Envelope · Method
How to calculate a U-value
A U-value is 1 divided by the total thermal resistance: add the resistance of every layer (its thickness in metres divided by its conductivity λ) to the two surface resistances, and take the reciprocal. That is the whole method for a wall, a roof or a floor, to EN ISO 6946. Glass and windows are worked out differently: a sealed unit by what crosses each cavity (EN 673), a window by sharing its area between glass, frame and the spacer edge (EN ISO 10077-1). So there are three calculators, one per case, and the result is always in W/(m²·K): the lower it is, the less heat gets through.
The wall method, step by step
-
List the layers
From the inside face to the outside face, with the thickness of each: plaster or board, block or brick, insulation, cavity, outer leaf, render. The order does not change the U-value, but it matters later for condensation.
Thickness in millimetres on paper; it becomes metres in the next step.
-
Find each layer's conductivity, λ
λ (lambda) is how well a material conducts heat, in W/(m·K). Take the manufacturer's declared value for insulation and blocks; the calculator lists typical values for common materials and lets you type your own.
Insulation has a λ around a twentieth of brick's, which is why it does almost all the work.
-
Resistance of each layer: R = d / λ
Thickness in metres divided by λ gives the layer's thermal resistance in m²·K/W. In the example below, the insulation layer (Mineral wool, 100 mm, λ 0.037) gives R 2.703.
An unventilated air cavity has a fixed resistance instead of a λ.
-
Add the surface resistances
The thin layer of still air on each face resists heat too. For a wall the calculator uses Rsi 0.13 inside and Rse 0.04 outside; roofs and floors have their own inside value.
Leave them out and every wall looks better than it is.
-
U = 1 / R total
Add every resistance, surfaces included, and divide 1 by the sum. That is the U-value, in W/(m²·K).
Resistances add, U-values do not. Always sum the R and invert at the end.
-
Compare with the target, and size the insulation
Set the U-value your project or your local rules ask for. The calculator says whether the build-up reaches it and how thick the insulation layer would have to be to get there.
Thicker insulation is almost always the answer; thicker masonry almost never is.
Worked example: a masonry wall
The wall the calculator opens with, layer by layer, with the values the calculator gives: thickness in mm, λ in W/(m·K), R in m²·K/W.
| Layer | mm | λ | R |
|---|---|---|---|
| Inside surface, Rsi | — | — | 0.13 |
| Cement render | 10 | 0.9 | 0.011 |
| Fired clay brick | 115 | 0.77 | 0.149 |
| Mineral wool | 100 | 0.037 | 2.703 |
| Aerated concrete block | 100 | 0.18 | 0.556 |
| Plasterboard | 13 | 0.21 | 0.062 |
| Outside surface, Rse | — | — | 0.04 |
| Total resistance | 338 | 3.651 |
U = 1 / 3.651 = 0.274 W/(m²·K)
Against the calculator's starting target of 0.25: Above the target, and it would take 113 mm of the same insulation instead of 100 mm to get there. Take the “Mineral wool” layer out altogether and the same wall is 1.055 W/(m²·K).
Worked example: a double-glazed unit
A 4-16-4 unit: two panes with a 16 mm cavity (fill: Argon 90 %) and a low-E coating of emissivity 0.03. The calculator works out the heat crossing the cavity by radiation and by the gas, then adds the glass and the two surfaces.
| Part | Calculator result |
|---|---|
| Radiation across the cavity, hr | 0.153 W/(m²·K), 11 % of the cavity |
| Gas conduction across the cavity, hg | 1.218 W/(m²·K) |
| Ug of the unit | 1.10 W/(m²·K) |
| The same unit with no coating (emissivity 0.837) | 2.63 W/(m²·K), radiation 75 % of the cavity |
The coating does more than the gas: without it, radiation carries most of the heat straight across the cavity.
It opens on this very unit.
Worked example: the window that unit goes in
A 1230 × 1480 mm window with a 70 mm frame face, frame Uf 1.30, spacer Ψg 0.060 W/(m·K), and the Ug of 1.10 from the unit above. The window U-value shares the window's area between glass and frame and adds the edge of the glass along its whole perimeter.
Uw = (Ag·Ug + Af·Uf + lg·Ψg) / Aw
| Term | Calculator value |
|---|---|
| Window area, Aw | 1.8204 m² |
| Glass area, Ag | 1.4606 m², 80.2 % of the window |
| Frame area, Af | 0.3598 m² |
| Glass edge length, lg | 4.860 m |
| Uw | 1.30 W/(m²·K), +0.20 over the glass alone |
The frame and the spacer edge make the window worse than its glass. That is normal, and it is why a window's U-value, not the glass's, is the one to compare.
Questions
What is the formula for a U-value?
U = 1 / (Rsi + R1 + R2 + … + Rse), where each layer's R is its thickness in metres divided by its conductivity λ. This is EN ISO 6946 for walls, roofs and floors without significant thermal bridges.
What is the difference between a U-value and an R-value?
R is a resistance, in m²·K/W: higher is better, and resistances add up layer by layer. U is the reciprocal of the total resistance, surfaces included, in W/(m²·K): lower is better. An R-value on an insulation board is for that board alone, not the wall.
Why is a window's U-value worse than its glass?
Because the frame usually insulates less than modern glass, and the spacer at the edge of the unit leaks heat along the whole perimeter. In the worked example the glass is 1.10 and the window 1.30.
Can I calculate a U-value for a wall with steel or timber studs this way?
Not exactly. The layer method assumes each layer is uniform. Studs, ties and junctions are thermal bridges; the thermal bridges calculator adds them as linear losses on top.