U-Value Calculator for Walls, Lofts, Roofs and Floors
Work out the U-value of a wall, loft, roof or floor by adding its layers, from the plaster to the brick. See how it compares with the building regulations, how much insulation would bring it up to standard, and what the heat lost through it costs you each year.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your U-value
U-Value Calculator
Build up a wall, loft, roof or floor layer by layer, from the inside out. See its U-value, how it compares with the building regulations, and what the heat lost through it costs you each year.
1 What are you checking?
2 Its layers, inside to outside
Start from a common build-upPick the timber option for insulation fitted between joists, rafters or studs: the wood lets more heat through than the insulation around it.
3 Running cost (optional)
How cold your winters are. The more degree days, the more heat escapes over a year.
U-value
R-value (higher is better)
Heat lost a year
Cost a year
Against the building regulations (England)
| Standard | Needs | Yours |
|---|
Where the resistance to heat comes from
How it's worked out: each layer's resistance R = thickness ÷ thermal conductivity (λ). Add the layers and the thin films of still air on each surface, then U = 1 ÷ total R. Where insulation sits between timbers, the tool works out the heat flow through the wood and the insulation separately and averages the upper and lower limits, as BS EN ISO 6946 does. Conductivities are typical values from BS EN ISO 10456 and BRE's BR 443; check the maker's data sheet for your product. Heat lost a year = U × area × degree days × 24 ÷ 1.000. Targets: Approved Document L 2021 (England), Tables 4.1 and 4.3.
Estimates only. Real walls lose more heat at gaps, wall ties, fixings and corners, and floors on the ground lose less (the soil helps). For building control, use a U-value calculation from the insulation maker or a qualified assessor.
How it works
Add up the layers, then turn it over
A U-value is how many watts of heat escape through each square metre for every degree of difference between inside and outside. Lower is better. The sources and assumptions behind it are on our How we calculate page.
Each layer resists heat
A layer's resistance R is its thickness in metres divided by its thermal conductivity (λ). 100 mm of PIR board (λ 0.022) gives R 4.5; 100 mm of brick (λ 0.77) only R 0.13. Thin films of still air on each surface add a little more.
U = 1 ÷ total R
Add up every layer's R and divide 1 by the total. Timber between the insulation lets heat past, so for joists, rafters and studs the calculator works out both paths and averages them, as the standard method (BS EN ISO 6946) does.
From U-value to cost
Heat lost a year = U × area × degree days × 24 ÷ 1,000 kWh. Degree days measure how cold your winters are: about 2,000 in most of the UK and Ireland. Multiply by what a kWh of heat costs you to see the money.
Examples
Typical U-values for UK homes
Common walls, lofts and floors, worked out with the calculator above, and the target when you renovate them under the building regulations in England.
U-values in W/m²K. Heat cost: 2,000 degree days and 9p a kWh of heat (gas in a boiler). Targets from Approved Document L 2021, Table 4.3. Real walls lose a little more at gaps, ties and corners.
Tips
Getting the most from insulation
A U-value tells you where your home loses most heat. These help you act on it.
Start with the worst
Going from 2.1 to 0.3 W/m²K saves far more than from 0.3 to 0.15. An uninsulated loft or an empty cavity wall is usually the cheapest heat you'll ever save; see what your whole home loses with the Heat Loss Calculator.
Keep it continuous
Insulation only between joists or studs leaves cold lines of timber. A layer across them, even a thin one, makes a big difference. Gaps between boards matter too: tape the joints.
Watch out for damp
Insulating solid walls on the inside can make the wall behind colder and wetter. Use a vapour control layer on the warm side or a breathable system, and check the risk with the Damp & Mould Check.
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Common questions
Frequently asked questions about U-values
Common questions about U-values, insulation and the building regulations.
What is a U-value?
A U-value is the rate heat passes through a wall, roof, floor or window, in watts per square metre for each degree of difference between inside and outside (W/m²K). The lower the U-value, the better the insulation.
How do I calculate a U-value?
Work out each layer's resistance: thickness in metres ÷ thermal conductivity. Add them up with the surface resistances (0.13 inside and 0.04 outside for a wall) and divide 1 by the total. Where insulation sits between timbers, work out both paths and average them, as BS EN ISO 6946 does.
What U-value does a wall need for building regulations?
In England, a new home's walls must be 0.26 W/m²K or better. When you renovate or replace an existing wall it should reach 0.30, or 0.55 when you fill a cavity. Scotland, Wales, Northern Ireland and Ireland set similar but not identical values.
What is the U-value of a solid brick wall?
About 2.1 W/m²K for a 215 mm (one brick thick) wall with plaster inside. An empty cavity wall is about 1.4, and a filled one about 0.5.
How much loft insulation do I need?
270 mm of mineral wool loft roll, laid as 100 mm between the joists and 170 mm across them, gives about 0.16 W/m²K, the target for a loft in England. 100 mm alone gives about 0.42.
What is the difference between a U-value and an R-value?
The R-value is resistance to heat: the higher the better. The U-value is the heat that gets through: the lower the better. For a whole wall, U = 1 ÷ total R. Insulation packs often quote R per board, so you can add them up.
Does a lower U-value always save money?
Yes, but by less each time. Taking a wall from 2.1 to 0.3 W/m²K cuts its heat loss by six times; going on to 0.15 saves only a little more. Check what each step saves with the cost section of the calculator before you pay for it.