Wall U-value calculator: dew point, mould, cooling and fins
Enter layers, temperatures and sizes and get the wall U-value, the dew point, the cooling time or the fin efficiency. The calculation runs in your browser, and each of the nine tabs downloads as CSV.
Opens without an account 3 times a day. A free account removes the limit.
- 9 tabsenvelope, body, transfer, time
- 13 materialsconductivity, density and heat capacity from references
- 3D viewin six of the nine tabs
- CSVa table from every tab
What the wall U-value calculator works out
The wall U-value calculator takes layers of 13 materials. Beyond the wall, the EduFacturing tool works out how a part cools and how much a fin, a heat sink and radiation carry away. It covers moisture too: dew point, mould and condensation inside the wall by the Glaser method.
The nine tabs sit in four groups: envelope, body, transfer and time. They are wall, Glaser, bridge, cooling, warping, fins, convection, radiation and cycle.
Wall, dew point, thermal bridge, fins: what you get
Calculates wall U-value, dew point and mould
For layers of 13 materials it gives U, the heat flux, the boundary temperatures and whether the surface is dry, mouldy or condensing. Glaser shows where it condenses inside, and the bridge is a 2D field up to 220 × 220 cells.
Calculates part cooling and warping
It compares the simple calculation with the full equation for a plate, a cylinder and a sphere. Warping gives shrinkage and stress for PLA, PETG, ABS, ASA and nylon.
Calculates fins, heat sinks, convection and radiation
Fin efficiency and heat sink power, h for a plate, a pipe and a tube bank, and radiative exchange between two plates with a view factor.
Calculates a daily and a seasonal cycle
The amplitude and delay of a daily or seasonal cycle at a given depth.
When you need a wall U-value or the cooling of a part
A heat question is more often where the temperature falls than how much insulation.
Checking a wall U-value
Wood 20 mm and EPS 60 mm at 20 °C inside and −5 °C outside give U = 0.504 W/(m²·K) and 126 W across 10 m². The surface is 18.4 °C against a 10.7 °C dew point: dry.
Cooling a thick part
With an anodised surface, still air and 20 °C around, an aluminium sphere of 25 mm radius cools from 200 to 60 °C in 26.1 min. In PLA the Biot number is 3.74: the surface reaches the target in 23.5 min, the centre in 62.4 min.
A heat sink with fins
10 aluminium fins of 2 × 25 × 40 mm with a fan and a 40 K excess: 93% efficiency, about 50 W.
How the U-value of a wall and the dew point are calculated
The calculations are closed formulas or the tool's own numerical solver.
Wall
The resistances of the layers and surfaces add up like resistors in series, and the temperature falls across each layer in proportion to its resistance.
Moisture
The dew point uses the Magnus formula. The surface is dry, mouldy (80% humidity or more next to it) or condensing. In Glaser the vapour pressure falls linearly with Sd = μ·d and is compared with saturation.
Part
Below a Biot number of 0.1 one temperature describes the whole part. Otherwise the heat equation is solved by the control volume method.
Transfer
Radiation is linearised exactly and added to convection. Convection uses the Reynolds and Prandtl numbers, with its own correlation for a plate, a pipe and a bank.
| Applies to | Formula | Meaning |
|---|---|---|
| Wall | R = d/λ; U = 1/(R_in + ΣR + R_out); q = U·ΔT | the surface resistances are 1/h |
| Cooling (Newton's law) | T(t) = T∞ + (T₀ − T∞)·e^(−t/τ); τ = ρ·V·c/(h·A); Bi = h·R/λ | Bi uses the total h (convection plus radiation), R is the radius or half the thickness; the lumped model needs h·(V/A)/λ < 0.1 |
| Radiation and fin | h_rad = ε·σ·(T₁ + T₂)·(T₁² + T₂²); η = tanh(mL)/mL; m = √(2h/(λ·t)) | T in kelvin; L is the corrected length L + t/2 |
A bridge and moisture that are solved, not looked up
The thermal bridge is solved, not looked up
ψ comes from the flux of the 2D field, while in the Wall tab it is a catalogue value from EN ISO 14683.
Moisture on the surface and inside the wall
For the default wall, at 80% humidity inside and a membrane of Sd = 100 m on the cold side, the vapour after the insulation is at 1808 Pa against 438 Pa saturation, and Glaser shows where it condenses inside.
It calculates the loss through a wall, not through a whole house. The wall and Glaser are steady-state, and Glaser is a single profile with the same μ for every layer and no yearly balance under EN ISO 13788. Full cooling is one-dimensional: plate, cylinder, sphere. The bridge is a corner of two equal walls.
Material properties are reference values, and h for the wall is a choice of five values. Warping gives a comparative indicator and the free shrinkage in mm, not how far a corner will lift. No geometry for a printer is exported.
Questions about the U-value, dew point and mould
How do I calculate the U-value of a wall?
A layer has a resistance d/λ, added to the surface resistances 1/h, and U = 1/R. For wood 20 mm and EPS 60 mm: 0.154 + 1.667 + 0.125 + 0.04 = 1.986 m²·K/W, U = 0.504 W/(m²·K).
What is the dew point, and when does mould start on a wall?
The dew point is the temperature below which vapour in the air condenses: 10.7 °C at 20 °C and 55%. Mould starts earlier, at 80% humidity next to the surface.
What is a thermal bridge, and why does ψ from the field differ from the catalogue value?
A thermal bridge is a place in a wall, such as a corner, where heat passes more easily. The catalogue value of ψ is one number for a typical corner, while the field is solved for your angle: for the default wall ψ is 0.276 at 60°, 0.127 at 90° and zero at 180°.
When is one average temperature not enough for a part?
When the Biot number exceeds 0.1: the core lags and the simple calculation gives a lower bound. With an anodised surface and still air a PLA sphere of 25 mm radius has a Biot number of 3.74, the aluminium one 0.003.
Is the calculator free?
Yes. Without an account it opens 3 times a day. A free account removes the limit.
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Calculate your wall
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