U-Value Calculator
Build up a wall layer by layer to calculate its U-value (thermal transmittance) and see how the insulation level affects surface temperature and condensation risk. Free, no sign-up.
Layers run outside to inside. Surface resistances for a wall (Rsi 0.13, Rso 0.04) are included automatically.
Total thermal resistance 1.01 m²K/W. Lower is warmer: a new-build wall is typically around 0.18 to 0.30 W/m²K. Higher U-values mean colder internal surfaces and a greater risk of condensation and mould.
Bring the fabric into your report.
SurveyMate records construction and thermal detail against each element and ties it to the condensation and HHSRS assessment.
See the inspection appGuidance only. Uses typical published conductivities and wall surface resistances; a formal U-value calculation should use declared material figures and account for bridging, fixings and moisture.
U-value, cold surfaces and mould
Heat leaves a building through its fabric. The U-value quantifies how quickly, so a poorly insulated wall with a high U-value has a cold internal surface. When that surface drops below the room's dew point, water vapour condenses on it, and a few days of that is all mould needs to establish.
That is why insulation and condensation are two sides of one problem. Pair this with the condensation risk calculator to see where a wall will attract moisture, and the glossary for the terms.
U-value calculator FAQs
What is a U-value?
A U-value measures how readily heat passes through a building element, in watts per square metre per kelvin (W/m²K). A lower U-value means better insulation, a warmer internal surface, and less risk of condensation.
How is the U-value calculated?
The tool adds the thermal resistance of each layer (its thickness divided by its conductivity) to the internal and external surface resistances, then takes the reciprocal of the total. Air cavities are included as a fixed resistance.
Why does U-value matter for damp and mould?
A high U-value leaves the internal surface cold. When that surface falls below the room's dew point, condensation forms and mould follows. Poorly insulated walls and thermal bridges are the classic cold spots where mould appears.