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Magnus dew pointSurface checkFree

Surface Condensation
Screen

Calculate indoor dew point, then compare it with one entered surface temperature. The result predicts liquid condensation for that steady-state input only—not mould, interstitial moisture, or compliance.

Who it's for: Architects, energy consultants, and building physicists screening whether a specific wall, roof, or glazing surface will reach the dew point at a given indoor air condition — this is a steady-state screening tool for design development, not a full hygrothermal analysis.

Magnus dew point
γ = ln(RH/100) + 17.62·T/(243.12+T); Td = 243.12·γ / (17.62 − γ). Condensation risk where surface temp ≤ Td. (A screening check — interstitial condensation needs the ISO 13788 Glaser method.)
Dew Point
Risk check

How to use this calculator

1
Enter indoor air temperature
The dry-bulb air temperature inside the conditioned space, in °C. Typical heated dwellings run 18–22°C; higher temperatures raise the dew point and widen the risk window.
2
Enter relative humidity
The fraction of moisture the air carries relative to saturation (0–100%). Occupied residential spaces typically run 40–60%. Values above 70% create mould risk before liquid condensation appears.
3
Enter surface temperature
The temperature of the specific surface to check — a wall face, glazing pane, cold lintel, or roof soffit. Obtain it from a thermal model, an infrared thermometer reading, or a thermal-bridge simulation. Leave blank to get the dew point alone.
4
Read the dew point
The result shows the dew point Td at your indoor air condition. Any surface whose temperature is at or below this value will accumulate liquid water.
5
Check the risk badge
If you entered a surface temperature, the badge shows ✗ Risk (surface ≤ dew point, condensation forms) or ✓ Safe (surface sits above the dew point). The detail panel also shows actual and saturation vapour pressures.

The formula

The Magnus approximation converts indoor temperature and relative humidity into a dew point by inverting the saturation vapour-pressure curve. Condensation forms wherever a surface sits at or below that temperature.

γ = ln(RH / 100) + 17.62 · T / (243.12 + T)
Td = 243.12 · γ / (17.62 − γ)

T = indoor air temperature (°C)
RH = relative humidity (%)
γ = Magnus intermediate argument (dimensionless)
Td = dew point temperature (°C)
17.62 and 243.12°C = empirical constants from Alduchov & Eskridge (1996), accurate to ±0.1°C from −40°C to +60°C
Risk condition: surface temperature ≤ Td

Worked example

Example
A residential room in winter has indoor air at 20°C and 55% RH. The Magnus formula gives γ = ln(0.55) + 17.62 × 20 ÷ 263.12 = 0.742. Dew point Td = 243.12 × 0.742 ÷ 16.878 = 10.7°C. Saturation vapour pressure at 20°C is 23.3 hPa; actual vapour pressure at 55% RH is 12.8 hPa. A north-facing masonry wall in the same room has a measured surface temperature of 10°C. Because 10°C ≤ 10.7°C, the tool flags ✗ condensation risk — moisture will form on that surface under these conditions.

When this estimate will be off

  • Surface condensation only — for moisture accumulating inside the wall assembly (interstitial condensation), use the ISO 13788 Glaser method, which steps through each construction layer.
  • Assumes steady-state indoor conditions. Diurnal humidity spikes from cooking or showering, occupancy-driven RH peaks, and thermal mass effects all shift the surface temperature and real risk level.
  • The Magnus formula covers −40°C to +60°C. Outside that range, or for high-precision psychrometric work, use the full Buck or Alduchov–Eskridge equations.
  • Mould growth can begin when surface RH exceeds roughly 70–80%, well before liquid condensation appears. This tool checks for liquid water only; mould-risk screening requires the temperature-factor (f-factor) method per ISO 13788.

Frequently asked questions

A common mistake: Surface condensation only — for moisture accumulating inside the wall assembly (interstitial condensation), use the ISO 13788 Glaser method, which steps through each construction layer.

The dew point is the temperature at which air at a given humidity becomes saturated — further cooling causes water vapour to condense into liquid. For a room at 20°C and 55% RH the dew point is about 10.7°C. Any surface cooler than that will accumulate moisture.

Surface condensation occurs on a visible face — wall, glazing, or lintel — when it drops below the dew point. Interstitial condensation occurs inside the building assembly, between layers. The two are assessed differently: this tool handles surface condensation; interstitial risk needs the ISO 13788 Glaser method, which traces the vapour-pressure profile through each construction layer.

Mould spores germinate when the relative humidity at the surface exceeds roughly 70–80% and the surface temperature stays below a critical threshold. That condition can exist well above the dew point. ISO 13788 defines a minimum temperature factor fRsi to prevent this; the critical surface humidity there is 80%, not 100%.

Not directly. It is a single-surface, steady-state screening tool. Code compliance and passive-house certification (Passive House Institute EnerPHit standard, national building regulations) require a full dynamic hygrothermal analysis — using tools such as WUFI or HEAT2 — or at minimum the ISO 13788 Glaser tabular method.

Sources

  • ISO 13788:2012 — hygrothermal performance of building components — surface temperature to avoid critical surface humidity and interstitial condensation (Glaser method)
  • Alduchov & Eskridge (1996) — improved Magnus-form approximation of saturation vapour pressure — constants a₁ = 17.62, b₁ = 243.12°C used in the WMO-endorsed approximation

Sources and review record

Sources and method basis

  • Alduchov and Eskridge (1996) Magnus-form dew-point approximation
  • Declared arithmetic or user-supplied method; no governing source URL is claimed for this bounded operation.

Limitations

  • Steady-state liquid surface-condensation screen only; mould, critical surface humidity, interstitial moisture, transient hygrothermal behavior, and compliance are excluded.

This free Condensation Risk Checker is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.