Insulation Calculator

The area that matters is the area of the building envelope you are insulating, measured along the construction. A pitched roof at 35° has 22 % more surface than the ceiling below it, and a wall with a bay window has more than its plan length suggests. Measuring the floor and using that number under-orders every roof in the house.

Where a drawing or a survey exists, enter the measured area directly instead of the room dimensions. The rectangular model in the calculator is a fallback for when you have a tape measure and nothing else, and it is only as good as the room is regular.

Insulation is supplied in fixed thicknesses, so 150 mm is a 100 mm layer plus a 50 mm layer, with the joints staggered between them. Staggering is not a detail: if the joints line up, the gap becomes a straight path for air, and the declared thermal resistance never appears in the finished wall.

Compressing mineral wool to fit a shallower space is the other common error. Squeezing 150 mm into 120 mm raises the density and lowers the lambda, so the U-value gets worse in a way that is invisible after the boards go back up.

Thermal bridging is outside this calculation. Timber studs occupy 10 to 15 % of a framed wall and conduct two to three times better than mineral wool, so the whole-wall U-value is worse than the material's. Steel studs and concrete lintels are worse still.

Vapour control, airtightness tape, fixings and the membrane are separate lines with their own 10 to 15 % overlap allowance. Skipping the tape to save money is the decision that turns a good insulation job into a condensation problem two winters later.