Thermal
Condensation
What happens when air contacts a surface colder than its dew point and water forms there. The mechanism behind window sweating, mold on exterior walls, and moisture accumulating inside assemblies.
In more detail
Condensation occurs when air meets a surface colder than its dew point and water forms there. Two variables control it: the moisture content of the air, which sets the dew point, and the temperature of the surface. Either can be the cause.
In houses it appears in three characteristic places. On glass, where a window's interior surface is the coldest thing in the room. On exterior walls, corners, slab edges and cantilevers, where thermal bridging cools the interior surface locally. And inside assemblies, where warm humid air reaches cold sheathing or a cold pipe — the most consequential case, because nothing about it is visible.
Because it is a comparison rather than a threshold, condensation is best diagnosed as a pair of numbers: the dew point of the indoor air, and the temperature of the surface in question. That reframes the fix. Lowering indoor humidity moves the dew point down; insulating, air sealing or covering the bridge moves the surface temperature up. Both work, and which is appropriate depends on which one is unreasonable.
Why it matters
Condensation is the mechanism behind most moisture damage in otherwise well-built houses — window sweating in winter, mold on one specific wall or corner, and hidden wetting inside assemblies that only surfaces as a smell or a stain years later. It is also why humidification has a ceiling: the level a house can carry in winter is set by its coldest surfaces, not by a comfort preference.
The common misunderstanding
That condensation means the house is too humid. It means a surface was below the dew point, and in a house with weak windows or unaddressed thermal bridging that happens at perfectly ordinary humidity. Chasing it with a dehumidifier while ignoring a cold surface treats the symptom, and in winter it can mean living drier than necessary.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.