Part L - Conservation of fuel and power

The Accredited Construction Details guides have now been archived; they are no longer the most up-to-date statutory guidance. They are available on the national archives website.¹

Accredited Construction Details (ACDs) have been developed to assist the construction industry achieve the performance standards required to demonstrate compliance with the energy efficiency requirements (Part L) of the Building Regulations.

The details and introductory section focus on the issues of insulation continuity (minimising cold bridging) and airtightness. They are not intended to provide any detailed guidance on other performance aspects such as vapour control, ventilation, etc which must also be considered by the design and construction team.

The details contain checklists which should be used by the Designer, Constructor and Building Control Body to demonstrate compliance.

The details have been grouped by generic construction type. It is strongly recommended that the introductory section is read in conjunction with the particular set of ACDs in order to better understand the principles underpinning the details themselves.

• Section 1 - Introduction and general theory of insulation continuity and air tightness² (PDF 1.23 Mb)
• Section 2 - Indicative detail drawings (see details below):
  • Steel frame details
  • Timber frame details
  • Masonry cavity wall insulation details
  • Masonry internal wall insulation details
  • Masonry external wall insulation details

Steel Frame Details

This section shows appropriate details for light steel frame construction. With this form of construction it is important that some insulation is placed outside the steel frame to provide a thermal break and avoid condensation.

• Warm frame construction where all the insulation is outside the steel frame.
• Hybrid construction, where insulation is included both outside the steel structure and in between the steel components (a minimum of 33% of the thermal resistance should be provided outside the steel.) With this form of construction a condensation risk analysis (in accordance with BS 5250) should be provided by the system manufacturer to ensure there is no risk of interstitial condensation. An internal vapour control layer is generally required.

The details drawn in this section are based on warm frame construction but apply equally to hybrid construction.

The depth of steel frame can typically vary from 75mm to 150mm and these details are appropriate for all such variations.

Insulation thicknesses have not been shown as these depend on the thermal properties of materials. However, it is important that appropriate tightly fitting materials are chosen. Generally, a rigid insulation material is required outside the frame that acts as an insulated sheathing board. Where the hybrid construction is used, with some insulation between the frame, a vapour control layer should be provided unless careful condensation risk analysis shows that this is not necessary.

Although the drawn details shown brick cladding, they are appropriate for a range of other claddings subject to suitable detailing.

• Steel Frame Illustrations³ (PDF 503Kb)

Timber Frame Details

The details within this section are valid for a range of timer frame wall thicknesses from 89mm up to 150mm stud size.

For this form of construction, details are given for the junctions with a range of roof, ground floor and internal floor types, in addition to details around window openings etc.

The nature of timber framed construction is that a variety of structural forms can be adopted, with variables such as stud centres, double or single head plates. The form of structure selected has an influence on the thermal performance of any given wall and so must be taken into account by those using these details.

Further variables are:

• Type of insulation chosen
• Type of sheathing
• Type and thickness of plasterboard (or other sheet material)
• Internal linings used and the outer cladding of the building

Products specified should be suitable for their intended purpose.

Insulation thicknesses for main building elements have not been provided as these depend on the thermal properties of the materials chosen together with the proposed U-value.

All details are shown with a brick outer lead for simplification. However, other types of claddings may be used without any loss of thermal performance or increased technical risk subject to suitable detailing. These include render on metal lath (on vertical battens fixed direct to frame), tile hanging on battens on frame, sheet panel systems on vertical counter battens fixed to frame, out leaf of other masonry.

• Timber Frame Illustrations⁴ (PDF 567Kb)

Corrections

Please note the corrections required to the following pages regarding the detail number on the drawings in the above PDF document:

• Page 18 of pdf: detail number on drawing should read TFW-RF-01 and would then be consistent with page footer.
• Page 24 of pdf: detail number on drawing should read TFW-WD-02 and would then be consistent with page footer.

Masonry Cavity Wall Insulation Details

The details within this section have been developed for a range of partial and fully filled cavity wall constructions.

For this form of construction, details are given for the junctions with a range of roof, ground floor, and internal floor types, in addition to details around window openings.

Insulation thicknesses for main building element have not been provided as these depend on the thermal properties of the materials chosen, together with the proposed U-value.

All details are shown with a brick outer leaf for simplification. However, other types of masonry materials may be used as a substitution, without any loss of thermal performance or increased technical risk- such as blockwork with render, tile hanging or weather boarding.

The suitability of full fill cavity construction is dependant on the exposure of the site and the nature of the outer leaf. Further information is given in BR262 ‘Thermal Insulation: Avoiding Risks’, NHBC Standards, and Zurich Building Guarantees Technical Manual.

• Masonry Cavity Wall Insulation Illustrations⁵ (PDF 630Kb)

Internal Walls

The details within this section have been developed for internally insulated cavity wall constructions.

For this form of construction, details are given for the junctions with a range of roof, ground floor and internal floor types, in addition to details around window openings etc.

Insulation thickness for main building elements have not been provided as these depend on the thermal properties of the materials chosen, together with the proposed U-value.

Due to the practicality of fixing insulated dry lining to blockwork these details limit the overall board-insulation thickness to 75mm.

All details are shown with a brick outer leaf for simplification. However, other types of masonry materials may be used as a substitution, such as blockwork with render, tile hanging or weather boarding, without any loss of thermal performance or increased technical risk.

• Masonry Internal Wall Insulation Illustrations⁶ (PDF 626Kb)

Masonry External Wall Insulation Details

The details within this section have been developed for a range of externally insulated solid masonry wall constructions.

For this form of construction, details are given for the junctions with a range of roof, ground floor and internal floor types, in addition to details around the window openings etc.

Insulation thickness for main building elements have not been provided as these depend on the thermal properties of the materials chosen, together with the proposed U-value.

All details are shown with a thin coat render system for simplification. However, a range of cladding may be used without any loss of thermal performance or increased technical risk. These include thick coat renders, brick slips, tile hanging, and other proprietary systems. It is recommended that insulating and cladding components are part of a system to ensure compatibility.

• Masonry External Wall Insulation Illustrations⁷ (PDF 713Kb)

1. https://webarchive.nationalarchives.gov.uk/ukgwa/20141202131824/http:/www.planningportal.gov.uk/uploads/br/accredconbk.pdf
2. https://webarchive.nationalarchives.gov.uk/20141202131824/http:/www.planningportal.gov.uk/uploads/br/accredconbk.pdf
3. https://webarchive.nationalarchives.gov.uk/20141202161427/http:/www.planningportal.gov.uk/uploads/br/steel_frame_illustrations.pdf
4. https://webarchive.nationalarchives.gov.uk/20141202161423/http:/www.planningportal.gov.uk/uploads/br/wood_frame_illustrations.pdf
5. https://webarchive.nationalarchives.gov.uk/20141202161419/http:/www.planningportal.gov.uk/uploads/br/masonry_cavity_wall_insulation_illustrations.pdf
6. https://webarchive.nationalarchives.gov.uk/20141202161421/http:/www.planningportal.gov.uk/uploads/br/masonry_internal_wall_insulation_illustrations.pdf
7. https://webarchive.nationalarchives.gov.uk/20141202161425/http:/www.planningportal.gov.uk/uploads/br/masonry_external_wall_insulation_illustrations.pdf