Use of timber frame construction approaches has grown rapidly in the UK over recent years. In addition to the promise of faster construction speeds, the transition to timber construction is also seen as key as we look to decarbonise the industry. In this article, we’ll look at the thermal performance targets when insulating timber frame walls, and how more thermally efficient insulation can be used to reduce the overall construction depth.
What U-value should new timber frame walls achieve?
The notional building specifications provide a helpful starting point when deciding what U-value to target for timber frame walls. These specifications are set out in the guidance documents which accompany the sections of the Building Regulations and Standards which deal with energy efficiency. These are known as:
- Approved Document L 2021 in England (ADL 2021)
- Approved Document L 2022 in Wales (ADL 2022)
- Technical Handbook 6 2022 in Scotland (TH6 2022)
The guidance documents provide key metrics which buildings must meet, including for expected carbon emissions and energy demand. The notional building specification is applied to a theoretical building of the same dimensions as the actual one. The targets for each metric are then set based on the outputs. Project teams are free to vary from this specification providing they meet all of the target metrics.
The notional building values for external walls on new projects are shown in the table below:
Area weighted U-values (W/m2K) | |||||
|---|---|---|---|---|---|
| Domestic | Non-Domestic | |||
|
| Side-lit and unlit | Side-lit and unlit | Top-lit | |
England - ADL 2021 | 0.18 | 0.18 | 0.18 | 0.26 | |
Wales - ADL 2022 | 0.13 | 0.22 | 0.26 | 0.26 | |
Scotland - TH6 2022 | 0.15 | 0.15 | 0.15 | 0.15 | |
What are the minimum U-values for new timber frame walls?
ADL 2021, ADL 2022 and TH6 2022 also set minimum requirements for fabric performance in the form of allowable ‘area weighted’ U-values for each element, which all properties should aim to achieve or better. Keep in mind these should not be treated as targets. In practice, it is very difficult to achieve compliance if using these values. It is also important to note that it is possible for specific areas of the element to achieve poorer U-values than these, providing the overall average performance for that element compensates for that.
Area weighted U-values (W/m2K) | ||
|---|---|---|
| Domestic | Non-Domestic |
England – ADL 2021 | 0.26 | 0.26 |
Wales – ADL 2022 | 0.18 (dwelling house), 0.21 (flats and apartments). | 0.26 |
Scotland – TH6 2022 | 0.17 | 0.21 |
What thickness of timber frame insulation should I install?
This depends on the U-value you’re targeting and the thermal conductivity of the insulation material you’re installing. Insulation materials with lower thermal conductivities are more effective at preventing heat loss. This means it may be possible to achieve your desired U-value with a slimmer overall construction. For example, Kooltherm K112 Framing Board has a thermal conductivity of just 0.019 W/mK.
Find out more about thermal conductivities.
In the table below, we’ve looked at a couple of different options for achieving the U-values for the new domestic properties within the notional dwelling/building in England, Scotland and Wales.
Construction | Insulation measures | U-value |
102.5 mm brick, 50 mm cavity Foil faced breather membrane 18 mm OSB 140 mm timber studs | 120 mm Kooltherm K112 Framing Board between studs 37.5 mm Kooltherm K118 Insulated Plasterboard internally with skim | 0.15 |
120 mm Kooltherm K112 between studs 52.5 mm Kooltherm K118 internally with skim | 0.13 |
It is possible to meet the value used within the notional building/dwelling in England and Scotland by fitting 120 mm Kooltherm K112 between the 140 mm studs, with a foil-faced breather membrane and a further 37.5 mm thickness of Kooltherm K118 internally. This can be easily raised to the notional dwelling value within ADL1 2022 (Wales) by increasing the thickness of Kooltherm K118 to 52.5 mm.
U-value calculations
If you’re looking to check what thickness of insulation you need for your project, use our U-value calculator. This allows you to easily adjust the construction type to find what insulation you need.
Use our U-value calculator for timber frame walls.
