Is someone trying to pull a fast one on me? Part L vs Psi Calculations

[Great_scot_selfbuild]

Are detailed Psi value calculations required for building control, and how expensive are these to get done?

A quote for SAP calculations etc. includes a large amount required for detailed Psi calculations, but I thought this was only relevant if we were aiming for Passivhaus certification (because it came up in discussion). I want to know what the minimum is that we require for energy calculations.

The quote in the proposal below mentions "The new Part L 2021 has removed the ability to use Accredited Construction Details for thermal bridging calculations. This means that the dwelling will need to use calculated Psi values for all thermal bridging junctions that occur in the dwelling ", but the way I read para 4.18 in Part L (see attached screenshot) implies there are standard values that can be used? (table K1 etc.)

 

Situation:

We're building a new house from scratch (no existing buildings).

Seeking high performance structure, 0.6acph (MVHR etc) but not going for passive certification.

Planning required 'as designed' SAP calculations to be submitted prior to approval - the figures submitted were estimates as the detail of material selection/exact suppliers for windows etc hadn't been decided on (in case we didn't get through planning).

 

SAP Proposal received:

******************

Part L - Our fee for carrying out SAP energy rating calculations and providing a Part L1A (2021) BREL compliance report and a Predicted Energy Assessment to support a Building Regulation submission together with the provision of an Energy Performance Certificate upon completion is £x plus VAT. I have attached the guidance for photographic evidence which is required on all new build SAP Calculations under the new Part L 2021. The photos taken on site will need to line up with the psi value calculations which I will outline below.

 

Part G - Our fee for carrying out the Part G Water Efficiency Calculations is £x plus VAT.

 

Part O – Our fee for carrying out Part O Compliance Calculations under the ‘dynamic method’ is £x plus VAT.

 

Psi Value Calculations – The new Part L 2021 has removed the ability to use Accredited Construction Details for thermal bridging calculations. This means that the dwelling will need to use calculated Psi values for all thermal bridging junctions that occur in the dwelling (I have attached a rough diagram which show you where these are). With cavity construction there are most of these details available to download for free depending on the method of insulation - https://www.recognisedconstructiondetails.co.uk/ . As this project is timber frame/SIPS I can produce a full set of Psi value calculations for project . Once calculated these psi values can be used on every project going forward so long as the construction remains the same. It is difficult to ascertain exactly how many junctions there will be at this stage. But roughly there will be:

E2 – Lintels

E3 – Window Sills

E4 – Window Jambs

E5 - Ground Floor (Normal)

E6 - Intermediate Floor

E11 - Eaves at Ceiling Level

E12 – Gable at Ceiling Level

E13 – Gable at Rafter Level

E14 – Flat Roof

E16 – Corner wall detail

E17 – Corner wall (inverted) detail

R1 – Rooflight Head (We can likely use the default figure and not calculate this junction)

R2 – Rooflight Sills (We can likely use the default figure and not calculate this junction)

R3 – Rooflight Jambs (We can likely use the default figure and not calculate this junction)

R6 – Flat Ceiling

R4 – Vaulted Ceiling

 

Depending on the final construction method I would expect a couple of the junctions not to be needed.

 

Roughly there will be 13 (16 with rooflights) junctions on the project. Our fee for producing bespoke calculations is £x plus VAT per junction (this will not be required for traditional constructions in line with the recognised details link above). So roughly speaking the fee for this work if it’s a timber frame project will be around £x plus VAT.

 

I appreciate there is a lot to take in on the new regulations so please feel free to give me a call If you would like to discuss anything.

A unique SAP calculation is produced for every dwelling to meet the requirements of Part L (2021) of the Building Regulations. Our fee includes advice on cost effective compliance and value engineering based on client needs; the provision of Predicted Energy Assessments (PEA’s) and BREL Compliance Reports signed by the SAP assessor to support a Building regulation submission. Once construction is complete, we will produce the As Built BREL Compliance Reports, and offer a same day turnaround for generating Energy Performance Certificates (EPC). Arcadian Architectural Services Ltd issue EPCs via an electronic link, from which you can access the EPC and download for your records, or we can provide pdf copies. If alterations to the issued design stage BREL Compliance Reports are required as a result of specification or layout/design changes, additional fees may be incurred.

 

Our fee may be subject to change under Approved Document L 2021, pending Government guidance on the implementation of photographic evidence required for thermal bridging. With effect from the 15th June 2022 to achieve construction approval, Building Control require evidence, through photographs, throughout the build process of all thermal bridges. Photographic Evidence is the responsibility of the builder/developer and to be provided to your Energy and Sustainability assessor as construction progresses. For more information refer to Appendix B of the revised regulations: Approved Document L: Conservation of fuel and power.

******************

Appreciate your experience / advice.

[ADLIan]

Psi-values are required as part of the SAP calculation input. The accredited construction details can no longer be used so option is to use default psi-values (which makes BR compliance very tough) or calculated psi-values (or mixture of both). There is lots of published data on masonry and timber frame psi-values - see trade associations, building product manufacturers. For non-standard construction such as SIP, insulated formwork, light steel frame etc bespoke calculations may be only option, they are complex and time consuming hence costly!

[Alan Ambrose]

If I remember rightly, SAP is mostly a nonsense calc anyway with a black box algorithm and the ‘assessors’ are allowed to ‘assume’ significant details about construction. It would be much better if we used PHPP calcs which are, at least, open and transparent (well as long as you’re happy to trawl through German-language research papers).

 

So you have a couple of options:

 

+ do as they say.

+ ask them to use the fallback default value first and see what the result is and whether you (and BC) are happy with that. Both of you may well be if the build is to a good spec and you’re not worried about boasting about your great EPC rating.

+ you can also do the psi calcs yourself using Therm or similar - at least to see how different the results are from the default numbers. Not trivial, but not hard either. BC may not accept your values in the official calcs (as you’re not an accredited ‘expert’) but at least it gives you some control over the situation (and you may learn something useful things about your construction also).

 

Standing back, this often seems to be how these new rules get introduced:

 

(a) industry organisations / academics steer government to introduce new ‘rules’,

 

(b) a new little industry is minted to milk housebuilders by creating or supporting a new monopoly industry organisation that promptly develops a new accreditation scheme to fleece new wannabe ‘experts’,

 

(c) often accompanied by an actual or virtual or contractural monopoly - often for a private equity company, Capita etc.

 

At least this rule-making process and economic milking process need to be made a lot more transparent.

[Nic]

  On 05/12/2024 at 21:44, ADLIan said:

Psi-values are required as part of the SAP calculation input. The accredited construction details can no longer be used so option is to use default psi-values (which makes BR compliance very tough) or calculated psi-values (or mixture of both). There is lots of published data on masonry and timber frame psi-values - see trade associations, building product manufacturers. For non-standard construction such as SIP, insulated formwork, light steel frame etc bespoke calculations may be only option, they are complex and time consuming hence costly!

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just had the same issue. Our very insulated, timber frame build triple glazed open plan with some internal steel uprights etc required a psi done before sap could be completed, all to comply with our building regs 🤷🏼‍♂️

[SteamyTea]

BRE has this 2010 document with some values.

 

https://files.bregroup.com/bre-co-uk-file-library-copy/filelibrary/SAP/2009/SAP-2009-Appendix-K.pdf

 

And the LABC has this one.

 

https://www.labc.co.uk/sites/default/files/resource_files/zch-thermalbridgingguide-screen.pdf

 

ESE

https://www.energy-saving-experts.com/wp-content/uploads/2011/07/Thermal-Bridging-Part-L1A-landscape-version-.pdf

 

Should be enough there to work it all out yourself and save a few quid.

[SBMS]

  On 06/12/2024 at 08:32, Alan Ambrose said:

ask them to use the fallback default value first and see what the result is and whether you (and BC) are happy with that

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Your build will almost certainly fail minimum sap requirements using the default values. That’s pretty much why they removed ability to use standard details. 
 

We went to our cavity insulation provider and their in house team calculated all the requisite psi values for most of the E values. Warmcel Then provided psi values for many of the R values. 
 

most sap providers will want custom values and won’t chance ‘making them up’ as they may face audit. 
 

what’s your construction makeup? Ours was a 200mm cavity so there were t any readily available. 

[Great_scot_selfbuild]

Thanks for all the responses - happy that the feedback is consistently confirming that we need them, and like much of the BR, it's something we've just got to 'suck up' and get done. The company we used for the as designed SAP during planning proved competitive then and we got more quotes for the construction phase and they're still competitive, as well as being good to deal with. Although the technical detail and calculations interests me, I think this is something that will be good to have done professionally for us and add value to the huge amount of data we'll have compiled by the time it's built - I figure it'll add value in the end.

 

We're just going through the process of selecting our timber frame manufacturer atm, and finding that the budget cost juggling is a complex relationship of interdependencies but we're getting there and still on track for our target of starting in March. At the moment...

[SBMS]

  On 07/12/2024 at 17:02, Great_scot_selfbuild said:

Thanks for all the responses - happy that the feedback is consistently confirming that we need them, and like much of the BR, it's something we've just got to 'suck up' and get done. The company we used for the as designed SAP during planning proved competitive then and we got more quotes for the construction phase and they're still competitive, as well as being good to deal with. Although the technical detail and calculations interests me, I think this is something that will be good to have done professionally for us and add value to the huge amount of data we'll have compiled by the time it's built - I figure it'll add value in the end.

 

We're just going through the process of selecting our timber frame manufacturer atm, and finding that the budget cost juggling is a complex relationship of interdependencies but we're getting there and still on track for our target of starting in March. At the moment...

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Might be worth asking your timber frame manufacturer if they have psi values. 
 

why timber frame out of interest? Have you considered masonry or had any quotes yet?

[PhilC]

Trying to work through this detail myself for a timber frame new build. @Great_scot_selfbuild who did you go with for the calculations?

 

I take it most SAP assessors are not doing the calculations themselves, I would imagine it's down to the assessor but if I can point them in the direction of an accredited junction as per 4.18 section b, then they shouldn't need to do that particular junction right?

[Great_scot_selfbuild]

  On 18/03/2025 at 10:32, PhilC said:

Trying to work through this detail myself for a timber frame new build. @Great_scot_selfbuild who did you go with for the calculations?

 

I take it most SAP assessors are not doing the calculations themselves, I would imagine it's down to the assessor but if I can point them in the direction of an accredited junction as per 4.18 section b, then they shouldn't need to do that particular junction right?

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Not done it yet but we’re planning on using Alex Pelling at Arcadian Architectural Services - they did the SAP input for us at planning stage and were very good to deal with and came in at a fair price compared to the others we approached.

[Alan Ambrose]

I don’t know if it helps any (as they probably want an ‘expert’) but I didn’t find the freeware Therm hard to use.

[JohnnyB]

  On 26/03/2025 at 20:10, Alan Ambrose said:

I don’t know if it helps any (as they probably want an ‘expert’) but I didn’t find the freeware Therm hard to use.

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I'm having the same issue, needing thermal modelling. If you didn't find it hard to use would you be able to give some help in working out how to use it?

 

I have been told that if we can model a standard junction and come up with the same results as the 'experts' then they can accept our figures.  It sounds like there are a lot of junctions to model and I'm getting prices of £150-300 plus vat per joint. It is sounding expensive if I have 5 plus joints to model.

[SBMS]

  On 27/03/2025 at 09:41, JohnnyB said:

I'm having the same issue, needing thermal modelling. If you didn't find it hard to use would you be able to give some help in working out how to use it?

 

I have been told that if we can model a standard junction and come up with the same results as the 'experts' then they can accept our figures.  It sounds like there are a lot of junctions to model and I'm getting prices of £150-300 plus vat per joint. It is sounding expensive if I have 5 plus joints to model.

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What’s your construction method? I asked our insulation supplierto model most of the junctions which they did for free..

[JohnnyB]

I don't think that's going to happen for us, we are using hempcrete with a stick built timber frame.  The hempcrete is mixed on site by a small company. It's a basic process using mostly natural materials and when we have come up againest these type of questions they haven't been asked them before.

[Alan Ambrose]

For Therm you need to get the 2D CAD details of the junction into the package together with the thermal values of the components. Then you apply the boundary conditions (i.e. temperature) on the inside and outside of the structure. Run the calc and it’ll give you the heat flow across the junction. Now, you need to take the heat flow in the idealised junction used in your overall heat calcs. Subtract Therm’s number from the idealised number and you end up with the psi value - that is, the extra heat loss per linear metre of junction that you need to add to your idealised calc to cope with the actual junction details.

 

Once you’ve got the hang of it, the hardest part is getting the CAD into Therm (either redraw in Therm or figure out how to transport DWG/DXF from wherever your drawings are now). The Therm UI is a bit basic but OK. It does produce some pretty pictures  which are interesting and enable you to better understand how the junction is working thermally.

[PhilC]

  On 27/03/2025 at 09:41, JohnnyB said:

I'm having the same issue, needing thermal modelling. If you didn't find it hard to use would you be able to give some help in working out how to use it?

 

I have been told that if we can model a standard junction and come up with the same results as the 'experts' then they can accept our figures.  It sounds like there are a lot of junctions to model and I'm getting prices of £150-300 plus vat per joint. It is sounding expensive if I have 5 plus joints to model.

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Was it building control who said they would accept it?

 

I can model with Therm and would be happy to save the cost, but I don't want to do all the work just to be rejected that I am not accredited. 

[PhilC]

Dont support anyone has already modelled some timber frame junctions in therm they would be happy to share?

[JohnnyB]

  On 29/03/2025 at 07:42, Alan Ambrose said:

For Therm you need to get the 2D CAD details of the junction into the package together with the thermal values of the components. Then you apply the boundary conditions (i.e. temperature) on the inside and outside of the structure. Run the calc and it’ll give you the heat flow across the junction. Now, you need to take the heat flow in the idealised junction used in your overall heat calcs. Subtract Therm’s number from the idealised number and you end up with the psi value - that is, the extra heat loss per linear metre of junction that you need to add to your idealised calc to cope with the actual junction details.

 

Once you’ve got the hang of it, the hardest part is getting the CAD into Therm (either redraw in Therm or figure out how to transport DWG/DXF from wherever your drawings are now). The Therm UI is a bit basic but OK. It does produce some pretty pictures  which are interesting and enable you to better understand how the junction is working thermally.

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For me the easy bit will probably be the CAD as I know how to use that.  If Therm is fairly easy to use I would be interested in understanding how to use it and work out how good the junctions should be, whatever the EPC shows. 

I'll see if I can make any sense of it. Would you be prepared to show me a little of how it works, as we are both in Suffolk? I find it easier to learn by doing/watching than trying to understand instructions. I did have a bit of a look last year and hadn't a clue what to do, I think that was understanding where to start, what was needed and feeling like I hadn't got the time to learn what was needed.

 

  On 29/03/2025 at 09:20, PhilC said:

 

Was it building control who said they would accept it?

 

I can model with Therm and would be happy to save the cost, but I don't want to do all the work just to be rejected that I am not accredited. 

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Thank you, that's very kind. As above I'll have a look and see if I can make any sense of it. Is there a simple instruction set to start out with that would help me start to understanding the software?

 

It was one of the SAP guys who said about proving the method. He and a colleague were looking into software used for thermal modelling to see if they could do it theirselves.  He said he needed to be able to prove his models by modelling an approved model first and getting the same results as the approved results. I was reading up since posting and wonder if it is that straight forward.  I'll speaking to the two people I've been in contact with about the SAP calcs and I'll ask them both what they need.  My understanding is they are responsible for what they sign off (and are happy to prove if they are inspected) and BC are happy as long as they have an EPC.

 

My timber frame isn't standard, using hempcrete makes it quite different so I'm not sure other timber frame models would work for this, I guess it would be a starting point . If I know what is needed for Therm I can draw it in CAD, I might already have it from the BC drawings.  Examples of DWG drawings would be helpful to start with.

[SteamyTea]

I downloaded Them as got a new laptop.

It then wanted me to download extra programs, .net and a Fortran on I think.

Downloaded them, installed them, got asked for a serial number.

Don't seem to remember having that trouble last time.

What am I missing?

[Alan Ambrose]

>>> I can model with Therm and would be happy to save the cost, but I don't want to do all the work just to be rejected that I am not accredited. 

 

There’s the rub. If they won’t, then apart from general interest / investigating design options / double checking - there’s an option which says ‘if I’m going to be paying anyway, I might just as well let the ‘expert’ do it in the 1st place.

 

@JohnnyB - yeah, can do. I’ll see if I can’t find the time to put up an example on here as well. Do have a simple example we can use as a test?

[JohnnyB]

  On 31/03/2025 at 07:21, Alan Ambrose said:

>>> I can model with Therm and would be happy to save the cost, but I don't want to do all the work just to be rejected that I am not accredited. 

 

There’s the rub. If they won’t, then apart from general interest / investigating design options / double checking - there’s an option which says ‘if I’m going to be paying anyway, I might just as well let the ‘expert’ do it in the 1st place.

 

@JohnnyB - yeah, can do. I’ll see if I can’t find the time to put up an example on here as well. Do have a simple example we can use as a test?

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The first SAP assessor I spoke to (who doesn't have time to take on my project for health reasons) when asked about the modelling and using PSI values I came up with, or someone else, he said can't see a reason why he couldn't accept my figures.  I have asked the person who will probably be doing my calcs if she can use my figures, or those from someone else, but via email and I am waiting for a response.

 

I can't see any 'qualifications' I could gain, just courses for using software, and the below screenshot is from the Elmhurst website

 

 

 

 I tried to attach a basic DXF of my wall build up above the window but a DXF file isn't an allowed file.  How do I post/send it?  I have added a screenshot and one of the 3D to show what we have built.  I have also attached a spreadsheet with figures I was given to work out the U values.

 

There will be timber fixed to the main timber frame to fix the window in place but it will be in short lengths at 3 or 4 points around the opening to reduce the amount of timber needed and that can conduct heat.  There will be an approx 10mm gap between the window frame and the hempcrete that will be filled with Illbruck FM330 expanding foam but gaps don't seem to be shown on the models I have seen so I haven't included it.  I would assume that helps the thermals is it is an insulating material and something is needed to fill the gap.

 

 

U value calculator.xlsxFetching info...