Roof Insulation U-Value Calculator

About this calculator

A roof's U-value measures its overall rate of heat loss, expressed in watts per square meter per degree Kelvin (W/m²K), and is calculated as the reciprocal of the total thermal resistance (R-value) of all the roof's layers combined: R equals the insulation thickness divided by its thermal conductivity (lambda, in W/mK), plus the resistance contributed by every other layer — plasterboard, roof deck, air gaps, internal and external surface resistances — added together, then U equals 1 divided by that total R.

Architects, builders and energy assessors calculate roof U-value to confirm compliance with building regulations, which typically cap roof U-value at a maximum figure (commonly around 0.15-0.18 W/m²K for new builds in much of Europe), and to model expected heating energy use in an EPC (Energy Performance Certificate) or SAP calculation — a roof is usually the single biggest source of heat loss in a home because hot air naturally rises, so getting this number right matters more here than for almost any other building element.

This calculator takes your insulation thickness, its thermal conductivity, and the resistance of the roof's other layers and returns the resulting roof U-value.