Checking a surface against the dew point
Enter the indoor dry bulb temperature and relative humidity and the page returns the dew point of that air, which is the lowest surface temperature that stays free of condensation. The surface temperature is optional: leave it blank and only the dew point is shown, or add a measured surface temperature for the margin against the dew point and a condensation verdict, plus an outdoor temperature for the temperature difference ratio. Temperatures are shown in °F with °C in parentheses. With a surface temperature entered, the page also reports how far the indoor relative humidity would have to come down to avoid condensation at the surface temperature entered.
Saturation vapor pressure uses 6.112 x exp(17.67T / (T + 243.5)), reported in secondary material as accurate to roughly 0.5 percent between -45 and 60 degrees Celsius, which is -49 °F to 140 °F. Other sources publish different coefficient sets, so the set used here is stated. Readings outside the range are blocked with a message. Figures are current as of October 2026.
Insulation thickness sizing and supply air temperature setting are not covered. This calculator is for reference and does not replace design work; review by a qualified engineer is required.
Frequently Asked Questions
The moment a surface drops below the dew point of the air touching it, water vapor condenses there. That makes the dew point the boundary where condensation begins.
It is the room to surface temperature difference divided by the room to outdoor difference. A smaller value means the surface sits closer to the room air.
It is the dehumidification target when the surface temperature cannot be raised. It holds only for the surface temperature entered, so recalculate when that changes.