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Foam and Foam Board Insulation

Many variables affect the final R-value of foam insulation: the initial density of the foam; the blowing gas used (CFC, HCFC, CO₂, air, or a number of other gases); how the panels are handled (dents and chips in the panel adversely effect the R-value); the type of facing (if any) used, and the conditions under which the panels are installed.

There are several methods of installing foam insulation. You can spray it as a chemical mixture from a small aerosol container, or apply it in bulk from a pressure sprayer. As the chemical mixture cures, it fills and seals odd-shaped spaces and wall stud cavities. You can also apply the foam as a liquid that flows over obstructions before it expands and cures. Finally, you can pour solidified plastic foam as loose beads into concrete blocks or install it as rigid panels that can be cut to size.

Foam insulation is an alternative if there is insufficient space to install non-foam products, or when an equal R-value of non-foam insulation would require structural changes to install it. Although many insulation products, such as fiberglass and cellulose, cost less per given thickness, they require more material to achieve the same R-value per inch as foam. Foam insulation also controls air infiltration more effectively than most other forms of insulation.

Foam insulations are made with one of three materials: molded expanded polystyrene (MEPS), extruded expanded polystyrene (XEPS), or one of several types of urethane, either polyurethane, polyisocyanurate, or a related chemical mixture. These materials are described below.

Molded Expanded Polystyrene (MEPS) Boards

Expanded polystyrene starts out as a small bead of foam. When formed into boards, the MEPS beads become beadboard. This is the closed-cell bead foam molded into many everyday items, such as coffee cups, picnic ice chests, and shipping materials.

To make beadboard, loose, unexpanded foam beads containing liquid pentane are mixed with a blowing agent and poured into a mold. A controlled amount of heat (200°F, 93.3°C) expands the beads to 30 times their original size. The beads are then injected into a vacuum mold and, under more heat and pressure, further expand and become blocks (about 4 x 16 feet x 30 inches tall [1.2 x 5 meters x .76 meters tall]). After curing, the blocks are cut into the required sizes. The physical properties of MEPS vary with the type of bead used, but the density of the board is usually one pound per cubic foot (16.3 kilograms per cubic meter.)

The R-value of beadboard depends on its density. Beadboard is manufactured at various densities, which depend on how it is used. Beadboards used as roofing materials have to be dense enough to walk on without damage. Wall insulation boards are several times less dense than roof boards. R-values range from 3.8 to 4.4 per inch (2.54 centimeters [cm]) of thickness. Since beadboard absorbs water vapor, you should install a vapor diffusion retarder. (Vapor diffusion retarders are materials that inhibit the movement of water vapor, such as polyethylene sheets.) Thin beadboard warps and chips easily, so you should not use it for high use applications, such as moveable window insulation.

Extruded Expanded Polystyrene (XEPS) Boards

Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation similar to MEPS. The raw materials are mixed with various chemicals to liquefy them. A pressurized blowing agent is injected into the mixture, forming gas bubbles. The foaming, thick liquid is forced through a shaping die. When cooled, the panel is cut as required. Foam densities are typically 1.5 pounds per cubic foot (.21 kilograms per cubic meter).

Extruded expanded polystyrene is more expensive than MEPS but has a higher R-value (about R-5 per inch [2.54 cm]). It is also much more consistent in density. XEPS has a higher compressive strength than MEPS, making it especially suitable for use on roofs or as facing for interior or exterior basement walls. Extruded polystyrene has a low permeability to water vapor and excellent resistance to moisture absorption.

Like MEPS, the R-value of XEPS depends upon the density of the material. Both types of polystyrene insulate concrete slabs, foundation walls, exterior walls for sheathing, and certain kinds of built-up roof assemblies.

Urethanes: Polyurethane and Polyisocyanurate

Polyurethane and polyisocyanurate foams are similar. Both are closed-cell foams that contain a low-conductivity gas in the cells. The high thermal resistance of the gas gives these foams an R-value of between R-7 and R-8 per inch (2.54 cm). Two advantages of polyisocyanurate over polyurethane are that it is more fire-resistant and has a slightly higher R-value.

Polyurethane and polyisocyanurate foams can be formed into rigid boards. They can also be made into laminated panels with a variety of facings, including metal foil, plastic, paper, or plywood. The foams are also sprayed or poured, depending on the type of chemical mixture used. Foamed-in-place applications are usually cheaper than installing foam boards. Be sure you use a contractor with plenty of experience with spray foam installations.

Like MEPS and to a lesser extent XEPS, polyurethane and polyisocyanurate board and spray foam absorb moisture, reducing their R-value.

Compared to other types of foam insulation, polyurethane and polyisocyanurate foam insulation have high R-values. When the foam cures during the manufacturing process, it traps small bubbles of gas (usually one of the HCFC or CFC gases, which have twice the R-value of air).

Over time, however, the R-value of the foam drops as the gas slowly escapes and ambient air replaces it. This phenomenon is known as thermal drift. When manufactured, the R-value of polyurethane foam is roughly R-9 per inch (2.54 cm). This value decreases until it stabilizes at about R-7 per inch (2.54 cm). Experimental data on polyurethane foams indicate that most thermal drift occurs within the first two years after manufacture.

Foil and plastic sheet facings on foam boards inhibit escape of gas from the cell structure. Laboratory and field testing data suggest that the stabilized R-value of rigid foam with metal foil facings remains unchanged after 10 years. Some rigid polyurethane and polyisocyanurate foam insulations have a carbon black coating within its cell structure that increases its R-value. This is due in part to its reflective qualities. Panels with foil facings also have higher stabilized R-values of 7.1 to 8.7 per inch (2.54 cm) than similar products.

Common Applications of Foam Insulation

Spray foam and foam boards insulate roofs, curtain walls, foundations, entry and overhead garage doors, pipes and tanks, and climate-controlled storage units. They also serve as sheathing and insulation on the inside surface of a basement or crawl space wall, beneath a basement slab, or slab-on-grade floor. Exterior applications are inside or outside foundations, crawlspaces, and slab-on-grade foundation walls. Sometimes foam insulation is installed horizontally beneath the foundation. Foam insulation sprayed or placed in wall and floor cavities both insulates and soundproofs.

Since most plastics degrade over time when exposed to the sun's ultraviolet rays, you should protect all these products from the elements. In roof applications, add protection by applying various liquid spray coatings, such as acrylics, silicones, rubbers, and a variety of other roofing compounds. You can also cover the foam with rubber/plastic membranes or layers of asphalt and roofing felt.

There are several ways to incorporate foam insulation in concrete or masonry walls: pouring loose foam beads into masonry block; manufacturing concrete blocks to accommodate rigid foam inserts; placing rigid foam insulation inside a cast-in-place wall, and using interlocking rigid foam blocks as permanent forms for concrete foundations. Some contractors may also use lightweight or insulating concrete blocks that have polystyrene beads in the concrete mixture.

Potential Moisture Problems

In cold weather, warm inside air containing water vapor can pass through the wallboard and wall insulation and condense on the cold exterior wall surface. If this condensation cannot escape, moisture-related problems occur. For this reason, building codes often require installing a vapor diffusion retarder on the heated side of the wall cavity.

In some areas of the United States, foam insulation boards are placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. This worries some builders in case foam insulation board applied in this way acts as a second vapor diffusion retarder, preventing indoor moisture that may enter the walls (through small defects in the wall finish) from escaping to the outside. Studies have shown, however, that condensation rarely occurs in any significant amount. In most areas of the country, the inside surface of the foam board stays warm enough to keep the water vapor in its gaseous state long enough for it to escape.

To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulking. If some wall finish areas show signs of moisture problems (i.e., peeling paint, warped wood, and stained or moldy walls), try to locate the source of the moisture or drill small holes in the insulation boards around these areas to vent the wall at the trouble spots.

Insect Problems

Although foam insulation offers no food value to insects, it provides protective cover and easy tunneling for termites, carpenter ants, and other pests. Insect tunnels in foam insulation reduce the R-value and structural integrity of the insulation. Exterior foundation insulation is particularly vulnerable to insect infiltration. Some foam board manufacturers treat their products with an insecticide or issue guidelines to help prevent infestation. Many building jurisdictions also mandate treating the earth around the structure with insecticides. Although the thermal performance of the wall is better with the foam board on the exterior of the wall, interior applications of insulation prevent ground-dwelling insect infestation in areas where it is a problem.

Fire Protection

Most types of foam insulation are relatively hard to ignite but when ignited, they emit a dense, black, toxic smoke containing, in some cases, deadly hydrogen cyanide gas. The combustibility characteristics of all foams vary according to the combustion temperatures, as well as different chemical formulations and other variations used in manufacturing.

Because of the dangers described above, most foams used in interior building construction require an adequate fire barrier. Half-inch (1.27 cm) gypsum wallboard is one of the most common fire barriers. Foam boards and wall board may also be treated with a fire retardant. Some building codes, however, do not require a fire barrier for certain metal-faced laminated foam products. Check with your local building code, fire officials, and insurers for each specific application, design, and building occupancy. You may also wish to consult the manufacturers' specifications and installation instructions.

EREC is operated by NCI Information Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of Energy. The statements contained herein are based on information known to EREC at the time of printing. No recommendations or endorsement of any product or service is implied if mentioned by EREC.

Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com

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