Condensation risk and dew point calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Room temperature | 20 °C |
| Relative humidity in the room | 60 % |
| Outdoor temperature | 0 °C |
| U-value of the wall, roof or window | 2.1 W/m²K |
| Inside surface resistance for condensation checks | 0.25 m²K/W |
Surface 9.5 °C vs dew point 12.0 °C: condensation likely
- Outside the guidance Inside surface temperature vs dew point: 9.5 °C UK-wide: BS EN ISO 13788 (surface condensation). Check with the source
- Out of scope Condensation inside the build-up (interstitial) Not covered here: ask the insulation maker for a condensation risk analysis. UK-wide: BS EN ISO 13788 (Glaser method). Check with the source.
| Result | Value |
|---|---|
| Surface condensation | Likely: the surface is at or below the dew point |
| Dew point of the room air | 12 °C |
| Inside surface temperature | 9.5 °C |
| Temperature factor fRsi | 0.475 |
Show the working
- Dew point: γ = ln(60 ÷ 100) + 17.62 × 20 ÷ (243.12 + 20); Td = 243.12γ ÷ (17.62 − γ) = 12 °C
- Surface: 20 − 2.1 × 0.25 × (20 − (0)) = 9.5 °C
- fRsi: (9.5 − (0)) ÷ (20 − (0)) = 0.475
How it works
- Dew point (Magnus): γ = ln(RH ÷ 100) + a·T ÷ (b + T); Td = b·γ ÷ (a − γ), with a = 17.62 and b = 243.12 °C
- Inside surface temperature θsi = θroom − U × Rsi × (θroom − θoutdoor), with Rsi = 0.25 m²K/W for condensation checks
- Temperature factor fRsi = (θsi − θoutdoor) ÷ (θroom − θoutdoor)
- If the surface is at or below the dew point, water condenses on it
Mould can grow before water appears: surfaces only a little above the dew point stay damp. Ventilate and heat steadily.
Condensation inside the wall or roof (interstitial, Glaser method) isn't covered: insulation makers offer a condensation risk analysis.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Inside surface resistance for condensation checks | 0.25 m²K/W | Our assumption Not yet checked against the source | Our choice: change it to suit your job Inside surface resistance for condensation checks, brief v2 I4 (BS EN ISO 13788). Researcher to cite the clause. |
Regulations and standards
- BS EN ISO 13788 (surface temperature to avoid condensation and mould)
Explained: BS EN ISO 13788 (condensation).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.