Thermal Modelling and Detailing: What’s wrong and what does the future look like?

Detail eye

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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Jon Ducker is a qualified energy assessor working for Kingspan Insulation Ltd. He has an extensive knowledge of energy efficiency, renewable energy systems and sustainability in buildings with an expert knowledge of the relevant sections of buildings regulations and standards and their interactions with SAP. He provides authoritative advice regarding energy assessments for a wide range of public and private sector clients.

0 comments on “Thermal Modelling and Detailing: What’s wrong and what does the future look like?
    • Thanks Trevor,

      Interesting to see how assessors in the Republic of Ireland are looking at competency. Thank-you for the links!

      Whilst I can understand the thinking behind the NSAI registration requirements and auditing, I do think that the NSAI fee structure for becoming ‘certified’ looks a bit pricey, especially considering the niche nature of the field, the complexity and time involved in modelling details, the cost of the software and the training. It all needs to be balanced to drive competency, whilst keeping creating thermally modelled details accessible – there is already considerable money and time cost involved in getting details modelled in the first place.

      The issue is that there are literally thousands of possible details and variations upon them; the cost of modelling a large number would already be high, with a risk to manufacturers of product substitution after all their hard work, so it’s unrealistic to put all the cost on them; Housebuilders are not going to be keen on absorbing additional costs and modellers certainly aren’t.

      Experienced practitioners are already considered competent in the UK, so are not going to be happy with additional expense and a lot of hassle to get someone else to agree that they know what they are doing (especially if they’re not convinced of the competency of whomever is auditing them!). At the very least, a cheap, simple APEL route is needed for those who have been undertaking calculations for many years.

      The On Construction Assessor accreditation model provides a pointer; with these schemes, you can join one or more accreditation scheme’s by undertaking a qualification and a competency portfolio, which various training providers offer at a cost; or, the schemes also allow for a check of acquired previous learning (the APEL route).

      The existing accreditation schemes make their money from training and from the small lodgement fees for EPC’s, most don’t charge any yearly fee. Something similar could work here, a one off charge for training and competency may be reasonable, a lower one-off charge for APEL assessment of competency may also be accepted, but on-going costs needs serious thought.

      One option might be for details having a small optional ‘lodgement’ charge to become ‘assured details’, uploaded to a scheme with supporting information for anyone to look over and challenge (peer review). Non-assured details could then eventually have confidence factors applied, as was originally proposed in the 2010 approved documents. If this approach were adopted, a condition of maintaining competency might be for a minimum of 3 details (or variants of details) a year be ‘lodged’. Such an approach could lead to a large, growing pattern book for everyone to use being slowly developed and expanded upon cheaply.

      Some form of monetisation will have to come into it (no-one is going to run things for free), but it doesn’t have to be overly burdensome.

      Personally, I think an Association, with a good open forum would be an excellent approach for the UK at the heart of whatever is set up, something with minimal or nil cost to join, access and participate that allows a ‘crowd-sourced’ approach to discussing and peer reviewing and challenging details and that allows modellers to discuss issues and ask for and give advice to other practitioners.


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