Using acceptable construction details for compliance
To make best use of the ACDs, the following will be helpful.
• Detail drawings (sections and plans) identifying the line of the air barrier
• List of Acceptable Construction Details incorporated into the design
• List of the designer/builder’s own details incorporated into the design
• Specification of the air barrier materials / elements
• Details of air barrier junctions and interfaces including means of sealing service penetrations
• Evidence of Site Quality Control during construction (photos, check
sheets, etc.)
Use of Acceptable Construction Details
Part 2 of this publication provides a series of Acceptable Construction Details showing typical
junction interfaces for various construction types.
Section 1 - Cavity insulation
Section 2 - External insulation
Section 3-Internal insulation (when combined with section 1, 4 or 5 construction
types)
Section 4 - Timber Frame
Section 5 - Steel Frame
Section 6 - Hollow Block Internal Insulation
Section G - General (applicable to all construction types)
At the start of each section there is an introduction outlining the details described in that section and the specific psi values associated with each detail. This also describes the range of U-values of the flanking elements making up the junction which can be used. Outside of these ranges, the Table D psi-values should not be used.
Acceptable Construction Detail Sheets
The ACD sheets comprise thermal insulation and air barrier checklists together with an indicative illustration and general annotation. The purpose of the illustration (used in conjunction with the checklists and general notes) is to provide generic guidance on the key features that must be incorporated into the actual designs. They do not provide a complete solution to airtightness or insulation continuity on any particular project.
The general notes outline important issues regarding performance of each junction in terms of insulation continuity and airtightness, and raise potential conflicts such as ensuring adequate ventilation to roof voids. The notes and the comments are not exhaustive and users must satisfy themselves as to the fitness of their own designs for the intended purpose.
The checklists enable the builder to make reasonable provision for ensuring insulation continuity and airtightness. It is recommended that the design and construction of each dwelling be reviewed at key stages during the process by reference the ACD and other checklists, as appropriate.
Substitution of Contractors Own Designs and Proprietary Designs
If details other than Acceptable Construction Details are proposed for use in construction, they should meet the alternative requirements given in Paragraph 1.3.3.2 in Building Regulations 2021 TGD L – (Dwellings).
DEAP Calculations
Where all ACDs are adopted for all key junction details for a dwelling type and are installed as per the ACD checklists, the dwelling fabric design as a whole will meet the guidance provided in Par 1.3.3.2 in Building Regulations 2021 TGD-L (Dwellings).
The linear thermal transmittance can be calculated for each of the key junctions for the specific dwelling using the psi values given in Tables D1 to D6 in Appendix D of TGD L 2021 and assembled into a y-factor calculation to take account of the overall thermal bridge heat loss in the DEAP calculation, see example in Appendix A.
Psi values (heat loss per metre of junction length) for details which are not Acceptable Construction Details should be calculated and certified in accordance with Building Regulations 2021 TGD-L (Dwellings) Par 1.3.3.2. Certified details may be provided by a third party certification body such as Agrément, or equivalent, or certified by a member of an approved thermal modellers scheme, or equivalent.
Heat loss through thermal bridging can be accounted for in terms of a factor (y) multiplied by the exposed surface area of the building. Where Acceptable Construction Details are used for all key junctions the value of (y) can be taken as 0.08.
Where this is not the case, but this method of accounting for thermal bridging is used, the default value of (y) is taken to be 0.15.
