Printable Version

Insulation

Properly installed insulation is designed to keep heat where it's wanted. Adequate insulation is essential for controlling home cooling and heating costs. Any interior insulation type that has vapor permeability is acceptable, including cellulose, fiberglass, and foam. Foam can also serve as an air retarder, but air sealing must be accomplished by a separate component or system when cellulose or fiberglass is used.

The following descriptions of insulation types were adapted from DOE's Fact Sheet on Insulation.

Blanket Insulation Blankets Blankets in the form of batts or rolls are flexible products made from mineral fibers, typically fiberglass. They are available in widths suited to standard wall, floor, and attic framing spaces. Continuous rolls can be hand-cut and trimmed to fit. They are available with or without vapor retarder facings. High-density fiberglass batts are about 15 percent more effective than traditional batts. Even if you choose to use other types of insulation, such as blown or sprayed in cellulose or foam, batts can be installed in areas that may become inaccessible as construction unfolds. These areas could include behind-shower inserts, stairs, or rim joists. Batts also make good dams in attics around access points or other areas where blown-in insulation should be held back. When fiberglass batt insulation is specified, use high-density, unfaced batts. Batt facing is a vapor retarder and can trap moisture inside walls.

Blown-In Insulation Blown-In Blown-in, loose-fill insulation includes loose fibers or fiber pellets that are blown into building cavities or attics using special pneumatic equipment. Another form includes fibers that are co-sprayed with moisture or an adhesive that allows them to set in walls and makes them resistant to settling. The blown-in material can provide some resistance to air infiltration if the insulation is sufficiently dense.

Foamed-in-Place Insulation Foamed-in-Place Foamed-in-place polyurethane foam insulation can be applied by a professional applicator using special equipment to meter, mix, and spray into cavities. Polyurethane foam makes an excellent air seal and can be used to reach hard-to-get-at places.

Rigid Insulation Rigid insulation is made from fibrous materials or plastic foams that are pressed or extruded into sheets and molded pipe-coverings. These provide thermal and acoustical insulation, strength with low weight, and coverage with few heat loss paths. Such boards may be faced with a reflective foil that reduces heat flow when next to an air space. Foil facing also makes the board nearly impervious to water and vapor and so should be used with caution. Rigid foam insulation may be used in combination with other insulation types, such as on the exterior of walls that are filled with cellulose or fiberglass. Foam sheets that may be in contact with the ground should be borate-treated for termite resistance.

Reflective Insulation Reflective Insulation Systems Reflective insulation systems are fabricated from aluminum foils with a variety of backings such as roof sheathing, kraft paper, plastic film, polyethylene bubbles, or cardboard. These systems are not recommended for cold and very cold climates. If a single reflective surface is used alone and faces an open space, such as an attic, it is called a radiant barrier. Radiant barriers are also not recommended for cold and very cold climates.

Additional information on radiant barriers is available from the DOE's Radiant Barrier Attic Fact Sheet.

How Much Insulation Do You Need?

This is an excellent question to ask a home energy rater professional. The answer depends on the home's location, the overall design, and the efficiency of other building features. The ENERGY STAR^® website contains Builder Option Packages (BOPS) that recommend insulation levels on a county by county basis. The ENERGY STAR BOPs provide examples of how insulation can be traded off with other features such as efficient windows and HVAC systems. DOE can also help with insulation recommendations for each zip code. Additional information is available from the DOE's Insulation Fact Sheet. These recommendations are only guidelines, however, and are limited in scope. The more complex or advanced the home design, the more important it is to rely on specific calculations.

Slab Foundation Insulation

• Slabs may be insulated at the perimeter with borate-treated foam board or rigid glass fiber insulation with an insulating value of at least R-8 in milder climates and up to R-10 in colder areas. Use only insulation approved for below-grade use. Some code officials may require a gap between exterior insulation and wood foundations elements to provide a termite inspection area.

• A shallow, frost protected slab foundation may be used in areas subject to seasonal ground freezing. With this approach, foundation footings need not be placed below frost depth. However, rigid insulation, approved for below-grade use, must be placed vertically on the exterior of the grade beam, and must be placed to extend away from the foundation horizontally at the base of the grade beam for a distance equivalent to frost depth. Rigid insulation is also needed vertically on the inside of the grade beam, and must extend horizontally under the slab, on top of the gravel capillary break, for two feet. Code officials may require that a structural engineer review and approve specific plans.

• Slab perimeters may be insulated on the interior side. This approach requires that rigid insulation be placed between the slab and the foundation wall, and under the slab, as required by local code.

Two good resources on shallow foundations are the American Society of Civil Engineers standard (number 32-01), Design and Construction of Frost-Protected Shallow Foundations, and the University of Minnesota, Building Foundations Research Program's Frost Protected Shallow Foundations Design Specifications.

Crawlspace Insulation

Two methods are in use for insulating crawlspaces. The first, in common use over the last several decades, is to insulate the underside of the building floor, and provide outside air vents in foundation walls. Research has shown that this approach can lead to moisture problems, especially in areas with cold or humid air.

• Conditioned crawlspaces are a better approach. Within this type of system, foundation side walls are insulated on either the interior or exterior (or both), and no outside air vents penetrate the foundation wall.

• The preferred approach is to install insulation on the exterior foundation wall. Exterior insulation will help to protect the foundation from the freeze-thaw cycle and a warmer wall is less likely to condense moisture. 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 fiberglass insulation does not insulate as well as foam but provides a drainage plane. 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.

• If placed on the interior, wall insulation must extend down the wall to a depth at least two feet below grade level. Polyisocyanurate insulation with an aluminum facing is a good interior insulation choice. If the crawlspace wall extends less than two feet below grade level, then the remaining insulation must be placed horizontally along the ground at the base of the wall. A sealed ground cover is installed over the entire area of the crawlspace. Install a system to provide conditioned air to the crawlspace.

More information is available at Building Energy Codes Resource Center, Details for Mechanically Vented Crawlspaces - Code Notes (includes information on sizing the mechanical ventilation), and at Advanced Energy's Web site about crawlspaces.

Basement Insulation

Wall insulation in basements is similar to the approaches described for crawlspaces. And basement floors are insulated in ways similar to slabs. 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 fiberglass 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 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.

• 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 in 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 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 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.