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New Part L Building Regs - no more standardised psi values for thermal bridging in timber frame buildings


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Hi all,


We are into a new build and have a very thermally efficient and airtight design but have been told by our SAP assessor that for the new Part L regs they cannot use standardised psi values as they used to in the past for the thermal bridging calculations of timber frame builds.  They perhaps can work them out manually but this may work out to cost around £5k…  I can’t imagine we are the first to do this since the new Part L regs came out and have been trying to find values from the structural timber association and our manufacturer as has the SAP assessor.  Has anyone come across this before or have an idea on a source for these potentially?  I have looked at other forums, there is an interesting one from 2021 on Part L but nothing that seems to help.  



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15 hours ago, SteamyTea said:

Thanks I did look at this post originally but think it is now generating more questions.  

This post is for ICF and Isotex and they were after the same info as I am for timber frame.  The formula you offered can be used for this calc in timber and is useful, I’m assuming that would be the same the assessor would use to calculate for each junction?  If so I can see why it could cost £5k, I suppose I could do it myself though it is probably above me dealing as I do with data protection and information security not big sums…


The bit from Part L that concerns me is:


 4.18 Thermal bridges should be assessed in a new dwelling using one of the following methods.

a. Use construction joint details calculated by a suitably competent person following the guidance in the Building Research Establishment’s BR 497 and the temperature factors set out in the Building Research Establishment’s Information Paper 1/06.

b. Use junction details from a reputable non-government database containing independently assessed thermal junction details, such as Local Authority Building Control’s Construction Details library.


I expect that calculation you provided in the Isotex post would be used to cover part a of 4.18 from Part L but competent persons are expensive and that will cost me £5k!  

Are there tables for timber frames that would meet part b.? I can find them for masonry but not timber…


It seems this new rule in Part L has come in since February and I can’t imagine we are the first to build a timber frame house since then, where have they get their figures from or, has each new timber frame build since Feb gone up by approx £5k?  Is this now a set of calculations that all builds will need and has it not been done before?   I have asked the STA but they have got nothing yet.



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Thanks, much appreciated, no I haven’t but will do...

I think it is reasonable now I understand the work involved!  It’s just upsetting that there seems to be data for other construction methods, masonry definitely, and not timber, the STA are saying go to the manufacturer, I have and they have nothing so far.


I strikes me that if there isn’t a central database for timber this potentially adds an extra uplift to the cost of a timber frame that other building methods with the tables of figures won’t have.

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  • 2 months later...
  • 2 months later...

Hi Piers, sorry I missed this message.  Yes after a fashion I used a company called ASAPS and the majority of the junctions they calculated though there are a few more that need to be done.  If you are still looking for a solution I can dig out the contact for them?



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