A properly insulated basement ceiling has two major features designed to decrease your energy costs. A well insulated basement ceiling combines insulation with an air barrier that prevents cold basement air from infiltrating the living areas of your home. Basement ceilings are only insulated in homes with an unheated basement or crawlspace. If your basement is heated, insulating the walls of the basement is more effective.
Ceiling insulation in the basement is designed to create a thermal barrier between the living area of your home and the basement. Home insulation is rated using the R-value system. The R-value of insulation defines how effectively 1 inch of insulation prevents the transfer of heat. Most homes in the United States only require insulation with an R-value between 11 and 18. The most common form of insulation used to insulate basement ceilings is batt insulation made from fiberglass. Batt insulation is a rolled rectangular blanket that is hung between the floor joists of your basement ceiling. This insulation has an R-value between 2.9 and 3.8 per inch.
Before you begin installing insulation you should check your basement ceiling for gaps that could allow air into the living areas of your homes. Gaps commonly occur where pipes, ducts and conduits penetrate the subfloor of your home. Even a small gap can allow significant amounts of cold air into your home. You can seal small gaps using a can of spray foam. Seal larger gaps using a piece of foam board cut to size, with spray insulation applied around the edges to form a complete air seal.
Batt insulation is installed by hand and held in place using wires strung between the floor joists. Each batt should fit snugly between the rafters without stretching or compressing. Install enough wire supports so that the batt does not sag or bend away from the floor. Most homes will only require between 3 and 4 inches of batt insulation to properly insulate the basement ceiling. Northern climates with extremely cold winters require up to 5 inches of insulation to effectively insulate the basement ceiling.
Excessive moisture in the basement will promote mold growth and can damage the framing or insulation of your home. If your home has a crawlspace with a dirt floor, you should cover it with a vapor barrier to keep moisture from the ground out of your crawlspace. A plastic sheet stretched across the floor and sealed with tape is a common and effective form of vapor barrier in most cases.
Avoid insulating gaps in the basement ceiling around electrical fixtures or other heat-producing objects since this will create a potential fire hazard. The Department of Energy recommends maintaining a gap of at least 3 inches between insulation and heat-producing fixtures. Insulating the ceiling of the basement often leaves the water pipes of your home exposed to cold air. If your area routinely experiences temperatures below freezing, you should consider insulating your pipes to protect them from freezing. Homes with a significant portion of their basement walls above ground level are more susceptible to cold weather and are more likely to suffer burst pipes when the temperature drops. These homes will benefit from a well insulated basement ceiling and water pipes.
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