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:

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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.

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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]

19 hours ago, 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!

 

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]

12 hours ago, 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

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]

3 hours ago, 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...

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?