Basement Insulation for Cold Climates

Author: 
Demartment of Energy Building America
Summary: 

This resource covers technical best practices for basement insulation in Cold climates.

Document:

Basements are a common foundation system in the cold and very cold climate. Wall insulation in basements is similar to the approaches described for crawlspaces. And basement floors are insulated in ways similar to slabs. 

Best Practice: Exterior wall insulation is preferred over interior approaches (Broniek 2003; Yost and Lstiburek 2002). Exterior insulation will help to protect the basement wall from freeze-thaw cycles and will help make the wall warmer, giving condensation less chance of forming and improving thermal comfort. Exterior insulation’s position outside of damp proofing makes it less likely to contribute to problems of trapped moisture inside basement walls. Exterior wall insulation must be approved for below-grade use. Products such as borate-treated foam board or rigid glass fiber insulation work well. Extruded polystyrene (R-5 per inch) is durable and moisture resistant. Expanded polystyrene (R-4 per inch) is less expensive, but it has a lower insulating value. Rigid fiber glass insulation does not insulate as well as foam but it is the only insulation option that provides a drainage plane for foundation walls. Some code officials may require a gap between exterior insulation and wood foundations elements to provide a termite inspection area. Insulation that is exposed above grade must be covered with a protective coating such as flashing, fiber cement board, parging (stucco type material), treated plywood, or membrane material. Exterior insulation is an especially good choice in areas with high water tables or poor draining soils.

If interior insulation is used it is important to consider moisture control, insulation flame spread rating, and moisture compatibility. Yost and Lstiburek (2002) discuss three requirements for interior basement insulation systems.

·         It must dry to the interior if wetting occurs because the below grade portion of the wall cannot dry to the exterior. This requirement means that interior polyethylene vapor barriers or any impermeable interior wall finishes such as vinyl wall coverings or oil/alkyd/epoxy paint systems should not be installed.

·         The wall system must be tightly sealed to keep interior air from reaching the cool foundation wall. The system must have either an effective interior air barrier (see the section on structural air sealing), or rigid insulation could be installed directly on the interior concrete or masonry surfaces.

·         Material is contact with the foundation wall and the concrete slab must be moisture tolerant. A capillary break must be placed between materials that transport moisture and moisture sensitive materials.

Yost and Lstiburek go on to present three strategies for interior basement insulation. The first system uses foil-faced polyisocyanurate rigid insulation attached directly to the upper portion of the basement wall. Extruded or expanded polystyrene can be attached to the below-grade portion of the wall. The polystyrene would require a gypsum board or equivalent covering. Extending gypsum board up the entire wall, leaving at least a half-inch gap at the floor to avoid wetting, provides a finished wall.

A second system has either expanded or extruded polystyrene foam board attached to the entire foundation wall. Extruded polystyrene is more moisture tolerant and should be used if there are any doubts in the external drainage system. Additional insulation can be added to a frame wall built on the interior of the foam insulation. If no additional insulation is desired, wood furring strips can be attached over the foam and gypsum board attached to the furring strips. A similar approach is described in the masonry wall section of this report, suggests the installation of 2x4 furring, against the basement wall, at the intersection with the ceiling, if a firestop is required. Check with local code officials about required fire ratings and stops. Gypsum board should be held at least a half inch above the basement floor to avoid wetting.

A third approach uses pre-cast concrete foundation walls that come with 1 inch of rigid foam insulation attached to the interior.

Broniek suggests that a blanket insulation with a perforated facing (to allow drying of the wall to the inside) can also be used on the inside face, but it is best used in combination with exterior insulation and in conditioned basements.

For more information on basement insulation see the following:

·         Broniek, John. 2003. Builder System Performance Packages Technical Report. Prepared by IBACOS for the Building America Program. Available at www.buildingamerica.gov in the publications section.

·         Yost, Nathan, and Joseph Lstiburek. 2002. Basement Insulation Systems. Prepared by the Building Science Consortia for the Building America Program. Available at www.buildingamerica.gov in the publication section.

·         DOE, Office of Building Technology. 2002. Technology Fact Sheet: Basement Insulation. Available at www.buildingamerica.com in the publication section.

·         IBACOS. 2002. Don’t Forget About the Basement. Prepared by IBACOS for the Building America Program. Available at www.buildingamerica.gov in the publications section.

·         University of Minnesota, Building Foundations Research Program. Minnesota Energy Code Building Foundation Rule: Amendment Proposal Development Project Final Report, www.buildingfoundation.umn.edu/FinalReportWWW/Section-A/A-recs-main.htm

For more information, please visit Building America or download the full reports below:

Cold and Very Cold Climates

Hot-Dry and Mixed-Dry Climates

Hot and Humid Climates

Marine Climates

Mixed-Humid Climates

OCEAN Resource: 
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