How much insulation for pitched roof?

A pitched roof usually needs enough insulation to reach the target U-value without exceeding the available build-up depth.

Best for

Worked example

How much insulation is needed to reach 0.18 W/m²K?

Roof calculations are sensitive to the existing roof deck resistance, so this page keeps the example simple and direct.

Example build-up

Target U-value 0.18 W/m²K, existing roof resistance 1.70 m2K/W, insulation lambda 0.035 W/mK.

Result

Required insulation thickness is approximately 10.6 cm.

Roof notes

The rafter depth may limit the thickness you can fit.

Some roof types need a ventilated gap that changes the build-up.

The target value is not only about thickness; details around junctions matter too.

Open the calculator for a custom roof build-up.

Read the roof-specific thickness guide.

Compare with the general roof target-U page.