Does Vapor in a House Usually Rise?

Water Vapor

• Air inside the home has differing degrees of moisture content, depending on the relative humidity inside the home and other factors. The moisture inside the air is called water vapor, and it is invisible to human eyesight. When that air makes physical contact with a surface that is colder than the air's temperature, the vapor condenses and forms physical droplets that can be observed, called condensation. Water vapor can therefore make its way into the home from the outside through numerous means, including open doors and windows, venting and ductwork and cracks and crevices in the home's exterior. People also add water vapor to the indoor environment through cooking, dishwashing, showering, doing laundry and even breathing.

Vapor Movement

• The water vapor and all the other gases that make up indoor air all exert an unique pressure, and air will always move from areas of high pressure to areas of low pressure. Therefore vapor in the home will only rise if there is an area of low pressure above the vapor's current position. But vapor is no more likely to rise than to move in any other direction; the difference in relative pressures between two areas is the most important predictor of water vapor movement in any environment.

Temperature

• Of course, the temperature of the vapor and air plays a role in the vapor's movement. Many homeowners install vapor barriers in the attic, for instance, because warm air inside the home will naturally rise to the attic where vapor can quickly become a problem if it is not controlled. Water vapor tends to rise if it is significantly warmer than the indoor ambient temperature.

Controlling Vapor

• In addition to vapor barriers installed in basements, crawl spaces and inside interior walls, installing and maintaining insulation throughout your home, controlling indoor humidity with ventilation and dehumidifiers and eliminating leaks and condensation as soon as they are discovered can all greatly assist homeowners in their efforts to stem excessive indoor moisture before it can cause problems.