Thermal Modelling and Detailing: What’s wrong and what does the future look like?
Out of the Zero Carbon Hub’s end of term report, Construction Detailing was identified as a particular area requiring improvement. This post looks at what is happening now, and what an Accredited Thermal modelling scheme may look like.
At the moment, this is what typically happens:
- Someone tells an energy assessor that they are following the ACD’s (whether they are or not) and then those values are used in SAP calculations
- Savvy developers look into using better details (such as our Kingspan TEK set of details)
- Developers model their own (or get someone to do it for them)
- The y-value default (y=0.15) is used and this poor value is compensated for elsewhere in the build
Or, at worst:
- They wrongly claim a universal y-value better than y=0.15 and no-one picks them up on it!
There are issues about whether details are then actually followed or not during construction; whether the modelled or ACD details psi values are correct for the construction used; whether the energy assessor then enters them into the calculation correctly or not; and whether the person calculating the Psi-values has done so competently (modelled correctly in accordance with appropriate guidance and standards).
With the current values for ACD’s in SAP, there is often no benefit to modelling some junctions (as anyone with any knowledge is aware that modelling those details – even the ACD detail itself -, following current building performance levels will result in a value worse than the approved detail column of SAP Appendix K). As a consequence, people continue to claim the ACD values, as doing so gives them no disadvantage, or even an advantage over what the modelled performance would actually be.
Some details are more significant than others to the overall performance of the building and these are the ones most in need of better detailing.
A better understanding is essential for all involved:
- Developers need to understand the importance of following energy efficient, buildable details
- SAP assessors need to correctly use modelled values in their energy calculations
- Assessment schemes need to start ensuring that what goes into calculations has an evidence basis and start picking up on assessors who use values without anything to support them
- Thermal Modellers need to do things in a repeatable, open, way following universally understood guidance and conventions – so that there is confidence in the values claimed
There also needs to be confidence that the details are then correctly followed on site – so quality assurance and checking needs to come into it all somewhere.
Given the number of potential details and variations of those details, one of the first steps needed is for a modeller to be provably ‘competent’. Which means having confidence that they are doing the modelling correctly (and in the same way as other modellers) – effectively confidence that we’re all working on a level playing field!
Thermal modelling can be time consuming and complex, it requires considerable attention to detail, correct application and understanding of measurement and calculation conventions and an understanding of heat paths as well as knowledge of what is buildable.
An Accredited Thermal Modelling (ATM) competency scheme could help to ensure that competency is achieved and maintained by:
- Helping to develop a National Occupational Standard (NOS) for new Thermal Modellers, (against which training providers can then assess competence)
- Developing an APEL (Acquired Prior Experiential Learning) route to allow existing modellers to provide evidence to show that they are competent
- Administering some form of simple, low cost on-going competency check
- Serving as an avenue to develop a set of Modelling Conventions, (which might include ‘typical’ thermal performance of materials that modellers should use where an actual value isn’t known as well as ranges of values for which a detail might be acceptably deviated from, whilst still using the modelled value)
- Offering an avenue to ask questions of other modellers and providing a route for modellers to offer advice and guidance to others
- Producing supplementary guidance such as recommended formats for process sequences
- Hosting copies of the relevant technical documents and standards for modellers to use
- Providing a place to access relevant CPD
- Providing a route to challenge details and give feedback as to buildability or to recommend means of improving details
A scheme probably shouldn’t try to cover the ‘has it actually been built like that?’ side of things, but should consider whether a detail is actually buildable.
A central repository or ‘book’ of details could be set up and run by one or more suitable operators (or the scheme itself), in such a way that modellers could choose to ‘lodge’ their modelled details for feedback, but this should perhaps be an optional approach (and certainly one with minimal, or no cost). Until such time as there is something in place to ensure that what is built matches the detail claimed, I can’t see a cost element being justifiable for any third party checking – when the market will more than likely happily pick up on any issues or errors for free! A crowd-sourced approach if you like.
The checking of whether details actually are followed does need looking at, but it was this element of the process that shot down the idea when CLG first considered it, back when the 2010 regulations came in. As it turned out, the whole idea was side-lined then, as it was considered overly burdensome – so simplicity and minimal cost will be essential to a successful scheme.
If you would like some advice on SAP please contact our Technical Services Department who will be happy to help you.
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