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Sometimes joints between walls and floors allow open passage of air between the heated part of the house and the attic area or outdoors. Look for such joints in your attic or in the space over a porch ceiling. This air leakage path is commonly found in
You Must Prevent Moisture Accumulation
Moisture control is a major concern associated with installing thermal insulation. The warm air inside your house contains water vapor. If this vapor passes into the insulation and condenses, it can cause significant loss of insulating value. If moisture becomes deposited in the building structure, it can cause mold growth, peeling paint, and eventual rotting of structural wood. To guard against moisture problems, use vapor retarders and provide adequate ventilation for the house. If you have a crawl space you should place a vapor retarder on the ground surface.
Vapor retarders are special materials including treated papers, plastic sheets, and metallic foils that reduce the passage of water vapor. Vapor retarders should be used in most parts of the country. In colder climates, place the vapor retarder on the warm side - the lived-in side - of the space to be insulated. This location prevents the moisture in the warm indoor air from reaching the insulation. If you live in an area where the climate is predominantly hot and humid, check with a local builder to determine the correct placement or need for a vapor retarder. More detailed guidance on regional differences in moisture control recommendations can be found in the Moisture Control Handbook published by US Department of Energy.
Batts and blankets can be purchased with a vapor retarder attached. However, if new material is being added to insulation already in place, use batts or blankets that do not have an attached vapor retarder. If this type is not available, be sure to remove the vapor retarder facing (or slash it with a sharp knife) between layers of insulation to allow any moisture which does get into the insulation to pass through.
For loose-fill insulation or for batts and blankets not having an attached vapor retarder, heavy-weight polyethylene plastic sheets are available in rolls of various widths for use as vapor retarders. In places where vapor retardant materials cannot be placed, such as in finished wall cavities being filled with blown-in insulation, the interior surface of the wall can be made vapor-resistant with a low-permeability paint, or with wall paper that has a plastic layer.
Adequate ventilation in your house is important for two reasons:
A well-insulated attic should be adequately ventilated to prevent moisture accumulation. Attics may be ventilated with a combination of soffit vents at eaves and continuous ridge vents. Attic vents may also be installed in gable faces. Many codes and standards require one square foot of unobstructed ventilation opening for each 300 square feet of attic floor area if a vapor retarder is included in the top floor ceiling. Twice as much ventilation is recommended if there is no vapor retarder. The net free area of a vent is smaller than its overall dimension because part of the vent opening is blocked by meshes or louvers. The openings should be equally distributed between the soffit and ridge vents or between each gable face. Never cover or block vents with insulation. Take care to prevent loose-fill insulation from clogging vents by using baffles.
Whether or not to ventilate a crawl space has been a controversial issue. Most building codes presently require installation of vents to provide ventilation with outside air, but a recent symposium on crawl space design organized by the American Society of Heating, Refrigerating and Air Conditioning Engineers concluded that there is no compelling technical basis for crawl space ventilation requirements. However, if the crawl space is not ventilated, it is crucial that all of the crawl space ground area be covered with a durable vapor retarder, such as heavy-weight polyethylene film. Other concerns that must be considered before eliminating ventilation to your crawl space are discussed in the Builder's Foundation Handbook published by the US Department of Energy.