For any flats or homes that are uninsulated, you should aim to insulate your flat roofs with insulation boards totalling around 150mm in thickness.
New government guidelines from 2021 that were updated in 2022 have stated that for flat roofs target U-value W/(m2-k) of 0.11 for new build dwellings, 0.15 for Extensions and 0.16 for refurbishments and 0.18 for other buildings that are not classed as dwellings.
Below is an example of how to understand how thick your flat roof insulation should be:
0.11 W/(m2·K) for a New Build Dwelling
- Foil Faced - TR26/EcoFix/EuroDeck would be 180mm
- Tissue Faced - TR27/EcoBond/Powerdeck F would be 210mm
- Rockwool Hardrock would be 335mm
0.15 W/(m2·K) for an Extension on a Dwelling
- Foil Faced - TR26/EcoFix/EuroDeck would be 130mm
- Tissue Faced - TR27/EcoBond/Powerdeck F would be 150mm
- Rockwool Hardrock would be 235mm
0.16 W/(m2·K) for a Refurbishment on a Dwelling
- Foil Faced - TR26/EcoFix/EuroDeck would be 130mm
- Tissue Faced - TR27/EcoBond/Powerdeck F would be 140mm
- Rockwool Hardrock would be 220mm
0.18 W/(m2·K) for non-dwelling
- Foil Faced - TR26/EcoFix/EuroDeck would be 120mm
- Tissue Faced - TR27/EcoBond/Powerdeck F would be 120mm
- Rockwool Hardrock would be 190mm
These calculations are based on a timber deck with Single Ply / EPDM Rubber Roofing.
For more information on U-Value Calculations, please click the following links for all associated brands
Check out the government guidelines here.