Target U-value
Roof intentHow much insulation for roof?
Roof insulation often needs to balance thermal performance with structural depth. This page gives a simple roof-specific reference point.
Best for
- Flat roofs
- Pitched roofs
- Warm roof build-ups
Worked example
How much insulation is needed to reach 0.18 W/m²K?
The roof example below uses the same target-U method as the calculator: target resistance minus existing resistance, then multiply by lambda.
Example build-up
Target U-value 0.18 W/m²K, existing roof resistance 1.80 m2K/W, insulation lambda 0.035 W/mK.
Result
Required insulation thickness is approximately 9.9 cm.
Roof notes
Why roof thickness changes quickly
Structure depth
Roof build-ups often have tighter height limits than walls.
Thermal target
The lower the target U-value, the thicker the insulation usually becomes.
Check the deck
Existing resistance from the deck and finishes can change the result materially.
Related pages
Next steps
Insulation thickness calculator
Open the calculator for a custom roof or wall build-up.
Open calculator