Excessive amounts of warm, moist air trapped in homes can cause considerable damage. Wall corners and closets collect mildew, while walls, floors and ceilings are stained and begin to decay. In time, much of the house may take on a damp or musty odor.
Moisture comes from many sources inside the home including cooking, washing, bathing, watering house plants and the respiration and perspiration of the occupants. When this warm, moist air comes in contact with cool air or a cool surface, it condenses and forms beads or a film of moisture. This condensation has a tendency to show up on a cool window, ceiling, wall or water pipe _ a condition known as sweating.
Water also enters from the outside, through pores or cracks in the foundation, a condition known as seepage. Before you can remove dampness from your home, first find its source.
To test for seepage, tape a piece of aluminum foil securely to the foundation wall where a damp spot frequently appears. Make sure all the edges are airtight. Leave it in place a day or two, then remove it. If the foil's face against the wall is damp, the problem is seepage. If the other face is damp, the problem is condensation.
During cold weather, warm, moist air tends to work its way into the colder wall spaces and, eventually, up into the attic. There it condenses on the colder inner faces of the wall or roof sheathing, ruining insulation, rotting wood framing and rusting nails.
Preventive measures include ventilation to remove excess moisture from areas where it will do damage and insulation and vapor barriers to keep warm, moist air separated from colder outside air.
A vapor barrier is a non-permeable film (as a paint or coating), foil, plastic sheet or treated paper. Often, it's used on the face of insulation. It forms a barrier to prevent moisture from passing through walls, ceilings or floors.
A vapor barrier always faces the warm side of the structure, though in an attic, insulation without a vapor barrier is installed over existing insulation that already has a vapor barrier.
In the case of a crawl space, often a polyethylene sheet is placed over the dirt floor and concrete is poured over that. Combined with venting, this prevents moisture from rising into the structure above it.
No vapor barrier stops moisture completely. Therefore, ventilation is needed to rid the attic of moisture rising from the living space.
Attic vents provide openings to allow moisture to escape outdoors before condensing. As a rule of thumb, provide 1 square foot of unobstructed vent opening per 150 square feet of attic floor space. The key word here is "unobstructed." Louvers or screens decrease the area of the vent opening by as much as 50 percent, so allow for this when selecting a vent.
If the attic has a vapor barrier, 1 square foot of opening per 300 square feet of attic floor space should be sufficient. The vents should be placed so they provide efficient cross ventilation.
Roof vents placed on the back of the house so that they are not visible from the street and ridge vents are other options to consider. These are especially useful if you have or want a finished attic that would close off the standard gable vents.
If the vents alone are not enough, install exhaust fans to speed the job along. These fans may be installed on the roof or gable end, above a vent or in place of one. Use one that operates on an energy-saving manual switch, a time-delay switch or a humidistat switch. Other places to consider installing exhaust fans that vent to the outside are in the bathroom, laundry room and kitchen.
Moisture can be removed from the air by a dehumidifier. Look for one that collects a minimum of a gallon of water from the air in 24 hours.