1st October 2020

Parts L and F of building regulations: Planned Changes

Committing to a target of net-zero carbon emissions before the middle of this century is challenging enough. Existing stock must be upgraded, and this will be time-consuming and costly. In addition to this, construction companies will need to ensure that new-builds are also up to scratch.
In the vanguard of legislation falling out of the ambitious zero-carbon target setting, will result in the publication of revised Parts L for new dwellings and F of the Building Regulations for new dwellings in England and Wales. When they come into force, these regulations will set out the requirements for the energy performance and ventilation of the property.
These documents have been designed as a natural step towards more stringent standards due to come into force in 2025. This raises the importance of a phased approach for the construction industry since they will have not only to ensure current regulations are adhered to, but also ensure they can up their game to comply with 2025 standards.
Here, we’ll consider the most critical sections in Parts L, and F consultations in England and Wales, in what ways they are limited, and in particular actions construction practitioners need to take to prepare for 2025 standards.


Control of Building Regulations is devolved within the UK. Recent consultations in England and Wales concerned themselves with parts L and F of the Building Regulations for domestic properties. The documents deal with different requirements. Each document contains recommendations on how to become compliant. So far, consultation has taken place solely on requirements concerning new domestic properties. Non-domestic and refurbishment standards will follow close on their heels.
The Part L requirements have been constructed around a core performance measurement, which must be achieved for every property. In the past, this target related to a carbon emissions target. Most recent consultations produced two alternatives for further reducing the emissions target. For England, 20% or 31% (preferred) reduction over the current requirements. For Wales, the targets exceeded that of England— 37% (preferred) or 56%.
This requires some explanation. The calculations indicate reductions in carbon emissions. However, these were constructed around a new metric–primary energy. The carbon emissions metric has not been discarded; it has been retained as a back-up metric.
This change proved necessary because they do not directly measure energy efficiency. While efforts continue to decarbonise fuel, and in the end, to power dwellings with zero-carbon electricity, low emissions figures reveal little of importance about the energy efficiency of property in regular use. The primary energy measurement has been designed to generate a more precise figure for total energy use, factoring in the energy required for fuel preparation and a property’s ultimate energy demands.

Primary Energy Calculations

Each fuel type is calculated to have a primary energy factor (PEF), starting with the amount of energy employed during upstream production.
Factors include:

  • Planting and cultivation of biofuel sources
  • Extraction
  • Processing and transformation
  • Transportation
  • Transmission

All PEF factors are worked out in advance and are contained in the Standard Assessment Procedure (SAP) specification. Energy demands are then worked out for specific uses. Examples include:

The energy required in each case is then multiplied by the PEF matching its fuel type. The addition of these figures provides the total primary energy demand for the property.
Here’s an example of how to calculate primary energy demand for heating:
(property energy demand/efficiency of heating technology) x PEF
Let’s start with a property heated by totally efficient electric powered panel heaters rated at 10,000kWh. We know that the fuel factor for electricity is 1.501, allowing us to derive primary energy demand as follows:
(10,000kWh / 1) x 1.501 = 15,010kWh
Primary energy calculations consider all renewable energy generated on-site to be subtracted from overall energy demand. For example, if we included a PV array producing 1,500kWh for exclusive use in the above-mentioned property, the calculation becomes:
([10,000kWh – 1,500kWh] / 1) x 1.501 = 12,759kWh
Dropping of the Fabric Energy Efficiency Standard
It is generally agreed that the move from carbon emissions as the primary indicator of how energy-efficient a property is, to Primary Energy Calculations, offers a more accurate snapshot. There have been critical voices, however, about the way in which is to be implemented in the draft in force in England.
The current situation is that the English Approved Document for new dwellings (ADL1A) uses the Fabric Energy Efficiency Standard (FEES). This sets minimum energy performance targets for the fabric elements of buildings. To simply the system when primary energy targets were introduced in the 2020 version of Part L, some argued for the removal of FEES. This meant that to exercise control over a building’s fabric performance would require the use of the worst-case backstop U-values. This is what currently happens in Wales because the FEES was never introduced in that country. However, the backstop U-values proposed for England are less onerous than their existing Welsh counterparts (as shown in Table 1 below).

Heat loss elementsCurrent England ADL1ACurrent Welsh ADL1AEngland 2020 proposedWales 2020 proposed
External walls0.30W/m²K0.21W/m²K0.26W/m²K0.18W/m²K (flats 0.21W/m²K)
Flat and pitched roofs0.20W/m²K0.15W/m²K0.16W/m²K0.13W/m²K

Table 1: Area weighted worst-case backstop U-values for new domestic buildings

This gives rise to an anomaly. Primary energy calculations permit the deduction of contributions from on-site renewable energy. Potentially, this could see homes constructed to these backstop U-values. Meaning homes constructed under 2020 Part L with renewable generation would require more heating than those constructed under current standards using FEES.
This seems to fly in the face of the very reasons for upgrading Part L. It could also worsen differences between the actual energy performance of dwellings and what the design predictions indicated, as a result of adopting heat pumps to meet the heating and hot water requirements.

In winter, heating and lighting demand is high. At this time, air-source heat pumps tend to underperform as temperatures dip. PV output falls too at a time of shorter days and lower sun angles. If there is energy waste in homes, carbon measurements could well end up higher than they currently are.
Fuel bills will be impacted as a result. The Committee on Climate Change (CCC) predicted that existing proposals could result in domestic bills that 50% above those for homes built to English requirements.
It is concerning that this estimate fails to factor in on-site technology running costs. In the end, this could leave many in fuel poverty. Affordability is best achieved by pursuing fabric efficiency to reduce demand to a minimum.
Construction companies, in England and Wales, need to consider such changes within the timescale of the next five years. The English version states that the 2025 requirement is supposed to require that carbon emissions from domestic assets be between 75% and 80% lower than at present.
At these levels, attaining a high degree of fabric performance is likely to become compulsory. If so, perhaps a better way forward than masking leaky homes with renewable technologies, it is vital to use the next five years to raise construction standards and to adapt supply chains for future solutions.
It may help to view this up-skilling process within the context of probable requirements in 2025.

2025 Part L and F requirements

English and Welsh approaches indicate a pressing need to combine low-carbon heating systems with an extremely high fabric standard specifications. It is likely; therefore, that triple-glazing will be required. Building elements will have to conform to the U-value limits shown in table 2.

ElementExpected U-value requirements in Welsh Part L 2025Expected U-value requirements in English Part L 2025
External wall0.13W/m²K0.15W/m²K

Table 2: Expected minimum U-value requirements for Part L 2025 in England and Wales

The CCC offers more detail still on how a net-zero home might perform. They say that homes must be extremely airtight and supported by timber-framed mechanical ventilation heat recovery (MVHR), and with a space heating demand of 15-20kWh/m2/yr.
To conceptualise this figure, the Passivhaus Trust estimates a typical UK home has a space heating demand of around 130-140kWh/m2/yr. This would represent a considerable change, yet it is achievable. From an international point of view, the net heating in the city of Brussels has been restricted to 15 kWh/m2/yr since 2015, and by 2020 Denmark’s demand for space heating in residential buildings will be reduced to 20 kWh/m2/yr.
Perhaps more significantly, the demand for space heating of 15kWh/m2/yr is also a requirement of the voluntary Passivhaus energy performance standard.
In common with CCC recommendations, the Passivhaus requirement is for highly airtight properties with an air-leakage rate of just 0.6 ach @ 50Pa and ventilation through MVHR. Even then, high levels of insulation won’t be enough—properties will have to be virtually thermal-bridge free. The primary energy demand must also be regulated to ≤ 120 kWhm2 / yr, and the specific cooling load must be regulated to ≤ 10 W / m2.
Properties may be certified only after careful assessment to ensure real-world outcomes match expectations. Such a rigorous approach supports high construction standards, and results show that Passivhaus properties regularly meet or exceed targets for energy performance. Traditionally, Passivhaus standards were considered more relevant to self-builds. It has become more common, however, for these standards to be applied to larger-scale projects.
In short, there is enough evidence, from various sources, to confirm that such requirements could be achieved when scaled up. We also have some idea of how to meet them. Offsite solutions like structural insulated panels (SIPs) work well for Passivhaus. The modular format of such panels lends itself to achieving faster build times. They provide superior thermal performance, airtightness and insulation continuity. This may reduce stress on site workers – a key component is given that there still does not seem to be an end to the current skills shortage.
The CCC’s data show that immediate action to investigate approaches and up-skill workers would bring the limitation of space heating on all new dwellings to Passivhaus levels (15 kWh/m2/yr) by 2025, eminently within reach.

2020 Part L and F

With an eye on requirements in the pipeline, a ‘fabric first’ approach sounds most practical for project teams, concentrating on getting quick wins. Were fabric-first methodologies to be adopted first, and quickly become standardised, it would be easier to upgrade building construction methods, so they take future requirements into consideration.
Although not necessary, the fabric criteria set out in Option 1 of the English consultation and Options 1 and 2 of the Welsh consultation tend to be a fair jump-off point, with U-values corresponding to the estimated limit values for the 2025 standards (see Table 3 below). The airtightness requirements within Welsh Option 2 offer a similarly reasonable path to 2025. Project teams could begin to gain a thorough understanding of the new airtightness approaches and the effective installation of MVHR systems.

2020 England proposal
Option 1
2020 Welsh proposal
Option 1
2020 Welsh proposal
Option 2
External walls0.15W/m²K0.13W/m²K0.13W/m²K
Flat and pitched roofs0.11W/m²K0.11W/m²K0.11W/m²K
Thermal bridgingGlobal value Y = 0.05; or individual values from SAP table R2 (Option 1 values)Global value Y = 0.05; or individual values from SAP table R2 (Option 1 values)Global value Y = 0.05; or individual values from SAP table R2 (Option 1 values)
Air permeability553 + mechanical ventilation heat recovery
Photovoltaics installedNoYesYes

Table 3: Selected notional dwellings building parameters

Both consultations highlighted the need to stop thermal bridges degrading the overall performance of the dwelling. With this in mind, they recommend that the Approved Construction Details (ACDs) are removed from new versions of Part L as these are inappropriate when engaging with the new fabric standards.
Universal backstop thermal bridging level values used within SAP will also get worse. Yet, avoiding them will require project teams to calculate their own thermal bridging values or use model construction details from non-government databases which contain independently assessed thermal junction data. This will require careful attention to detail if compliance is to be achieved.

Chris Nicholls
Commercial Director

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