Refer to the Air Sealed, Insulated Basements guide for best practices. While this guide is not intended to represent a “best practice” installation, it does represent an improvement to typical basement blanket insulation installations that will result in improved thermal performance and moisture durability. (See the guide Termite-Resistant Foundations and Walls for more information.) In high-termite-risk areas, a termite inspection strip (a 4-inch horizontal gap of bare wall) may be required at the top of the foundation wall consult your local code department. Also see the following guides for information on moisture control and indoor air quality: Water Management of Existing Basement Floor, Whole-House Ventilation Strategies for New Homes, and Whole-House Ventilation Strategies for Existing Homes. See the Whole-House Dehumidification guide for information on maintaining proper humidity levels. This insulation approach will not compensate for poor moisture management. The strategy presented is appropriate for new or retrofit construction in conditioned or semi-conditioned, fully or partially below-grade basements where reasonable humidity levels are maintained. Figure 1. Water vapor in humid indoor air that gets past the vapor barrier on conventional plastic-covered blanket insulation can condense when it reaches the cold basement foundation wall causing moisture issues (Source: Building Science Corporation 2006).įigure 2. Moisture from a concrete foundation wall that hasn’t had time to properly cure can migrate into conventional plastic-covered blanket insulation and cause moisture problems (Source: Building Science Corporation 2006).įigure 3. If the basement foundation lacks good moisture management, moisture can migrate from below the foundation into the basement wall that may lead to moisture issues if the interior of the wall is covered with plastic-covered blanket insulation (Source: Building Science Corporation 2006). The third source is moisture below the foundation, that can move from beneath the footer through the foundation wall via capillary draw and dries to the interior (Figure 3). The second source is moisture that is within the concrete wall when it was constructed, that can move via diffusion towards the drier air of the basement or outside (Figure 2). The first source is moisture vapor (i.e., humidity) in basement air from human activity or from humidity in the outdoor air that can condense on the cold interior surface of the foundation walls if that surface temperature drops below the interior dew point (Figure 1). Typically, there are three sources of moisture that can result in condensation issues on basement foundation walls that are constructed of poured concrete or concrete masonry units (i.e., CMU block). This simple installation method has led to condensation where the blanket insulation meets the surface of the concrete, resulting in potential durability and indoor air quality problems. These blankets are available in widths of 48 inches and 72 inches and are typically installed in a horizontal fashion directly against the foundation walls with the vapor retarder or barrier facing towards the living space. The facer can be either solid or perforated. Some builders meet the basement insulation code requirement by using a pre-manufactured insulation “blanket” that is made up of a large fiberglass batt consisting of either a reinforced polypropylene vinyl or foil facer (FSK) laminated to one surface of the insulation to create a vapor retarder or barrier. (See Compliance tab for insulation requirements.) Basement blanket insulation is not recommended for homes in Climate Zones 7 and 8 where more extreme temperature variations across the wall assembly could lead to condensation issues. This guide is intended to provide an effective strategy to combat thermal- and moisture-related issues in conditioned and semi-conditioned basements that are constructed in International Code Council (ICC) Climate Zones 3, 4, 5, and 6. The International Residential Code (IRC) requires basement foundation walls to be insulated in Climate Zones 3 through 8 (see the Climate tab for clarification on Climate Zone 3).
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