Dew Point & Condensation Risk Calculator

Quick Answer

If the surface temperature falls to the dew point, condensation becomes possible even when your room-level RH sounds acceptable.

That is why this tool is not really about comfort. It is about figuring out how much margin you have before windows, walls, or cold corners start collecting water.

Most useful when You can point to an actual cold surface problem and want a safer RH ceiling instead of a generic humidity recommendation.

Inputs

Temperature Unit

Indoor Air Temperature

Indoor Relative Humidity (RH)

Surface Preset (optional)

Surface Temperature (optional)

Preset Assumption Level

Results

Dew Point

9.3°C

If a surface falls to or below this temperature, condensation becomes possible.

Condensation Risk (at Surface Temp)

Medium

ΔT (Surface - Dew Point)

+2.0°C

Smaller ΔT means higher condensation risk. ΔT ≤ 0° indicates likely condensation.

Safe RH Ceiling (for this Surface Temp)

45% RH

Targeting at or below this RH helps reduce condensation risk on the selected surface.

Open Target Humidity Tool

Who This Calculator Is For

Homes with window condensation, damp wall corners, or other cold-surface moisture problems
Users trying to decide whether a lower RH target is needed for condensation control
Shoppers who want to connect indoor RH, surface temperature, and dew point before changing equipment or settings

How This Calculator Works

This calculator uses a common dew point approximation (Magnus formula) suitable for typical indoor ranges.
If you enter °F, calculations are performed in °C internally and converted back for display.
Surface presets are only typical examples. If you can measure surface temperature, use that for best accuracy.
For persistent condensation, also address ventilation, air leakage, and cold bridging.

When This Calculator Is Not Enough

Use mold risk score when leaks, stains, odors, or visible growth suggest a broader moisture problem than surface condensation alone.
Use target humidity when the goal is a general room setpoint and not one cold-surface risk calculation.
Use troubleshooting when the current dehumidifier may be failing to control RH despite a reasonable target.

Frequently Asked Condensation Questions

What is the dew point in simple terms?

The dew point is the temperature at which air becomes saturated and water vapor condenses into liquid. Warmer air holds more moisture, so the same room can feel dry in summer and sweat in winter when surfaces cool below the dew point.

Why does condensation form on my windows but not my walls?

Glass cools faster than drywall, so window surfaces drop below the dew point first. The same indoor air will condense on walls, pipes, or floors once those surfaces are cold enough — usually a sign the indoor humidity is too high for the room temperature.

What indoor humidity prevents condensation?

Keeping indoor RH between 40-50% prevents most window and wall condensation in normal conditions. In cold climates, you may need to run lower in winter — around 35-40% — to keep cold glass and corner walls above the dew point.

Best Next Step

Use the surface risk to adjust the room target, then check whether the broader moisture problem is bigger than condensation alone.

If condensation is only one symptom, move next into mold triage, troubleshooting, or a room-specific setup guide.

Open Target Humidity Tool Open Mold Risk Score

Sources & Methodology

Dew point calculation uses the Magnus formula, the standard psychrometric model for indoor air
Surface temperature presets based on typical residential construction and common window types

Last reviewed: 2026-08. Information is periodically reviewed and updated as standards, testing procedures, and field data evolve.