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Condensation Risk Calculator — Dew Point & Surface Condensation Check
Calculate the dew point temperature and check whether your wall surfaces are at risk of condensation. Covers solid, cavity and insulated cavity wall constructions.
Typical UK living room: 18-21°C
40-60% is normal; >70% is high risk. The simplified dew-point formula is reliable for RH ≥ 50% and over-estimates (errs safe) at lower humidity
UK winter average: 2-7°C
Internal surface resistance in m²K/W. 0.13 is the BS EN ISO 6946 design value (open wall). For a sheltered worst case — a corner or behind furniture — BS EN ISO 13788 uses 0.25, which gives a colder, more cautious surface temperature.
Divisor in simplified Magnus formula (default 5). Higher = lower dew point estimates
Margin above which risk is considered low (default 5°C)
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How We Calculate This
This calculator estimates whether surface condensation will occur on your walls by comparing the wall’s internal surface temperature with the dew point temperature of the room air.
Dew point calculation
The dew point is calculated using a simplified version of the Magnus formula:
Td ≈ T − (100 − RH) ÷ 5
Where Td = dew point (°C), T = room temperature (°C), RH = relative humidity (%). This approximation is reliable to within about ±0.5°C for RH ≥ 50%; below that it over-estimates the dew point (so it errs on the side of caution and never hides real condensation).
Surface temperature estimation
The internal wall surface temperature is estimated from the room temperature, external temperature and wall U-value:
Tsi = Ti − (Rsi × U × (Ti − Te))
Where Tsi = internal surface temperature, Ti = internal air temperature, Te = external temperature, Rsi = internal surface resistance (0.13 m²K/W), U = wall U-value
U-values used
- Solid wall (uninsulated): 2.1 W/m²K — typical pre-1919 construction
- Cavity wall (unfilled): 1.5 W/m²K — typical 1930s–1980s construction
- Insulated cavity wall: 0.3 W/m²K — typical retrofit cavity-fill (note: the current Part L limit is 0.26 W/m²K for a new-build wall and 0.18 W/m²K for an extension, so a brand-new wall must do better than 0.3)
Risk assessment
- Low risk: Surface temp more than 5°C above dew point
- Medium risk: Surface temp 0–5°C above dew point
- High risk: Surface temp at or below dew point — liquid condensation likely
The “high risk” band uses the dew point (100% surface saturation), i.e. the point at which visible liquid water forms. The 5°C low-risk buffer is a practical convention rather than a code figure. For mould growth specifically, BS EN ISO 13788 uses a stricter test — a critical surface relative humidity of 80% (factor φsi,cr= 0.8), meaning mould can develop on a surface that is warmer than the dew point but still cold enough to sit above 80% RH. Treat a “medium” result as a prompt to improve ventilation and insulation, not an all-clear.
Standards
Condensation risk assessment should follow BS 5250:2021 (Management of moisture in buildings) and Approved Document Part C. For detailed interstitial condensation analysis, the Glaser method or dynamic simulation tools (WUFI) are used. This calculator provides a simplified surface condensation check only.
Working figures for this are collected in our UK Vapour Resistance Values Reference reference table.
Frequently Asked Questions
The dew point is the temperature at which air becomes saturated and water vapour condenses into liquid water. If any surface in your home is at or below the dew point temperature, moisture will condense on it. At 20°C room temperature and 60% relative humidity, the dew point is approximately 12°C. The higher the humidity, the closer the dew point is to the air temperature.
Condensation occurs when warm, moist indoor air meets a cold wall surface that is at or below the dew point temperature. Cold spots are common on solid walls, behind furniture against external walls, in corners and around window reveals. Poor ventilation (trapped moisture from cooking, bathing and breathing) and inadequate insulation (cold wall surfaces) are the main contributing factors.
The recommended relative humidity for UK homes is between 40-60%. Below 40% can cause dry skin and irritation. Above 60% significantly increases condensation risk, especially in winter when external temperatures drop. Bathrooms and kitchens can temporarily reach 80-90% during use, which is why adequate extract ventilation is essential in these rooms.
Wall insulation raises the internal surface temperature of walls, making condensation less likely. An uninsulated solid wall has a U-value of about 2.1 W/m²K, meaning the internal surface gets very cold in winter. An insulated cavity wall (U-value ~0.3 W/m²K) keeps the internal surface much warmer. However, internal wall insulation can move the dew point into the wall structure, creating interstitial condensation risk, which requires a vapour control layer.
Approved Document Part C (Site preparation and resistance to contaminants and moisture) covers moisture resistance including condensation. BS 5250:2021 (Management of moisture in buildings) provides detailed guidance on condensation assessment and control, including interstitial condensation analysis. Part F (Ventilation) sets minimum ventilation rates to control moisture levels. Part L (Conservation of fuel and power) sets maximum U-values which help keep surfaces warm.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.