Example Data Table
| Scenario | Indoor (°C) | Indoor RH | Surface (°C) | Dew Point (°C) | Expected Outcome |
|---|---|---|---|---|---|
| Comfortable winter room | 21 | 40% | 17 | ~7 | Low risk |
| Humid bedroom, cool window | 22 | 65% | 13 | ~15 | Condensation likely |
| Bathroom after shower | 24 | 80% | 18 | ~20 | Very high risk |
| Improved glazing, same indoor RH | 22 | 55% | 18 | ~13 | Moderate to low |
Formula Used
- Dew point (Magnus): Computes dew point from indoor air temperature and relative humidity. It is widely used in building moisture checks for practical ranges.
- Condensation condition: Condensation becomes likely when the interior surface temperature is at or below the dew point.
- Surface relative humidity: Uses indoor vapor pressure and saturation pressure at the surface temperature: RHₛ = 100 × e / eₛ(Tₛ). Mold risk can rise when RHₛ stays high.
- Surface temperature estimate (optional): Tₛ ≈ Tᵢ − (Tᵢ − Tₒ) × (Rsi / Rtotal), with Rtotal based on the provided U-value.
How to Use This Calculator
- Select temperature units, then enter indoor temperature and humidity.
- Enter outdoor temperature for U-value estimation mode.
- Choose a surface input method: measured surface temperature or U-value estimation.
- Click Calculate Risk to view results above the form.
- Download CSV or PDF to share moisture checks with your team.
Moisture patterns that trigger surface wetting
Surface condensation is a short-duration event with long-term consequences. When indoor air meets a cold surface, moisture can form as droplets, damp films, or recurring fogging on glazing. In occupied buildings, peak humidity often occurs in kitchens, bathrooms, laundry zones, and crowded rooms. Tracking these peaks helps explain complaints such as musty odor, stained paint, blistering finishes, and window puddling.
Dew point as a decision threshold
Dew point represents the temperature where air becomes saturated at the current moisture content. If the interior surface temperature falls at or below dew point, condensation becomes likely. The calculator also shows a safety margin, defined as surface temperature minus dew point. Margins near 0 indicate immediate risk, while small margins suggest that minor humidity spikes can cause wetting.
Interpreting surface relative humidity outputs
Surface relative humidity (RHs) is calculated from indoor vapor pressure and the saturation pressure at the surface temperature. Even without visible droplets, RHs can remain high near thermal bridges, corners, and window frames. Sustained RHs above a chosen threshold, often 80%, supports mold growth on dust layers and porous finishes. Use RHs to prioritize mitigation where condensation is intermittent.
Using U-value mode for early design screening
When surface temperature is not measured, U-value mode estimates the interior surface temperature from indoor and outdoor temperatures and resistances. Lower U-values usually keep interior surfaces warmer and increase the safety margin. This screening is useful for comparing assemblies, checking glazing upgrades, and evaluating retrofit concepts. For critical junctions, measure surface temperatures and review thermal bridging separately.
Field actions to reduce risk and document results
Reduce risk by lowering indoor humidity, increasing ventilation during moisture events, and improving surface temperatures through insulation continuity and draft control. Confirm improvements by re-running the calculator after changes and exporting the CSV or PDF for site records. Repeated entries across seasons reveal whether risk is a rare spike or a persistent condition requiring envelope or HVAC upgrades.
FAQs
1) What does the dew point tell me?
Dew point is the temperature where the current indoor moisture content reaches saturation. If a surface is at or below dew point, condensation can occur. A larger safety margin means a more robust buffer against wetting.
2) Should I use measured surface temperature or U-value mode?
Use measured surface temperature when you can access the surface and conditions are representative. Use U-value mode for quick screening during design or early audits. Thermal bridges can still dominate local risk.
3) Why can surface relative humidity exceed 100%?
RHs can exceed 100% when the estimated or measured surface temperature is below the dew point, meaning the air adjacent to the surface is supersaturated. In practice, that condition tends to produce liquid water on the surface.
4) What are Rsi and Rse values used for?
Rsi and Rse represent interior and exterior surface film resistances. They influence the estimated surface temperature in U-value mode. Defaults suit typical still-air conditions, but high air speed or wind exposure can change these resistances.
5) How do I lower the calculated risk quickly?
Reduce indoor humidity during peak events, run exhaust fans longer, avoid drying clothes indoors, and maintain stable heating in cold zones. Improve surface temperature by sealing drafts and upgrading insulation or glazing, especially at corners and frames.
6) When should I consult a specialist?
Consult a building professional when condensation is persistent, mold is visible, finishes are failing, or occupants report health concerns. A specialist can assess thermal bridges, ventilation balance, moisture sources, and compliance requirements using measurements and modeling.