The air temperature inside the bathroom tends to be higher than the air temperature on the other side of the walls, floor, and ceiling. This is especially true, in winter, of exterior walls, and a ceiling with an attic above it. There is a natural tendency for warm air to move to cool air. This is nature’s way of trying to maintain equilibrium. In a bathroom, warm, moist air will tend to move through walls and ceilings, moving from warm to cool.
As the air moves through the wall or ceiling, it is cooled. When the dew point temperature is reached, condensation occurs (see Figure 3.12). This hidden condensation inside walls and attics can be a particular nightmare for homeowners. As building materials get wetter, deterioration and mold growth can become extensive before the problem is noticed.