How to use it
- Type each layer of the wall on its own line: its name, its thickness in millimetres and its λ, for example Clay brick 100 0.77.
- Keep the Inside surface resistance and the Outside surface resistance at their values for a wall, or type the values for your case.
- Type the λ of the insulation to add from its data sheet, and your Target U-value.
- Read U-value of the wall as entered, then Insulation to add (mm). The table shows the resistance of each layer.
How the U-value is worked out
Each layer resists heat in proportion to its thickness and in inverse proportion to its conductivity λ. The inside and outside surfaces add their own resistances, because a thin film of air sits against each side. The total resistance is the sum of them all, and the U-value is one divided by that total: a lower U-value means less heat through each square metre.
The insulation to add comes from the same sum. Its resistance, thickness divided by λ, must make up the difference between the total of the present wall and one divided by the target U-value.
R of a layer = thickness (m) ÷ λ
total R = Rsi + sum of the layers' R + Rse
U = 1 ÷ total R
thickness to add (m) = λ × (1 ÷ target U − total R)Surface resistances
The values 0.13 for the inside and 0.04 for the outside are the ISO 6946 values for a wall, with heat flowing horizontally, as the Wikipedia example of the ISO 6946 method uses them.
The value of a surface depends on the direction of the heat flow. A roof, where heat often flows up or down, has different values from a wall, so type the values for your case from the standard before you rely on the result.
Where the conductivities come from
The Engineering Toolbox table gives typical values: clay brick from 0.6 to 1.0 W/(m·K), dense concrete from 1.0 to 1.8, mineral wool and glass wool 0.04, and expanded polystyrene and polyurethane foam 0.03. Products differ, so use the λ on the data sheet of the material you buy.
What this calculator does not include
- Thermal bridges such as concrete frames, lintels and fixings, which the layer method does not include.
- Air gaps, damp and the drying of new plaster: the page assumes solid, dry layers.
- Ventilated cavities and the air spaces between panes of glass.
Worked example: a brick and concrete block wall of 200 mm, to a U-value of 0.30
The wall has 100 mm of clay brick (λ 0.77) and 100 mm of concrete block (λ 1.13). Its total resistance is 0.13 + 0.04 + 0.100 ÷ 0.77 + 0.100 ÷ 1.13 = 0.388 m²·K/W, so its U-value is 1 ÷ 0.388 = 2.57 W/(m²·K).
For a U-value of 0.30 the total must be 1 ÷ 0.30 = 3.333 m²·K/W. The insulation must add 3.333 − 0.388 = 2.945 m²·K/W; at λ 0.04 that is 2.945 × 0.04 = 0.118 m, which is 118 mm. With it the U-value is 0.30.
| Part | mm | R (m²·K/W) |
|---|---|---|
| Inside surface | 0.130 | |
| Clay brick, λ 0.77 | 100 | 0.130 |
| Concrete block, λ 1.13 | 100 | 0.088 |
| Outside surface | 0.040 | |
| Total | 200 | 0.388 |
Frequently asked questions
What is a U-value?
The U-value is the heat that passes through one square metre of a wall, roof or window for each degree of difference between the air on its two sides, in W/(m²·K). A lower value means better insulation.
Which λ should I use for my insulation?
Use the λ printed on the data sheet of the product you buy, because products made of the same material differ. The values in the table are typical only.
Why is the insulation thickness larger than I expected?
The page counts only the layers you type and the two surfaces. Thermal bridges such as frames and fixings are not included, so a real wall can need more. The standard's full method, or a surveyor, gives the figure you can rely on.
Does the order of the layers matter?
No. The total resistance is the sum of the layers, so for a flat wall the order from outside to inside does not change the U-value.
Are the surface values the same for a roof?
Not always. The value of a surface depends on the direction of the heat flow, so a roof, where heat often flows up or down, has different values from a wall. Type the values for your case.
Does the tool store what I enter?
No. The calculation runs in your browser and nothing you type is sent to us or stored.
Sources
- Wikipedia: Thermal transmittance (the ISO 6946 method and the wall example with Rsi 0.13 and Rse 0.04)
- Engineering Toolbox: thermal conductivity of building and insulation materials
- Wikipedia: R-value (insulation), surface resistances by direction of heat flow
Figures and text last checked on Tuesday, November 10, 2026.