Double Glazing Condensation Risk Calculator
Assess the risk of condensation forming on the inner pane of double glazing by comparing the inner glass surface temperature to the indoor dew point temperature.
Formulas Used
1. Dew Point Temperature — Magnus Formula (Lawrence 2005):
α = ln(RH/100) + (17.625 × Ti) / (243.04 + Ti)
Td = 243.04 × α / (17.625 − α)
2. Inner Glass Surface Temperature:
q = U × (Ti − To) [W/m²]
Tg = Ti − q / hi [°C]
Where U is the overall window U-value and hi is the indoor convective/radiative surface coefficient.
3. Temperature Factor (ISO 13788):
fRsi = (Tg − To) / (Ti − To)
A higher fRsi (closer to 1) indicates better thermal performance and lower condensation risk. ISO 13788 recommends fRsi ≥ 0.75 for most climates.
4. Condensation Risk Decision:
Margin = Tg − Td
Margin ≥ 3 °C → Low Risk
0 ≤ Margin < 3 °C → Moderate Risk
Margin < 0 °C → High Risk (condensation expected)
Assumptions & References
- The window is modelled as a simple thermal resistance; edge effects and frame conductivity are not included.
- Indoor surface heat transfer coefficient hi = 8 W/m²·K is the default per ISO 6946 (combined convection + radiation, still air).
- The Magnus formula constants (a = 17.625, b = 243.04 °C) are from Lawrence (2005), valid for −40 °C to +60 °C.
- Saturation vapour pressure uses the Buck (1981) equation for liquid water.
- Typical double-glazed unit U-values: argon-filled low-e ≈ 1.2–1.4 W/m²·K; air-filled standard ≈ 2.4–2.8 W/m²·K.
- ISO 13788:2012 — Hygrothermal performance of building components and building elements.
- ASHRAE Fundamentals Handbook — psychrometrics and surface condensation.
- Lawrence, M.G. (2005). The relationship between relative humidity and the dew point temperature. Bulletin of the American Meteorological Society, 86(2), 225–233.
- This calculator assesses internal surface condensation only; interstitial (cavity) condensation requires a full Glaser or dynamic analysis.