The basics of Part L Thermal Modelling Building Regulation compliance – Part 1

This post has come about after the 2 I wrote for BRegsForum on:

1. Are specialist Part L Ancillary Certifiers required (Part 1)

Part 2

Which in turn came about following the post by the admin team on:

Design Certifiers – 3 things about certifying Part L…

The purpose of this post is to outline the basics of how Part L Thermal Modelling Building Regulations compliance is achieved. The post is not dealing with levels of insulation, airtightness, renewables etc… However, the outline given below DOES give a generalisation on how Part L compliance is achieved.

So, here goes:

1. Currently Part L compliance relies on the figures calculated in the DEAP (Dwelling Energy Assessment Procedure) software. It is this software that Registered BER Assessors use in calculating the BER rating of the property. We have seen in the ‘3 things post’ that DEAP ‘may not be enough to meet the Part L requirements for dwellings (e.g. surface temperature missing)’

2. A critical aspect of heat losses that lead to condensation and mould growth is concerned with Thermal Bridging. I have written a Fact Sheet on how Passivhaus deals with Thermal Bridges HERE

3. The Building Regulation TGD for Part L states:

“To avoid excessive heat losses and local condensation problems, reasonable care should be taken to ensure continuity of insulation and to limit local thermal bridging”

This is achieved in one of four methods:

• Method 1:

Adopt Acceptable Construction Details as shown in the document “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” for all key junctions.

The problem here is that these Acceptable Construction Details (ACD) published by the Department of the Environment may not be Part L compliant (e.g. Frsi changed in February 2014, ACD not updated in 2011)

• Method 2:

Adopt the above document PLUS “other certified details (as defined below) for all key junctions”

The same problem occurs that these Acceptable Construction Details (ACD) may not be Part L compliant

• Method 3:

By using certified details which have been assessed in accordance, and comply, with Appendix D [of Part L TGD], e.g. certified by a third party certification body such as Agrément or equivalent or certified by a member of an approved thermal modellers scheme or equivalent for all key junctions.

The problem here is that although the Thermal Modellers Scheme exists, there are currently only two people registered in the whole of Ireland; one of which is an employee of an insulation company who don’t supply bespoke values directly. nb A 2D Y value calculator and Accredited details are however available from this company (Xtratherm): Thermal Bridging and Xtratherm Y-value calculator

• Method 4:

By using alternative details which limit the risk of mould growth and surface condensation to an acceptable level as set out in paragraph D.2 of Appendix D (Part L TGD) for all junctions.

This method requires you to calculate the internal surface temperatures for all junctions using the following formula:

fRsi = (Tsi – Te) / (Ti – Te)
Tsi = minimum internal surface temperature,
Te = external temperature, and
Ti = internal temperature.

For dwellings, the value of fRsi should be greater than or equal to 0.75, so as to avoid the risk of mould growth and surface condensation. For three-dimensional corners of ground floors this value may be reduced to 0.70, for all points within 10 mm of the point of lowest fRsi.

This is Part 1, Part 2 to follow…

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