Dew Point
The dew point is the temperature at which air becomes saturated with water vapor and condenses into liquid water. In building science, dew point matters because warm interior air carries more moisture than cold air; if interior air contacts a surface below the dew point (a cold window, a poorly-insulated rim joist, the cold side of a sheathing layer), water condenses there. Most condensation problems in buildings — mold, rotted sills, frosted windows — are dew-point problems.