U-value and dew point calculator
Choose the element — wall, floor or roof — the construction and the lining. The generator works out the thermal transmittance (U-value), shows the dew point in every layer, checks for condensation and gives the quantities by area.
For prices and pallets use the calculator. If the generator does not load, open it on a separate page.
What the generator works out
- The thermal transmittance U of the wall, floor or roof (W/m²K), the total thickness and the thermal resistance R; for an old wall also the U-value before insulation.
- The temperature in every layer and the dew point, worked out from the vapour permeability of the layers (sd).
- A condensation check by the Glaser method (DIN 4108-3): whether moisture collects in the construction and whether it can dry out in summer (the limit is 500 g/m²).
- The quantities of the layers by area — with 5–10 % waste, approximate.
How the U-value is worked out
U = 1 / R, where R is the sum of the resistances of the layers (thickness ÷ λ) and of the air on both sides (Rsi inside and Rse outside). Where a layer contains studs or rafters, the generator also takes the timber into account according to its share of the layer. The lower the U-value, the less heat passes through 1 m² per 1 °C of difference.
| Element | Rsi, m²K/W | Rse, m²K/W | Rse with a ventilated layer | U for comparison, W/m²K |
|---|---|---|---|---|
| Wall | 0.13 | 0.04 | 0.13 | 0.28 (Ordinance No. 7) |
| Floor | 0.17 | 0.17 | 0.17 | 0.40 (reference) |
| Roof | 0.10 | 0.10 | 0.10 | 0.25 (reference) |
Dew point and condensation
The generator calculates with 20 °C and 50 % humidity inside and −5 °C and 80 % outside over 90 days. The red line in the section is the temperature, the blue line the dew point; condensation occurs only where they touch. For internal insulation with clay and wood fibre no amount of condensate is shown, because the Glaser method does not account for these materials absorbing moisture and distributing it by capillary action.
The calculations are approximate and do not replace a design. λ and μ of naturheld and ELKA esb Plus are from the manufacturers; typical values are used for the other materials. How the layers of a wall, roof and floor are arranged is shown in the system.