16th March, 2020

SAP Knowledge Email – ACD Checklists

What impact will the removal of ACDs within SAP 10 have on SAP Calculations?

As part of the government’s consultation on Part L 2020 and SAP10, proposals include the removal of the SAP Appendix K Accredited Construction Details, otherwise known as PSI Values for use in thermal bridging calculations.

The FES Group are committed to helping our clients and on request are able to provide calculated bespoke PSI values.

Appendix K

The Appendix K ACD details have provided developers with a “fits all” solution to thermal bridging details provided that simple design and construction practices are implemented on a range of construction types, from cavity, solid and timber frame wall constructions to slab or beam and block floors.

The removal of these details will result in an increased requirement to have project specific, or specification specific PSI values calculated.

Alternative Schemes?

Some alternative detail schemes exist, such as the Concrete Block Association (CBA Details) the Aircrete Products Association (APA or Constructive Details) and the LABC have published a variety of details as well as some block manufacturers. However, all of these details require the development to meet a specific set of criteria, from thickness and thermal conductivity of both  blockwork and insulation to the type of floor construction or type of thermal break within window jambs. This often makes finding an accurate set of details that can be used on the development difficult.

New Junctions & Default Values

Several new junctions have been added over the past few years, including the R junctions and E24 & E25 junctions for exposed floors that were not given ACD values. Very few of these details at this time have been calculated and in these instances default values need to be used in place of calculated PSI values.

Default values are often significantly worse than actual and calculated PSI values specific to the construction being used. This issue is twofold, firstly as the calculations not being represented accurately the developer will be required to over design the dwelling in order to compensate for the lack of performance in the thermal bridging calculation. Secondly the default value is an inaccurate representation of the dwelling’s final as built performance (using default entries always is). This is represented with a lower rating and higher potential costs on the final EPC (making the dwelling less appealing to prospective purchasers).

The key points required for PSI Values 

The key points to keep in mind when looking at PSI Values are as follows:

  • Floor type; the floor type needs to be determined, is it beam and block or slab on ground?
  • Is the building timber frame or cavity. If the latter, will it be using full fill or partial fill insulation?
  • How thick is the insulation and what is the thermal conductivity?

It’s a minefield of potential errors with only one differing parameter in the specification potentially resulting in the PSI values not being valid for the build.

Let our suite of services work for you.

At the FES Group we can help; we can investigate if any of the aforementioned schemes, match a specification and we can issue these junction specific details to the design team to ensure compliance is met. We can also calculate bespoke PSI values calculations for all junction types. This means that in addition to Part L compliance reports we can provide a full set of calculations or calculate those junctions that haven’t been done yet.

For any immediate questions and advice, you can contact our technical team.

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