01What this calculator tells you
The U-value (thermal transmittance) of a wall, roof or floor is the rate at which heat passes through one square metre of it for every degree of temperature difference between inside and outside. It is measured in watts per square metre per kelvin (W/m²K), and lower is better — a low U-value means a well-insulated element that loses little heat. This calculator builds the element up layer by layer, adds the surface resistances, and returns the U-value, the total thermal resistance and the total thickness, then rates the result against recognised targets.
The method is the steady-state, one-dimensional approach set out in ISO 6946 and used by every major insulation manufacturer’s tool. It underpins the fabric-efficiency guidance in the U.S. Department of Energy Building Technologies Office and the sealing-and-insulating advice from ENERGY STAR Save at Home. For related building tools, see our embodied carbon calculator and energy efficiency upgrade calculator.
02U-value targets and rating bands
There is no single legal U-value for every situation — targets vary by element, country and whether the work is new-build or retrofit — but the ranges below are widely used benchmarks for opaque walls. This calculator maps your result onto them. Choosing lower-carbon, higher-performance insulation products, verified through EPA guidance on greener products, is how a build-up moves up the scale.
03What changes the result
The U-value depends on the layers you enter and a few standard assumptions. The main levers are:
- Insulation thickness and type. Resistance is thickness ÷ λ, so both matter. PIR (λ ≈ 0.022) gives more resistance per millimetre than mineral wool (λ ≈ 0.035), which in turn beats timber (≈ 0.13). Dense materials such as brick (≈ 0.77) and concrete (≈ 1.5) add mass but very little resistance.
- Surface resistances. This calculator uses the standard wall values Rsi 0.13 and Rse 0.04 m²K/W. Roofs and floors use slightly different figures because heat flows in a different direction; for a sheltered or reversed-flow element the surface resistances change.
- Air gaps and bridging. Unventilated cavities add a small resistance, while repeating thermal bridges (timber studs, mortar joints, wall ties) raise the effective U-value above the simple layered figure. A full ISO 6946 assessment uses upper and lower resistance limits to account for this.
- Moisture and workmanship. Wet or compressed insulation, and gaps at junctions, all worsen real-world performance compared with the calculated value.
For a broader picture of how a low-energy fabric fits into a net-zero design, the DOE zero-energy buildings programme is a good reference.
- List your construction from outside to inside — for example brick, insulation, blockwork, plasterboard.
- Enter each layer’s thickness in millimetres and its thermal conductivity λ in W/mK (use the reference values shown above the fields, or a product datasheet).
- Leave any unused layer completely blank — the calculator skips it.
- Press Calculate to see the U-value, total R-value, total thickness, a rating chip and the resistance breakdown.
- Increase the insulation thickness or switch to a lower-λ insulant to watch the U-value fall.
Fitting out the rest of the project? Our conduit fill calculator helps size electrical containment.
This is a clear educational estimate using the simple layered method. It is not a substitute for a full ISO 6946 assessment or approved software for building-control submission. In particular it does not account for:
- Repeating thermal bridges (studs, joists, mortar, wall ties) that raise the real U-value
- Ventilated cavities, reflective foils and non-standard surface resistances
- Roof and floor geometry, ground contact and edge effects
- Moisture movement, condensation risk and as-built workmanship
01The U-value formula
A U-value is built from one idea: the total thermal resistance of the element is the sum of the resistances of each material layer plus the resistances of the two air surfaces, and the U-value is one divided by that total.
Where:
- d= thickness of the layer in metres (a 100 mm layer is 0.100 m).
- λ= thermal conductivity of the material in W/mK — from a datasheet or the reference list.
- Rsi, Rse= internal and external surface resistances, ≈ 0.13 and 0.04 m²K/W for a wall.
- R_total= sum of every layer plus surface resistance, in m²K/W.
02Worked example
Take a cavity wall built from 102 mm brick (λ 0.77), 100 mm mineral wool (λ 0.035), 100 mm dense concrete block (λ 1.5) and 12.5 mm plasterboard (λ 0.25). Here is the calculation carried through to a rating:
At 0.305 W/m²K this wall lands just outside the typical new-build band. The mineral-wool layer alone provides about 2.86 of the 3.28 total resistance, so it is by far the most important layer — swapping it for 120 mm of PIR (λ 0.022) would raise its resistance to about 5.45 m²K/W and drop the whole-wall U-value to roughly 0.17 W/m²K. Verifying insulation choices against EPA greener-products guidance keeps the upgrade both efficient and low-carbon.