Below is a brief summary of the flat roof insulation requirements within Approved Document L 2022 in Wales. These requirements within ADL1 2022 came into force for domestic buildings from 23rd November 2022 whilst those within ADL2 2022 came into force on 29th March 2023.
New Dwellings – Approved Document L1 2022 (ADL1 2022)
Under ADL1 2022, 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
- minimum Energy Efficiency Rating – requiring the building to achieve a SAP rating of at least 81.
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. You 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 area-weighted average U-value for flat roofs, taken from the Notional Dwelling specification is 0.11 W/m²K. The worst acceptable area weighted average U-value for flat roofs is 0.13 W/m²K.
Existing Dwellings – Approved Document L1 2022 (ADL1 2022)
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 maximum 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 “Limiting U-values”.
New elements | |
Worst-case roof U-value | 0.13 W/m²K |
Existing elements | |
Maximum flat roof U-value | 0.16 W/m²K |
Limiting flat roof U-value | 0.35 W/m²K |
New Non-Domestic – Approved Document L2 2022 (ADL2 2022)
Approved Document L2 adopts a similar approach to Approved Document L1, taking the entire building’s performance and CO2 emissions into consideration. All new buildings other than dwellings must be designed and built with their Building CO2 Emissions Rate (BER) being no worse than the Target CO2 Emissions Rate (TER).
The TER is set from a Notional Building of the same size and shape to the actual building. There are three defined Notional Buildings; heated and cooled buildings (that are side lit or unlit), and toplit buildings. The specification for each building type is summarised in ADL2. CO2 Emissions Rates for buildings other than dwellings are calculated using the Simplified Building Energy Model (SBEM) or other approved software.
In addition to meeting the TER, a new Target Primary Energy Consumption (TPEC) rate has been introduced for new buildings other than dwellings. Put simply, this new target is aimed at two factors – energy efficiency of the fabric and its building services. The TPEC ensures these two factors are carefully considered and prevents renewable technologies being highly specified to counteract a lower performing fabric or services standards.
If the building is built to match the Notional Building specification, its BER should meet the TER. Note that other factors need to be taken into consideration including continuous building fabric insulation, air permeability, thermal bridging, performance testing and limiting the effects of heat gains in summer.
The area-weighted average U-value for flat roofs, used in the Notional Building specifications for setting the TER, is 0.18 W/m²K.
Although ADL2 is for new buildings other than dwellings, there are some exceptions if a dwelling is featured within the new building. For more details on these exceptions ADL2 should be referred to.
Special considerations are made for low energy demand buildings, modular/portable buildings, temporary buildings, industrial sites, workshops, and non-residential agricultural buildings. ADL2A should be consulted for further details.
Existing Non-Domestic – Approved Document L2 2022 (ADL2 2022)
Existing buildings do not require a Target CO2 Emissions Rate, however they do require a target U-value to meet for building elements (walls, floors and roofs). There is a new consideration for the character of the building, with buildings that are essentially domestic in character having more stringent targets to meet for extensions.
The target U-value for flat roofs on existing buildings other than dwellings are 0.15 W/m²K* / 0.18 W/m²K.
* Rooms for residential purposes (for example in nursing homes, student accommodation, etc.) are not dwellings, and so ADL2 will apply.
