How the U-value is worked out
Each layer resists heat in proportion to its thickness and in inverse proportion to its conductivity. Adding those resistances, plus the resistance of the air films on each face, gives the total resistance, and the U-value is one divided by that total.
The UK figures use the BS EN ISO 6946 surface resistances: 0.13 inside and 0.04 outside for a wall, 0.10 and 0.04 for a roof, 0.17 and 0.04 for a floor.
Layers with no thickness are ignored, so a three-layer form still works for a two-layer wall. Leave a layer at zero rather than guessing a value for it.
Worked example
A cavity wall with 200 mm of mineral wool at 0.035, a 100 mm brick leaf at 0.77 and 13 mm of plaster at 0.25 gives resistances of 5.71, 0.13 and 0.05. Adding the 0.13 and 0.04 surface resistances gives a total of 6.07 m²K/W, so the U-value is 0.165 W/m²K.
Change the insulation thickness and the U-value moves almost in proportion, which is why insulation thickness dominates a wall's performance.
What the target means
The U-value you are aiming for. Part L asks for about 0.18 W/m²K on an external wall.
The target is a comparison, not a compliance check. Building control looks at the whole dwelling, including thermal bridging, airtightness and the services, so a wall that meets its target can still sit in a home that does not.
This page reports a steady-state U-value for one element. It does not check interstitial condensation, which depends on vapour resistance and the order of the layers.
Limits of this calculator
It assumes one-dimensional heat flow through continuous layers. It does not account for repeating thermal bridges such as timber studs, wall ties or mortar joints, which lower the real performance.
It does not model the ground, the air gap resistance in a ventilated cavity, or the effect of reflective foil, all of which change the answer.
It does not check condensation risk or the suitability of the build-up, which needs a hygrothermal assessment.
To use the U-value in a whole-home calculation, go to the UK heat loss calculator.
Inputs for the U value calculator
Building element
Wall, roof or floor. The choice sets the surface resistances at each face.
Layer 1 thickness
Thickness of the main insulation in millimetres, 200 for a modern full-fill cavity.
Layer 1 conductivity
Thermal conductivity of the insulation, 0.035 W/mK for mineral wool.
Layer 2 thickness
Thickness of the inner leaf in millimetres, 100 for blockwork.
Layer 2 conductivity
Thermal conductivity of the inner leaf, 0.77 for brickwork.
Layer 3 thickness
Thickness of the finish in millimetres, 13 for plaster.
Layer 3 conductivity
Thermal conductivity of the finish, 0.25 for gypsum plaster.
Target U-value
The U-value you are aiming for. Part L asks for about 0.18 W/m²K on an external wall.
CALCULATOR QUESTIONS
UK u value calculator questions
What is a good U-value?
Lower is better. A modern wall is about 0.18 W/m²K, a roof about 0.15 and a window about 1.4.
Why do surface resistances matter?
The air films on each face resist heat too. They add about 0.17 m²K/W to a wall, which is worth more than 50 mm of brick.
Does this account for timber studs?
No. Studs, ties and mortar joints are thermal bridges, and the real U-value is worse than the clear-layer figure.
Is this a compliance check?
No. It compares one element with a target. Building control assesses the whole dwelling.