Below is a brief summary of the flat roof insulation requirements within Approved Document L 2021. These requirements came into force from 15th June 2022.
New Dwellings – Approved Document L1 2021 (ADL1 2021)
Under ADL1 2021, all new domestic properties must meet targets for:
• carbon emissions;
• primary energy demand – this considers both the energy used in the dwelling and in upstream activities such as extracting, transporting and refining the fuel; and
• Fabric Energy Efficiency Standards (FEES) – this is only affected by the U-values, thermal bridging and airtightness of the property.
To calculate specific targets for each, a standard specification is applied to a Notional Dwelling of the same dimensions as the proposed dwelling to see how it would perform. Project teams can choose to vary from this specification providing the finished building meets all three of the above targets and that the U-values are better than specified worst case U-values.
The U-value for flat roofs used within the Notional Dwelling specification is 0.11 W/m²K. The worst acceptable U-value for flat roofs is 0.16 W/m²K.
Existing Dwellings – Approved Document L1 2021 (ADL1 2021)
For existing dwellings, ADL1 2021 sets worst case U-values for new elements (such as extensions). These apply to flat roofs on existing buildings in cases where the original deck needs to be completely stripped and replaced. It sets separate “Improved U-values" when refurbishing the roof. If it is not practical to meet these requirements (see the Approved Document for specific guidance), project teams can choose to build to worse U-values but these should typically be at least as good as the “Threshold U-values”.
New elements | |
Worst-case roof U-value | 0.15 W/m²K |
Existing elements | |
Improved roof U-value | 0.16 W/m²K |
Threshold roof U-value | 0.35 W/m²K |
New Non-Domestic – Approved Document L2 2021 (ADL2 2021)
Under ADL2 2021, all new non-domestic properties must meet targets for carbon emissions and primary energy demand. The U-values of the different elements must also meet specified worst-case U-values.
As with for new domestic buildings, the carbon emissions and primary energy targets are set by applying a set specification to a building of the same dimensions as the proposed project and seeing how it performs. Different specifications are used depending on whether the project is primarily side-lit (such as offices, schools, hospitals etc) or top-lit (mainly warehouses and offices).
The roof U-value used within the Notional Building specification is 0.15 W/m2K for side-lit buildings and 0.18 for top-lit. The worst acceptable U-value for flat roofs is 0.18 W/m2K.
Existing Non Domestic – Approved Document L2 2021 (ADL2 2021)
For existing non-domestic buildings, ADL2 2021 again sets worst case U-values for new elements (such as extensions) and improved and threshold U-values for refurbishments. Where possible, project teams should look to meet the improved U-values in refurbishment applications but in all cases they should at least meet the threshold value.
New elements | |
Worst-case flat roof U-value | 0.18 W/m²K |
Existing elements | |
Improved flat roof U-value | 0.18 W/m²K |
Threshold roof U-value | 0.35 W/m²K |
