What is condensation? Condensation happens when warm, moist air meets a cold surface, such as a window or wall. The air condenses and runs down the surface as water droplets. Condensation is more of a problem in winter than in spring or summer, as the surfaces are often cooler, and the home is less likely to have windows and doors open for long periods. We all get condensation on our windows occasionally, but this isn't necessarily a problem if it clears up quickly.