Use the calculator below to work out the U-value of a wall, roof or floor build-up — including an Isover insulation layer — or to switch modes and find out how thick that Isover insulation needs to be to hit a target U-value. Add each layer of the construction with its thickness and thermal conductivity, and the tool applies the same layer-resistance method used in BS EN ISO 6946 to give you a U-value in W/m²K.
Isover U-Value Calculator
Build up a wall, roof or floor construction layer by layer to get its U-value, or work out the Isover insulation thickness you need to hit a target.
Ground floors also depend on the floor’s perimeter/area ratio and edge insulation, which this simplified layer method does not account for.
U-values are calculated using the layer-resistance method set out in BS EN ISO 6946, with standard surface resistances from BR 443. Declared thermal conductivity (λD) values for Isover products are taken from current Isover UK product datasheets; always confirm against the latest datasheet before specifying.
The calculator covers three element types (wall, roof and floor), each with the correct standard surface resistances for that heat-flow direction, and comes pre-loaded with declared thermal conductivity (λD) values for three common Isover insulation products — Cavity Wall Slab CWS 32, Cavity Wall Slab CWS 36, and Spacesaver loft roll — so you don't have to look them up separately. Every other layer in the build-up (plaster, blockwork, render, timber, and so on) takes a thickness and a λ value you enter yourself, since those vary by product and should come from the relevant manufacturer datasheet. Anyone specifying insulation, checking a build-up against Building Regulations, or comparing insulation thicknesses can use this page to see exactly how the number is calculated.
How a U-Value Is Calculated
A U-value (thermal transmittance) measures how much heat passes through one square metre of a construction element for every degree of temperature difference either side of it. The lower the U-value, the better the element performs as an insulator.
The Layer-Resistance Method
U-values are calculated from the thermal resistance of every layer in the construction, plus the resistance of the air films on each surface. The formula is:
U = 1 ÷ R Total
Where R Total is the sum of every layer's resistance, including the internal and external surface resistances:
R Total = R si + R 1 + R 2 + R 3 … + R se
Each material layer's resistance is found from its thickness and thermal conductivity:
R = Thickness (m) ÷ λ
Where:
- Thickness is the layer's thickness converted to metres
- λ (lambda) is the material's thermal conductivity in W/mK — a lower λ means a better insulator
- R si and R se are the internal and external surface resistances, which depend on the direction of heat flow
Standard Surface Resistances
BR 443 sets out standard surface resistance values (in m²K/W) for the three heat-flow directions used in this calculator:
| Element | Heat flow | R si | R se |
|---|---|---|---|
| Wall | Horizontal | 0.13 | 0.04 |
| Roof (pitched or flat) | Upward | 0.10 | 0.04 |
| Floor (exposed) | Downward | 0.17 | 0.04 |
A roof has a lower internal surface resistance than a wall because warm air rises more readily against an upward-facing surface, and a floor has a higher one because heat loss downward is naturally slower. The calculator applies the correct pair automatically when you pick the element type.
Why Surface Resistance Matters
Surface resistance isn't a physical layer you can touch, but it's a real part of the total resistance — a thin layer of still air clings to every surface and resists heat flow just like a material would. Leaving it out of the calculation would understate the true U-value of the element.
Worked Example: A Cavity Wall With Isover CWS 32
Take a masonry cavity wall built up from four layers: 13mm of plaster on the inside, a 100mm dense block inner leaf, a 100mm cavity filled with Isover Cavity Wall Slab (CWS) 32, and a 20mm render finish outside.
Step 1 — find each layer's resistance:
- Plaster: R = 0.013 ÷ 0.18 = 0.072 m²K/W
- Dense block: R = 0.100 ÷ 1.13 = 0.088 m²K/W
- Isover CWS 32: R = 0.100 ÷ 0.032 = 3.125 m²K/W
- Render: R = 0.020 ÷ 1.00 = 0.020 m²K/W
Step 2 — add the surface resistances for a wall (horizontal heat flow):
R Total = 0.13 + 0.072 + 0.088 + 3.125 + 0.020 + 0.04 = 3.476 m²K/W
Step 3 — invert to get the U-value:
U = 1 ÷ 3.476 = 0.29 W/m²K
That single 100mm layer of Isover CWS 32 accounts for the vast majority of the wall's total resistance — 3.125 of the 3.476 m²K/W — which is why the insulation layer, not the masonry, is what really controls the U-value of a cavity wall.
Reading the Result
A lower U-value means less heat escapes through that square metre of wall for every degree of temperature difference. There's no single “correct” U-value — what counts as good enough depends on the element, the building type, and whichever regulations or performance target apply to that project.
Finding the Insulation Thickness You Need
Switch the calculator to “Find Insulation Thickness” mode when the U-value is fixed — for example, a target set by Building Regulations or a specification — and you need to know how much Isover insulation to specify. Enter the other layers as normal, choose the Isover product for the insulation layer, enter your target U-value, and the calculator rearranges the same formula to solve for thickness instead of U-value:
R Insulation Needed = (1 ÷ U Target) − R si − R se − R Other Layers
Thickness (mm) = R Insulation Needed × λ × 1000
Worked Example: Solving for Thickness
Using the same plaster, dense block and render layers as above (R = 0.072 + 0.088 + 0.020 = 0.180 m²K/W) and a target wall U-value of 0.18 W/m²K:
R Insulation Needed = (1 ÷ 0.18) − 0.13 − 0.04 − 0.180 = 5.556 − 0.35 = 5.205 m²K/W
Thickness = 5.205 × 0.032 × 1000 = 167mm of Isover CWS 32
Since CWS 32 is manufactured in fixed thicknesses, you'd specify the next size up — 150mm falls short of the target, so 150mm would need supplementing, meaning the practical choice is a cavity wide enough to take a full 167mm or more. This is exactly the kind of check worth running before finalising a cavity width on a drawing.
Building Regulations Context
England's Approved Document L (2021 edition, with 2023 amendments) sets both limiting and notional U-values for new dwellings, and separate limiting values for extensions and retrofit work to existing dwellings:
| Element | New build — limiting | New build — notional | Existing dwellings/extensions |
|---|---|---|---|
| Wall | 0.26 | 0.18 | 0.18 |
| Roof | 0.16 | 0.11 | 0.15 |
| Floor | 0.18 | 0.13 | 0.18 |
The limiting value is the absolute maximum U-value that element is allowed to reach; the notional value is the tighter figure the SAP energy assessment for a new dwelling is actually benchmarked against, so new-build designs typically need to get closer to the notional column to pass overall. Regulations and their figures change between editions and differ across the UK's nations, so always check the current version that applies to your project rather than relying on this table alone.
Using the Calculator Against a Target
Once you know which figure applies to your project, use it directly in “Find Insulation Thickness” mode as the target U-value — the calculator will tell you the Isover insulation thickness needed to meet it for the specific build-up you're checking.
The Isover Products Built Into This Calculator
Cavity Wall Insulation
Isover Cavity Wall Slab (CWS) 32 and CWS 36 are glass mineral wool slabs designed for full-fill and partial-fill masonry cavity walls, sized to align with standard wall tie spacing.
- CWS 32 — λ = 0.032 W/mK, available in 65, 75, 85, 100, 125 and 150mm
- CWS 36 — λ = 0.036 W/mK, available in 50, 65, 75, 85, 100, 125 and 150mm
CWS 32 has the lower thermal conductivity of the two, so it reaches a given U-value in a thinner layer than CWS 36 — useful where cavity width is limited.
Pitched Roof Insulation
Isover Spacesaver is a pre-perforated glass mineral wool roll designed to fit between pitched roof joists, with a declared thermal conductivity of λ = 0.044 W/mK, available in 100, 150, 170 and 200mm.
Entering Your Own Materials
Every other layer — plasterboard, blockwork, brickwork, timber, screed, render, and so on — uses the “Custom / other material” option, where you enter the thickness and λ value yourself from the relevant product datasheet. This keeps every non-Isover figure in the calculation tied to a real source rather than a generic assumption, since material conductivity can vary between manufacturers and product ranges.
Practical Notes
Rounding and Precision
U-values are shown to two decimal places, which matches how they're normally quoted and specified. Required insulation thicknesses in “Find Insulation Thickness” mode are rounded up, since a thinner layer than calculated would miss the target — always round up again to the nearest thickness the chosen product is actually manufactured in.
Ground Floors Are More Complex
The floor option in this calculator uses the standard exposed-floor surface resistances (downward heat flow) and treats the floor as a simple layer stack. A true ground floor U-value also depends on the floor's perimeter-to-area ratio and any edge insulation, which this simplified layer method doesn't account for — for ground floors, use a calculation method that includes those ground-specific factors.
Cavities and Air Gaps
An unfilled air gap within a construction also contributes some thermal resistance, but it isn't automatically added by this calculator — if your build-up includes a genuine unfilled cavity as a separate layer, add it as a custom layer using the resistance value for that gap width from BS EN ISO 6946 or the relevant datasheet, rather than a thickness-over-lambda calculation.
Frequently Asked Questions
Calculation Questions
What's the difference between U-value and R-value?
R-value (thermal resistance) measures how well a single layer or a whole construction resists heat flow — higher is better. U-value (thermal transmittance) is its inverse, 1 ÷ R Total, and measures how much heat actually gets through — lower is better. The calculator works out R-value for every layer first, then inverts the total to give the U-value.
Why does the calculator ask for the element type before I add any layers?
The element type sets the internal and external surface resistance (R si and R se), which are added to the layer resistances as part of R Total. Walls, roofs and floors each have different standard values because heat behaves differently depending on whether it's moving sideways, upward or downward, so picking the right element type is essential to getting an accurate U-value.
Can I mix Isover products with other manufacturers' insulation in the same build-up?
Yes. Each layer is independent — select an Isover product where relevant for its declared λ value, and use “Custom / other material” for any other manufacturer's product, entering the λ value from that product's own datasheet.
Using the Calculator
Why is my required insulation thickness not a “real” product size?
The calculator solves the formula exactly, so the answer is rarely a round number. Isover products are manufactured in fixed thicknesses (for example, CWS 32 comes in 65, 75, 85, 100, 125 and 150mm), so always round the calculated figure up to the next available thickness — rounding down would leave the U-value short of your target.
Does this calculator account for thermal bridging?
No. This calculator gives the “clear wall” or “clear field” U-value of a uniform layer build-up only. Thermal bridging — at junctions, around openings, or through wall ties and fixings — is calculated separately and isn't part of a standard layer-resistance U-value calculation.
Where do the Building Regulations figures in this article come from?
They're taken from Approved Document L Volume 1 (2021 edition, with 2023 amendments) for England. Building Regulations are updated periodically and differ between the UK's nations, so treat the table above as a reference point and confirm the current figures that apply to your specific project before relying on them.