BASEMENT ATLASInternational basement experience exchange English

FREE BASEMENT TOOL

Basement dew point calculator: compare condensation risk

Could outdoor air form condensation on your cold basement wall? Enter air temperature and relative humidity indoors and outdoors, then the temperature of the cold surface.

Example values are filled in. Replace them with measurements from your own basement before interpreting the result.

Basement air
Outdoor air
Cold wall, floor or pipe

Measure the surface separately. Do not substitute the room's air temperature.

How to use the result

  1. Take indoor and outdoor temperature and humidity readings at about the same time. Protect sensors from direct sun, rain and immediate heat sources.
  2. Measure a relevant cold surface separately. The wall behind furniture may be colder than the room air.
  3. Compare each dew point with that surface, then compare indoor and outdoor dew points. These answer two different questions: possible condensation and the moisture effect of air exchange.

Download the moisture observation sheet (PDF, 1 page)

Why comparing humidity percentages can mislead

With 26°C and 65% humidity outside, 18°C and 70% inside, and a 15°C wall, the outdoor dew point is about 18.9°C and the indoor dew point about 12.4°C. The outdoor percentage is lower, but that air can condense on the wall. These are illustrative inputs, not measurements from a real project.

What this calculator does and does not show

This is a simplified comparison of air and a measured surface at one point in time. It does not model airflow, moisture stored in materials, leaks, mould growth, radon or building compliance. Do not disable required ventilation or combustion-air supplies based on this result. The 2°C surface margin and 1°C comparison band are explanatory cautions, not standards or guarantees. Values are not stored by this calculator.

Method and sources

Dew point is calculated using the Magnus equation over water: γ = ln(RH/100) + 17.62 × T/(243.12 + T); Td = 243.12 × γ/(17.62 − γ), with temperatures in °C. The coefficient form is documented by WMO and Vaisala. The tool uses ordinary near-surface air conditions and basement surfaces at or above freezing; it does not calculate frost point.

Understand the result

Basement condensation in summer: when opening windows adds moistureMeasuring basement moisture: a useful log, not just one numberBasement ventilation or a dehumidifier: choose by the problemDamp basement walls: find the cause before choosing a repair

Have readings that seem contradictory? Describe your country, measurement positions, weather and room use when asking the community.

Ask about your basement
?Ask Basement Atlas