Approved Document L and timber frame

Timber frame buildings have evolved over many years in response to changing thermal performance requirements in Building Regulations. Historically, timber frame buildings had substantially better thermal performance than their masonry counterparts; even early post-war ‘modern’ timber frame buildings contained some form of insulation between studs, where their masonry counterparts did not.

However, in more recent decades requirements to achieve ever tightening thermal performance targets saw that gap decrease, and today the U-values (thermal performance) of masonry and timber frame buildings will be governed by U-values in Approved Document L and the outcome of Standard Assessment Procedure (SAP) calculations. 

Early 'modern' timber frame external walls used approximately 90mm deep timber studs, but in the early 2000s, the industry increased wall thickness to 140mm to incorporate more thermal insulation between the studs. After the updates to Approved Document L in 2010, service voids with additional insulation or reflective vapour control layers were generally adopted to help further improve U-values as well as offering enhanced air tightness performance. 

The recent changes to Approved Document L were published in 2021 and came into effect in June 2022. These changes introduced prescriptive guidance above and beyond the SAP calculation/U-value targets. While the improvements in U-values and SAP targets are an incremental step change, building upon previous revisions, the prescriptive guidance is new and quite different to the information previously contained in Approved Document L.

BREL report

One overarching notable change is the introduction of drawing reviews and site audits encompassing items such as installation of insulation, thermal bridging and airtightness. During the design stage, the designer and installer should review drawings to check that prescriptive guidance in Approved Document L has been met and that the details are robust and buildable. During construction, photographic records should be taken of site works before they are enclosed by following trades. The final Building Regulations England Part L (BREL) report should then be provided to Building Control and the building owner to show that work complies with energy efficiency requirements.

This review and reporting may offer advantages for off-site construction, and for building designers working with timber frame manufacturers. Installing items such as insulation and airtightness layers in closely controlled factory conditions may make control of these works easier and reduce the risk of construction defects on site, for example.

Thermal bridging 

Timber frame buildings have historically had reasonable thermal bridging performance. Repeat thermal bridges (such as wall studs) are factored into U-value calculations, and non-repeat thermal bridging has generally been reasonable when compared to other construction materials (due to timber’s reasonable thermal conductivity). Under the recent changes to Approved Document L, there are a number of prescriptive guidelines which have the potential to result in considerable changes to timber frame.

Firstly, it is suggested that insulated plasterboard can be installed to the inside face of external walls to help further reduce thermal bridging from timber studs. While insulated plasterboard can provide that function, there are several potential issues with this type of construction. At present there is limited availability of suitable fire resistance test evidence for insulated plasterboard products used on timber frame which may limit or prevent their use. In addition, there are potential order of works issues with the installation of services, service voids and vapour control/airtightness layers. 

Where designers wish to incorporate continuous layers of insulation in timber frame external walls, it is generally more sensible to install a separate continuous layer over the inside or outside face of the studs prior to the installation of further finishes. However, these methods also have their advantages and disadvantages, including availability of fire test evidence, order of works and installation of cavity barriers. Designers must remember that there can be unintended consequences to their choices - a good choice for thermal performance reasons may bring with it added complications to fire performance and buildability, and it is important that these potential issues are identified and mitigated at the design stage. 

Secondly, Approved Document L suggests that in all construction types, insulated cavity closers should be installed around window and door openings. In timber frame, cavity closers must act as fire resistant cavity barriers. Historically, timber battens at least 38mm wide have been used to both close the cavity and act as a fire resistant cavity barrier, although it is unlikely that these would be considered 'insulated'. Therefore, it may be necessary for the industry to move away from timber battens and instead use polythene-sleeved mineral wool cavity barriers, or other proprietary insulated cavity barriers/closers (providing they can provide the required fire resistance). Polythene-sleeved mineral wool cavity barriers are commonly used in other locations in timber frame construction, but their use around windows will have a knock-on effect on window installation details for air and moisture tightness and for order of works. The industry may need to adopt a different approach to weather-sealing windows and doors into external walls, as historic detailing is not compatible with flexible cavity barriers.    

Airtightness 

Timber frame buildings have always offered good airtightness performance compared to other construction methods. The inclusion of a vapour control layer to the inside face of external walls can also be used as an effective air barrier, with lapping of membranes at junctions helping to reduce or eliminate unintended air infiltration into the building.

The updates to Approved Document L now discuss the use of self-adhesive tape to seal junctions and laps in the vapour control layer/air barrier. However, in our experience, the use of self-adhesive tape should be minimised as site application of tapes can be problematic and may not provide a long-term robust seal. Conditions on site are often less than ideal, and surfaces which are too cold, wet or dusty can limit the performance and robustness of adhesive-bonded tapes. 

Instead, junctions in the vapour control layer/air barrier should be mechanically lapped over studs and rails and then trapped and clamped by the service void battens. These types of mechanical laps are likely to provide a more robust and long-term seal than tapes, as well as being less reliant on quality of application. Combining mechanical lapping with the addition of taping may offer superior performance to either, in isolation. 

Approved Document L now also states that the air barrier should touch or be in contact with the insulation layer. The intention of this guidance is to reduce the likelihood of thermal bypass; that is movement of cold external air through the wall into a void or space between the air barrier and insulation on the warm side of the wall. This guidance will not be problematic for walls fully filled with 'soft' insulation materials (such as mineral wool, cellulose fibre, or other natural batt or quilt products). It may, however, require a change to material specifications where rigid board insulation products are used, as these often do not fill the full depth of the stud voids thereby leaving gaps, which should now be avoided.

Summary 

In summary, the recent changes to Approved Document L are a welcome step further along the path to reducing building energy usage (with the associated savings in cost and carbon emissions in use). However, as is often the case, building designers must carefully consider the potentially conflicting Approved Document sections and be mindful of potential unintended consequences which may inadvertently compromise some other aspect of building performance or safety.