Thermal
Dew Point
The temperature at which air becomes saturated and water vapor starts condensing into liquid. Unlike relative humidity, it describes the absolute amount of moisture in the air and does not change when that air is heated or cooled.
In more detail
Because dew point is tied to moisture content rather than to a ratio, it answers the question that matters in a building: is there any surface here cold enough for water to form on it? If the indoor dew point is 12°C, every surface below 12°C is a condensation site — a window frame, an uninsulated header, a duct running through an unconditioned attic, a cold-water pipe, the back face of a closet on an exterior wall.
That makes dew point the operative number in three otherwise unrelated problems. Winter window condensation is indoor dew point meeting cold glass, which is why the fix is either lowering indoor humidity or raising the glass temperature with better glazing, not wiping the sill. Condensation inside a wall assembly is the same event happening where nobody can see it, and it is why the position of insulation and vapor control layers is climate-specific rather than universal. Summer condensation on ducts and cool surfaces is outdoor dew point meeting air-conditioned surfaces, which is why humid climates punish poorly detailed mechanicals.
It is also the better summer comfort metric. Two days at the same air temperature feel entirely different at a 10°C dew point and a 22°C one, because evaporative cooling from skin depends on vapor pressure rather than on a ratio.
Why it matters
Nearly every moisture failure in a house is a dew point that met a cold surface and stayed there. Framing it that way makes the two available levers obvious: reduce the moisture in the air, or raise the temperature of the surface. That is the underlying logic of continuous insulation, better glazing, and thermal-bridge detailing. They are not only energy measures — they raise interior surface temperatures above the dew point and take condensation off the table.
The common misunderstanding
That relative humidity is the number to watch for condensation risk. It is not, because it moves with temperature while the moisture stays put. A house at 40 percent RH in a warm room can be condensing on a window in a cold one: same air, same dew point, different surface. When comparing rooms, assemblies, or seasons, dew point is the number that holds still.
Related terms
On Compound
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Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.