Thermal modelling and Psi value calculation
Heat losses due to thermal bridging at junctions have become increasingly important.
This is because more stringent government legislation and energy awareness is leading to increased insulation levels in the fabric of buildings. As of 1st October 2010 the Approved Documents L1A and L2A required that additional heat loss from thermal bridges be taken into account, with entry of detail lengths and Psi values for junctions being required to be entered into SAP and SBEM calculations (unless a worst case default is instead used). Previously a simple global y-value was allowed if Accredited Construction Details (ACD’s) had been followed – which is what most builders did.
The heat loss associated with these thermal bridges is expressed as a linear thermal transmittance, Ψ . The temperature factor ‘ƒ’ should also be calculated to determine the risk of condensation or mould growth, which can have significant health implications. Where construction details do not conform to generic Accredited Construction Details, they need to be evaluated using validated thermal simulation software and to follow agreed conventions and standards. Modelling of the additional heat loss at junctions in the building fabric should be carried out by ‘a person with suitable expertise and experience’ following the guidance set out in BR497 and BR IP 1/06.
Currently there is no standard certification of modellers, no central depositary of details and the onus is (and will likely remain) on the thermal modeller to be able to prove competency.
However looking ahead, there is scope for a qualification and an accreditation scheme for the thermal modeller and for the details he or she produces. Previous discussions have been had regarding how Accredited Thermal Modelling schemes might work, but it seemed a huge undertaking and there was considerable scepticism as to how it could work, who would check design and as-built and who would pay for it all.
Some of the current roadblocks in my opinion are:
- No-one is checking that given details have actually been followed and built (including ACD’s).
- No-one seems to sign off or check process sequences have been completed and followed, or that junctions have actually been built as per the details.
- There aren’t actually ACD’s available for a lot of junctions.
- The ACD details don’t actually achieve the psi values attributed to them for all construction types and particularly for all construction U-values.
- It’s a time consuming process and if modelled results come out worse than ACD’s (or worse than default values even), builders are likely to just claim the ACD’s Psi values.
- There is no easy way to check as-built performance of junctions match claimed psi values.
- There are so many combinations and potential details to model, that it gets a bit silly (and costly) requiring a fresh detail, process sequence and psi value for every possibility. Some common sense needs to be allowed.
- If someone spends a lot of time and effort producing their own details, what stops other people using their work, but substituting with a comparable product? If that happens, who would want to commit to the expense, knowing your details will be copied?
- There is no accreditation scheme for any of this at present and a business case to make a scheme work needs considerable thought.
- There is no-one to check that modelled details have been modelled correctly and achieve the Psi values claimed.
- There isn’t a qualification at present (although BRE were looking at this, but seemingly no impetus currently to rush to do so).
- People already doing this kind of work (and there aren’t currently a large number) would probably be a bit annoyed if they have to pay someone to agree they are competent via a new qualification.
- Some assessors are still just entering a y-value into SAP assessments, which then isn’t challenged by Building Control or picked up at EPC audit by OCDEA accreditation schemes.
This subject is one that needs further consideration moving forward, as the fabric heat loss becomes more and more important in the drive towards low carbon housing.
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