Condensation is the passing of a substance from a lighter to a denser physical state. In this instance, water passes from a gaseous (vapour) state to the denser liquid one. Because it is caused by too much humidity, condensation is also the symptom of a more serious problem – excessive moisture at work in your home’s atmosphere.
In seeking cooler, drier outside air, water vapour exerts pressure and forces its way through most building materials. Wood, brick, and drywall are too porous to stop moisture from migrating to the outside. Because window glass is denser and its surface colder than the surrounding walls, vapour is stopped by glass and condenses on it. While glass – the coldest, least porous (and most visible) material in a building – may show condensation first, it may also be noticed on exposed nail heads and other metal surfaces.