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.