demonstrate the dwelling’s construction quality while showing evidence of the commissioning of systems. Finally, the dwellings predicted CO2 emissions must be predicted. There are some things you can do to make you more likely to pass your SAP assessment. Keep reading for some tips to help you pass your SAP assessment. 

Get your SAP calculation completed at the design stage

Ideally you want to get your SAP calculation completed at the design stage, and definitely before you start any work on your site. Part L targets change regularly, so you don’t want to have to make changes to your development once you have already started the work and then find out too late you’re failing the SAP assessments. Having your assessment done in the design stage means you can plan your design to the specifications of the SAP assessment. 

Use low energy light bulbs

A quick and easy way to make a significant improvement on the SAP is to use low energy light bulbs. Technology in the lighting sector has moved on incredible amounts in recent years. Gone are the days where they were ugly and awkward to fit, they are now really straightforward and a great replacement to typical lighting fixtures. Ensure every light bulb in the dwelling is low energy (LEDs or fluorescent tubes) and avoid old fashioned tungsten or halogen lamps.

Use an accredited assessor

As regulations are constantly evolving, you want to make sure that the assessor you choose to use is completely up to date with the current regulations. You don’t want to go with somebody who is unsure and isn’t fully aware of the importance of getting your SAP assessment right. You might initially save a bit of money on a cheaper assessment but this could end up costing you thousands of pounds in the future if you are given poor advice. Briary Energy offer professional SAP calculations, with extensive knowledge of all current building regulations.

Improve the building fabric

For most dwellings, heat loss through the fabric accounts for the largest amount of heat loss. Lowering U-Values by increasing the insulation in the property will overall enhance the thermal performance and help with your SAP assessment. It is possible to achieve compliance with Part L without the need for renewable technology if you can reduce your U-Values with the right fabric.

Insulate party walls

To achieve best results in your SAP assessment, party walls (walls used by two properties) must be fully insulated and appropriately sealed to avoid any potential heat escaping between properties. This is particularly important in mid-terrace houses and apartments. You will be penalised on your SAP calculations if you build with partially insulated party walls, and uninsulated party walls are not allowed. 

Consider renewable energy

If you are struggling to get over the line to pass your SAP assessment, renewable energy could be the best option for you. Energy created from solar panels (photovoltaic), solar thermal (solar water heating), wind turbines or even hydro power can be incorporated into the SAP assessment. These options can sometimes be expensive to install, so do your research before committing to anything on your assessment. 

Thermal bridging is important

Thermal bridging is the heat loss that leaks out through the areas where external walls meet. As a minimum, developers should be following the generic Accredited Construction Details (ACD) guidelines, which allows for an improved psi value to be used in your SAP calculations. Using bespoke psi values for all or some connections can be a very cost effective way of improving thermal performance and will help achieve a low air pressure test.

If you are looking for an accredited company to perform your SAP assessment, or just want some more information about the process, get in touch with us.

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Electric Car
10th February 2020

Electricity? Really? The possible impact of the move to electricity for heating and cars

The latest attempt by the government to meet its near-zero carbon target by bringing forward the move to electric vehicles and all-electric heating is well-intentioned, but I believe it is considerably flawed.

One of the biggest problems is infrastructure, or to be precise, lack of it. Let’s take electric cars as a starting point. 25 million roadside car chargers would be required by 2035, meaning 4,000 charging point installs per day until 2035, starting last week. The cost aside in terms of install, the disruption alone would cost the economy with roads and pavements being affected on a greater scale than that of TV cable laying – what cost in terms of emissions from those works?
Heat Pump
Looking at the fact that 33% of the UK use cars to commute, what happens when they all arrive home and plug in? The head of Oxford University’s Energy and Power group, Malcolm McCulloch, has warned that the National Grid would require an extra 20 gigawatts of generating capacity – putting that into context, Hinkley point generates around a gigawatt per day. Furthermore, substations would need to be installed to cope with the power needed.

Michael Gove speaking on Radio 5 live, was asked by Rachel Burden, what happens if you live in a flat with no access to a charging point. The answer showed a complete lack of understanding of the issues. “Well, you go to a petrol station to fill up, so would go to a charging station to do the same” was the answer. Ah ok, so what if there is a queue with everyone taking 20-40 minutes to charge? Oh, and what do we all do while we are waiting? And this, unlike petrol is not once a week, it is every day!

Furthermore, what happens if we are stuck in a traffic jam, where the road has been closed for an accident and it is the middle of winter on a cold night. We need the heating on, but that will deplete the battery so we could have miles of stranded cars! Throw in the mix that the batteries do not perform as well in the winter and you have a recipe for disaster. Let’s not even get into the fact that there is a limited supply of Lithium for the batteries and to cap it all, Neodymium is only mined in China…. So we get rid of petrol, only to become beholden to another foreign power … could not make it up.

Ofgem has now got involved. Their ‘Decarbonisation Action’ involves households changing the way they use energy, meaning gas boilers to be removed and switched to a lower carbon source. ‘To meet net-zero, Britain will see changes to the way homes and businesses are heated,’ says Jonathan Brearley, chief executive at Ofgem. ‘This might include using hydrogen boilers or electricity to power heat pumps and may see more customers connected to heat networks.’ Aspirational, yes but in reality, it is not possible without a huge uplift in infrastructure. This has even bigger implications than the switch to electric cars.

New build sites would require a number of substations and potentially 3 phase electric supplies. But, in the same way, that commuters come home and plug in their electric car, heating time clocks will put the electric heating on at the same time meaning yet another huge shortfall in the energy supply needed. Large parts of the country could face the kind of blackouts we saw in the ’70s. The change from gas would mean higher bills for the 80% of existing dwellings that are not energy efficient homes, in fact up to 4 times higher bills.

What about hydrogen, you say? It has been suggested that we could pump hydrogen down the existing gas network, as a solution. Supplying hydrogen to industrial users is now a major business around the world. Demand for hydrogen, which has grown more than threefold since 1975, continues to rise – almost entirely supplied from fossil fuels, with 6% of global natural gas and 2% of global coal going to hydrogen production. As a consequence, production of hydrogen is responsible for CO2 emissions of around 830 million tonnes of carbon dioxide per year, equivalent to the CO2 emissions of the United Kingdom and Indonesia combined. So not that clean then? A 20% mix of hydrogen and natural gas is also being proposed, but that does not get around how Hydrogen is produced.

At the moment, hydrogen is mainly produced industrially from natural gas, which generates significant carbon emissions, a type is known as “grey” hydrogen. A cleaner version is “blue” hydrogen, where the carbon emissions are captured and stored, or reused. The cleanest one of all is “green” hydrogen generated by renewable energy sources with no carbon emissions produced. Currently, 3% of hydrogen is produced this way. The reality is that hydrogen on the kind of scale needed for domestic heating is 5 if not 10 years away and how much testing would be required to start pumping it down the existing gas network anyway? Where the hydrogen comes from is an important factor that cannot be ignored.

Hydrogen“Correct me if I am wrong but aren’t boiler manufacturers claiming they have hydrogen boilers?“ No, they claim they have “hydrogen ready” boilers which is not the same thing at all. The claim is that by ‘installing one you would not require a whole new central heating system when hydrogen becomes available.’ Given that there are 3 boiler elements that need changing when hydrogen becomes available, elements that are in fact already in natural gas boilers, it would be highly unlikely that you would require a “whole new central heating system”. Marketing hype and smoke and mirrors as usual.

What, therefore is the answer? Reduce the energy requirements of the home by taking a stringent fabric first approach. When the heating requirement is reduced, the prime energy use is domestic hot water and there are devices out there that reduce the energy required to produce hot water, no matter what the heating system is and I predict technology in this field will start to ramp up. In Sweden, some homes are heated by body heat alone because the fabric of the building is so efficient, that is all that is required.

Some facts to leave this post with. IVL Swedish Environmental Research Institute was commissioned by the Swedish Transport Administration and the Swedish Energy Agency to look into car battery production. The report says that each car battery produced releases as much Co2 as 8 years’ worth of driving a petrol vehicle. The manufacturing processes involved in building a car is more than 17 tonnes. Putting that in context, that is equivalent to 3 years’ worth of gas and electricity emissions for the existing UK home and more like 15 years’ worth for a typical new build home. However, there is an elephant in the room that the building industry needs to address. On average there are 50 tonnes of carbon emissions from the construction of one new dwelling. Off-site construction would not help much as each 2 storey home needs to be delivered by 2 lorries and with 100,000 homes a year predicted to be built this way that is 200,000 lorries on the already crumbling national road network. Maybe that is for another post!

About the author

Gary Nicholls the MD at Briary Energy.

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Heat-loss Building
3rd January 2020

Differences between new build EPCs and EPCs for previously constructed buildings

The letters’ SAP EPC’ reference Energy Performance Certification issued in accordance with SAP calculations for new building methodology and employing SAP software. Such certification is issued by an ‘On-Construction Domestic Energy Assessor.’

New Build SAP calculations and Energy Performance Certificates

SAP calculations are how the energy performance of new UK homes is determined. SAP assessments are a requirement for all new homes, converted dwellings or dwellings subject to a change of use. Specialist assessors use SAP calculations to create an EPC outlining the energy efficiency of each tested building.

Assessors derive part of their results from architectural drawings, together with construction, ventilation, and heating information.

EPC’s for Existing Dwellings

The majority of EPC’s relate to existing homes for sale or let. At one time, this sort of EPC formed part of the potential home purchaser’s or tenant’s Home Information Pack. At present, Home Information Packs are no longer used, and a new process for delivery of EPCs has been put in place.

First, a Domestic Energy Assessor visits the dwelling and carries out a survey. To produce the EPC, the assessor carries out a simplified energy assessment called an RDSAP (Reduced Data SAP). In cases where the inspection fails to yield precise details of how a home has been assembled, the RDSAP methodology may incorporate age-based values and assumptions to help assessors reach meaningful conclusions.

Essentially both types of EPC are identical. Both provide snapshots of the dwelling’s energy efficiency and costs. It’s just that one is constructed from highly detailed construction and service specs and architectural drawings (SAP EPC). The other type relies on on-site surveys. In general, EPCs for existing homes tend to be cheaper to purchase than those derived from SAP Calculations.

SAP Calculations and Extensions

Who doesn’t want plenty of light?

Part L1b of the current Building Regulations determines that new glazing must tot exceed twenty-five percent of the new floor area. Extension designs often propose higher percentages, and so render themselves non-complaint. This is where we can help. SAP Calculations are generated by energy assessors holding industry derived accreditations. These calculations can help demonstrate how your specific proposal may remain building reg compliant and still incorporate substantial amounts of glazing.

How do SAP Calculations for Extensions Work?

More heat escapes through glazing than is lost through roofs and walls. This explains why Building Regulations set limits. It is possible to reduce the negative effects of large-scale grazing by upgrading or over-compensating when constructing other areas. Or by demonstrating other solar gains conferred by additional glazing.

These are not the only options that take account of upgrades to an existing dwelling. ‘Change-of-use’ schemes may also fall under SAP regulations, with examples including flat conversions and commercial-to-residential conversions.

What’s Needed?

To carry out a full SAP L1b assessment for your extension, your assessor requires:

  • A set of plans for the existing property
  • A further set of plans for the proposed property (scaled)
  • A detailed summary schedule of all intended construction and services proposals

This is enough to allow the assessor to carry out a desktop assessment. No site visits or surveys will be required.

We specialise in SAP Calculations For Extensions

Every day, nationwide, we help 100s of contractors, architects, and homeowners make large numbers of SAP Calculations for extensions. We are therefore best placed to offer guidance and advice in such matters. If necessary, we can deal directly with your Building Control Officer, taking care of everything on your behalf.

EPC’s, SAP & BRUKL: A Planning Guide

It is the responsibility of an energy assessor to produce SAP, BRUKL, and EPC documentation for new buildings. Such documents offer evidence about how a new build is compliant with the regulations for lower carbon designs.

The EPC is the sole document audited by the accreditation body that governs Energy Assessors’ professional conduct. This renders the EPC the most comprehensive verification of the accuracy of the SAP or BRUKL document for third parties.

This short guide has been designed to help third parties understand the documentation and to make sense of what precisely the document says about a building’s energy performance. We’ll explain differences between SAP, BRUKL, and EPC reports, as well as what is meant by the terms’ design’ and ‘as built.’

What kind of information will I find in EPC, SAP, or BRUKL Documentation?

SAP & BRUKL reports verify compliance with Part L of the Building Regulations for energy conservation. Planning authorities occasionally reference these documents to check compliance against low-carbon targets. But perhaps the clearest distinction between Standard Assessment Procedure documents & Building Regulations UK, Part L documentation is that the former relates to dwellings. At the same time, the latter is concerned with non-domestic dwellings.

BRUKL documents may also be referred to as Simplified Building Energy Model reports. Both documents record a Dwelling Emission Rate (known as a DER) or a Building Emission Rate (known as a BER). Both figures predict the amount of carbon generated by the building in kilograms per square metre per annum and for new buildings must not exceed the TER (Target Emission Rate). Target Emission Rates are bespoke for each dwelling. They are the key deliverable of Part L of the Building Regulations.

Part L contains other rules which rely on SAP and BRUKL documentation, including a domestic Target Fabric Energy Efficiency and minimum scores for fabric and equipment performance.

EPC certification contains information targeted at consumers, which facilitates easy comparison between properties regarding running costs, amongst other things.

EPC, SAP, and BRUKL reports may be issued only by an accredited energy assessor working from government-sanctioned computer applications. These documents may vary in look from provider to provider; however, the information they contain is derived from identical data and methods. In some cases, specific assessors add bespoke summaries of their results and assumptions to the standard output.

What do the terms’ design stage and as built mean?

SAP & BRUKL reports are produced at the design stage of a project and are intended to be predictive, based as they are on a design team’s spec or an assessor’s recommendation. They do not confirm the final performance. Instead, they predict outcomes based on accurate dimensions and specs which remain unaltered throughout construction.

The term ‘As built’ refers to reports generated post-construction and intended to reflect the finished building accurately.

EPCs are generated along with ‘as built’ documents after verification — typically emerging from a mixture of on-site tests, certification from third parties, and self-declarations.

Energy Performance Certificates from Briary Energy

We issue EPCs just before the project comes to an end where residential new builds, conversions, and extensions are concerned. Our STROMA-accredited engineers will issue a residential EPC.

These following actions are carried to dot the I’s and cross the T’s:

  • We check that the Dwelling Emissions Rate (DER) meets the Target Emissions Rate (TER) determined by your SAP calculations.
  • We determine any changes or updates to the building’s fabric, cooling, heating, or ventilation systems.
  • We allocate an EPC rating between A–G, where A indicates the most energy-efficient and G, the least.
  • We Log the property in the government’s landmark register to officially complete the EPC certification process.

In general, dwellings are required to have an updated EPC once a decade. However, one may be necessary sooner where a major change has occurred — the introduction of a new heating system, for example.

Why Choose Us?

  • Discounts for SAP/SBEM Calculations
  • Full UK Coverage
  • Experienced and Accredited Team
  • Practical Advice
  • Same Day Certificates

Related SAP Calculation Services

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Passivhaus OSB Vapour Tight
17th December 2019

Air-tight OSB for timber-frame buildings

Passivhaus is a low-energy performance standard that can be applied to many types of buildings, from homes, care homes, schools, hotels, and supermarkets. This aims to reduce room heating and cooling energy demand while providing excellent indoor comfort levels. This is accomplished mainly by following a “fabric first” approach to construction, with very high airtightness, improved insulation rates and decreased thermal bridging, and the use of mechanical ventilation with heat recovery.
Airtightness is an integral part of building energy-efficient and a Building Regulations necessity. According to NHBC, home energy usage accounts for nearly 27% of UK carbon dioxide emissions. A 2002 BRE report found that air leakage can account for up to 40% of building heat loss. Buildings designed to meet the Passivhaus specification achieve a 75% reduction in space heating requirements compared to standard practise for new buildings in the UK. Therefore, the Passivhaus model provides a reliable approach to help the construction industry achieve the government’s goal of reducing carbon emissions by 80% by 2050.

Airtightness with Passivhaus

The key principle for achieving airtightness is to create a single, continuous, durable airtight layer covering the building’s heated area. It is typically located on the insulation’s warm side, thus also fulfilling the vapour control layer requirements.
To obtain Passivhaus certification, under test conditions, a building must achieve airtightness of less or equal to 0.6 air changes per hour. This is expressed as n50≤0.6h-1 @50 Pa, where n50 is defined as the number of air changes per hour at a 50 Pa reference pressure difference. The result is measured using the building’s internal air volume (m3) instead of its surface area (m2), thus the units are expressed as m3/m3.h, simplified to h-1. This must be the average pressure and depressurization conducted using a blower door sample. Meeting this level of airtightness is difficult but achievable with a clear design plan, with the final result being sensitive to on-site workmanship quality
Current Building Regulations require air permeability at 50Pa to be 7-10m3/h/m2, depending on whether a project is in England, Wales, Scotland or Northern Ireland, so the Passivhaus limit is about five times lower than the maximum allowed. A uniform comparison between air-permeability values and n50 air-change rate values is not feasible because there is no direct relationship and each uses different test and measurement protocols. But to put this in context, the airtightness rate of n50 ≤0.6 h-1 @ 50 Pa is roughly equivalent to a crack in the building’s envelope that is less than a 5p piece per 5m2. For contrast, a building that meets the restricting airtightness requirement for Part L (2013) of the Building Regulations (Section 6 of Building Standards, Scotland and Building Regulations Part F, Northern Ireland) would have a 20p piece gap per 1m2 of envelope.

Passivhaus Internal


Traditionally, air and vapour control layer (AVCL) membranes have been used to achieve airtightness, although there is surprisingly no industry standard or test process. The degree of vapour diffusion depends on the composition of the material and the quality of such systems depends on site design, installation and workmanship.
An alternative is to use certified air and vapour-tight oriented strandboard (OSB). Standard OSB (and other wood panel types such as plywood, particleboard and MDF) are not ideal as an air and vapour tight layer as their air and vapour permeability is variable and can vary greatly between production cycles and production sites. Air and vapour-tight OSB is a new technology: a structural OSB panel that has built-in vapour control and air-barrier properties for the timber frame industry.
The main British and European specifications regulating specification and use of OSB panels are BS EN 300 and BS EN 13986.
BS EN 300 provides a classification system specifying four OSB levels in terms of mechanical quality and relative moisture resistance.
These are:
• OSB1: general purpose boards and boards for interior fittings (including furniture) for use in dry conditions
• OSB2: Load-bearing boards for use in dry conditions
• OSB3: Load-bearing boards for use in humid conditions
• OSB4: heavy-duty load-bearing boards for use in humid conditions.
OSB3 is suitable for a variety of internal and protected external uses including roofing, sarking, flooring, site hoarding and sheathing for external walls, partition walls, internal walls and partitions, spandrel (gable) panels, warm walls, reverse walls and insulated structural panels.
Air and vapour tight OSB panels (classified as OSB3) offer an alternative to air and vapour control layer membranes. The panels are air-and vapour-tight, ensuring airtightness and preventing interstitial condensation within the timber frame structure without a separate membrane.
OSB is made of softwood wood strands such as spruce and pine, wrapped in three layers with a moisture-resistant, formaldehyde-free synthetic resin (bottom, core and top surface) and pressed under high temperature and pressure to form a rigid and dimensional stable wood board. The wood strands are alternately bent by 90 ° to the right angles to the outer layers. This distributes the strength, stiffness and spanning capacity of the finished OSB panels, which are approximately twice as strong in length (major axis) as in width (minor axis).
To address the unpredictable air permeability in conventional OSB, care is taken during production to minimise differences in density and air gaps. Most air and vapour tight OSB panels are accredited by the Passive House Institute (PHI) to achieve maximum air permeability of 0.01m3/(m2h) (PHI Class A).
To overcome inconsistent vapour permeability in OSB, a water vapour-resistant polymer coating coats the inside of the panels. It provides consistently high-water vapour resistance across the panel layer, and its smoothness also guarantees good airtight tape adhesion at panel joints.


In timber-frame buildings in cool temperate climates, the hot side (inside) of thermal insulation requires a vapour control layer to prevent excess water vapour from spreading into the building fabric. Warm air contains more water vapour than cold air and has higher vapour pressure. Vapor diffusion transfer occurs when the hot, humid air inside a building migrates (diffuses) through the building fabric to the low vapour pressure cold air (outside). If this vapour transfer is uncontrolled, condensation within the building fabric (interstitial condensation) can occur as moist air cools and vapour condenses on the assembly’s cold side. Therefore, the vapour control layer’s function is to limit the amount of water vapour entering the building fabric.

Air and vapour-tight OSB panels can also help prevent summer condensation by serving as a buffer. Materials made from cellulose fibres are hygroscopic and therefore can absorb and release small amounts of water vapour from the surrounding environment. In the limited number of cases of reverse vapour diffusion, OSB panels can help to prevent condensation within a structure. This is where moisture from a building envelope’s outer leaf is warmed by summer sun and permeates to the cooler inner layers, creating a possible condensation hazard unless managed or planned.
Research into the suitability of OSB panels as the air and vapour barrier in timber-frame structures found that installation reliability was as important as the panels ‘ intrinsic vapour resistance. This was quantified by the Fraunhofer Institute of Building Physics in 1989, when investigating the effect of a 1 mm tear in the plastic vapour control layer. The difference was found to increase vapour transmission from 0.5g/m2 to 800g/m2 (nearly two pints in 24 hours) as vapour was transported by convection and not by diffusion–convection was found to bring 1600 times more moisture into the system. Therefore, it is important for well-insulated buildings ‘ long-term quality that both airtightness and vapour tightness are considered in conjunction at the design stage and that appropriate, fit-for-purpose and approved products are chosen.

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Air Source Heat Pump Noise Issue
16th December 2019

Are heat pumps a noise nuisance in our garden?

Increasing the use of heat pumps following the release of the Future Homes Standard consultation may create acoustic problems that need to be planned.

Compressor and heat pump fans cause the main noise and vibration concerns.

As air source heat pumps (whether for individual homes or part of a heat network) are typically externally located, there is the potential to create noise nuisance in and around the building, disrupting residents. Careful attention will be given to the position of the plant in relation to noise-sensitive receptors and equipment attenuation means.

A BS4142:2014′ Methods for rating and assessing industrial and commercial sound ‘ assessments should be performed to ensure the plant does not impact the external background noise levels. Heat pumps can generate 60dB. Attenuation measures could include installing silencers and enclosing the ventilation heat pump with sound-absorbing panels and acoustic louvres. These add costs and require additional space, either in plan area or vertical height.

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Changing Specification
19th November 2019

The Dangers of Changing the Specification

Calamity, the merchant can’t get the Aircrete blocks specified for your build, but can offer you medium dense instead. A block is a block, right? Wrong for a variety of reasons that will determine whether your dwelling fails or passes Part L of the building regulations.

An Aircrete Block can have a lambda value as low as 0.11 W/m2K – which is the heat conductivity of a material. This value applies to thermal calculations on buildings and their thermal components. The lower the value the lower the heat loss. By comparison a medium dense block can be anything between a lambda value of 0.28 and 0.60 W/m2K meaning a greater heat loss an in turn higher heating bills.

You can thus see changing the blocks would have a serious impact on the SAP result due to that heat loss being greater. This is one example of where it is possible to fail, by making one decision at the merchants counter or from that call that says “sorry we can’t get those blocks for a week”.

Before making any changes at all, run the potential change past the SAP assessor to see the implications because it is likely that the changes will cost you more than you thought, but there may be an alternative solution that means the build is not held up.

Communication with your SAP assessor, across all aspects of the design and construction process, is vital to ensure the SAP calculation represents the design and construction of the dwelling. In both cases satisfies compliance. Commercial pressures can sometimes impact the quality and accuracy of assessments and information provided, so involving an assessor throughout the process can have significant benefits.

During the build process, communication back to the assessor, when making any changes that could affect the energy efficiency of the dwelling, is paramount. Check that the revised assessment will pass, before implementing any changes. Do not inform the assessor of requirements to get the As-built assessment and EPC produced. This will create issues with the audit process, carried out by the assessor’s accreditation body. The accuracy of the EPC will also be at question.

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U-Value Insulation
19th November 2019

What is a U-value?

U-values measure how effective an insulator is. When it comes to thermal performance, we look in detail at terminology and core concepts.

Although the main focus of the environmental performance of buildings is now on the use of carbon, the thermal performance of the building fabric still needs to be considered as a contributing factor. Thermal performance is measured heat loss and is expressed as U-value or R-value in the construction industry. U-value calculations will be required when designing construction strategies. Many terms have similar meanings, and conflicting interpretations can be found on the Internet. Various terminology and how they relate to each other are explained in this article.

U-value or thermal transmittance (reciprocal R-value)

Thermal transmittance, also known as U-value, is the frequency of heat transfer through a structure (which may be a single material or a composite material) divided by the difference in temperature across that structure. The measurement units are W/m2K. The better-insulated design, the lower the U-value. Workmanship and construction quality may have a significant impact on thermal transmission. If the insulation is designed with holes and cold bridges, the thermal transmittance can be higher than expected. Thermal transmittance takes into account the loss of energy due to conduction, convection and radiation.

How is U-value Calculated?

The basic calculation of the U-value is simple. The U-value can be determined by finding the reciprocal amount of each material’s thermal resistance that makes up the building component concerned. Remember that the internal and external faces, as well as the surface resistances, also have resistances to incorporate. These are fixed values.
A variety of requirements cover thermal transmittance measurement methods.
Simple U-value calculations can be made by considering layer-by-layer construction of the building component. Though, this does not compensate for cold bridging (e.g. by wall ties), air gaps around insulation, or the various thermal properties of e.g. mortar joints.

How do you continue to measure U-value?

Although design calculations are hypothetical, post-construction measurements are also possible. These have the advantage that they can compensate for workmanship. Heat flux metre can be used to measure thermal transmittance for roofs or walls. It consists of a thermopile sensor attached to the test area to control heat flow from inside to outside. Thermal transmission is derived from dividing average heat flux (flow) by average temperature difference (between inside and outside) over a continuous span of about 2 weeks (or over a year in the case of a ground floor slab due to heat storage in the soil).

U-value calculators

Because calculating U-values can be time-consuming and complex (especially where cold bridging needs to be accounted for, for example), many online U-value calculators were published. Many of these, though, are only available on subscription, and those free appear to be too simplistic. Another alternative is to request a u-value calculation estimate from Briary Energy.
Building Regulations Approved Documents L1A, L2A, L1B and L2B in England and Wales all apply to the publication BR 443 U-value calculation conventions for approved calculation methodologies, while the companion document U-value conventions in operation.

R-value, or thermal insulance (reciprocal of U-value)

Thermal insulance is the opposite of thermal transmittance; in other words, a material’s ability to resist heat flow. R-values are more used in some parts of the world (e.g. Australasia), as opposed to UK U-value choice. Thermal transmittance measurement units are m2K/W and, again, a higher figure suggests better performance (unlike the lower U-value figure).

k-value, or thermal conductivity (also known as lambda or λ value; reciprocal of thermal resistivity)

Thermal conductivity is a material’s ability to conduct heat. So, high thermal conductivity means a higher level of heat transfer across a material; note that this is also temperature-dependent. Thermal conductivity devices are W/m2k. Like U-values and R-values, yet, k-values don’t depend on the material’s thickness.

Y-value, or thermal admittance, or heat transfer coefficient

A material’s ability to absorb and release heat from an internal space as temperature changes in that space is called thermal admittance (or heat transfer coefficient) and is described in BS EN ISO 13786:2007 Thermal performance of building components. This also provides the basis for the Simple dynamic model in CIBSE Guide A: Environmental design, used to measure cooling loads and summer space temperatures. The higher thermal admittance, the higher the thermal mass. Thermal admittance is like thermal transmittance (using the same measuring units). Still, it tests a material’s thermal storage efficiency, i.e. a material’s ability to keep and release heat over time, usually 24 hours. Together with thermal transmittance, measuring units are W/m2K.

Note that ‘ Y-value ‘ thermal admittance should not be confused with ‘ y-value ‘ thermal bridging factor described in SAP Calculations (Standard Assessment Procedure) Appendix K as derived from linear thermal transmission.

Psi (Ψ) value, or linear thermal transmittance

Measurement of heat loss due to a thermal bridge is called linear thermal transmittance (as opposed to’ area’ thermal transmittance otherwise called U-value), with the units of measurement again being W/m2K. Psi values are used to

Thermal mass

Hitherto ignored in the UK construction industry, thermal mass (unlike thermal admittance) is derived from the specific heat capacity (the ability of a material to keep heat relative to its mass), density and thermal conductivity (how heat can pass through a material). SAP Calculations use thermal conductivity as the’ k’ (or kappa) factor in measuring the thermal mass parameter (TMP). The’ k’ value is the heat capacity per unit area of the building element’s’ active’ component (only the first 50 mm or so of the element’s thickness has a real impact on thermal mass, as it decreases the element’s depth by increasing; the effect is negligible beyond 100 mm). It should be noted that the’ k’ value is an estimate, as assumptions are made about the size of a material’s active volumes; yet, it ignores the effect of thermal conductivity in measuring the time during which heat is absorbed and released from the material. BS EN ISO 13786 offers a more effective thermal mass determination process. Isolation should not confuse thermal mass.

Why not try our U-Value resource.

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PSI Values
18th November 2019

What are PSI Values?

Psi values are a key component of low-energy building design, but what does PSI in building mean?

Historically, design teams have given them a cursory look, often leaving default values and’ assumed results’ in the region. But times change and an appreciation of Psi principles becomes important when an effective building project is being put together.

What are Psi Values?

Have you ever come across a u-value first of all? This is a heat loss calculation by a square metre of thermal component (e.g. a wall).

Okay, Psi values are a measure of heat loss along a metre of junction between two thermal elements, such as the line between a ground floor and an outer wall (see green line in the adjacent picture), and are calculated in W/mK. Collectively these junctions are known as thermal bridging.

Why are psi values important?

U-values compensate for heat loss by thermal components, but not the fabric’s total heat loss. There is additional heat loss at the junctions called non-repeating thermal bridges.

This is due to the geometry of the junction and also, in many cases, to the layout of the junction: the geometry since we calculate internal measurements in SAP and assign U-values to the calculated thermal elements (e.g. external wall and ground floor). Therefore, we underestimate total heat loss. Unless we calculated exterior measurements, we will overestimate total heat loss (assuming no psi-value of construction) The construction because the materials used at the junction often have a higher thermal conductivity than those used in the thermal elements (e.g. a block of concrete at the junction instead of continuous insulation around the junction).

What to do with Psi Values?

In SAP Assessments, we factor in PSI values for all junctions as well as U-values for all exposed elements in order to best model the energy efficiency of a dwelling. Psi levels must be kept to a minimum to satisfy the new building regs.

In reality, designers needed to follow existing schemes such as Accredited Construction Details and Enhanced Construction Details for a realistic shot at achieving compliance.

They have fixed junction configurations depending on the type of construction, i.e. a timber set, as well as a masonry set and a steel set. Nonetheless, these schemes are now obsolete and in the next update of SAP and Part L regs their use will be limited.

This ensures that designers are likely to either use or have their own measured approved make-ups (with related, predetermined Psi values attached) from manufacturers.

Many manufacturers of timber frames and SIPS provide their own super low Psi values that your assessor can use. Customers will need to model heat loss at junctions with custom-made Psi quality calculations on some very low energy builds, such as Passivhaus ventures.

There are also a few schemes set up by the manufacturers of insulation which allow you to demonstrate improved performance by using a combination of their goods.

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