Even though many foam insulation products are more expensive than other types of insulating materials such as fiberglass, cellulose, etc., they are commonly used in buildings where there are space limitations or where very high R-values are desirable. Foam insulation R-values range from R-4 to R-8 per inch of thickness (2.54 cm), which is two to three times greater than most other insulating materials of the same thickness. Also, if the installation is carefully installed, foam insulation may control air infiltration more effectively than other types of insulation.
Several variables affect the installed R-value of foam insulation, including the initial density of the foam, the blowing gas used (CFC, HCFC, CO2, air, or a number of other gases), how the foam insulation is handled (dents and chips adversely effect the R-value), the type of facing (if any) used, and the conditions in which the foam is installed.
Foam insulation is often made with one of three materials: molded expanded polystyrene (MEPS), extruded expanded polystyrene (XEPS) or polyurethane, polyisocyanurate, or a related chemical mixture. Some are installed as a liquid while other types come as factory-made panels or "foam board." The following information will go over all of these aspects of foam insulation in detail.
Liquid foam insulation can be applied from small spray containers as a liquid or in larger quantities as a pressure-sprayed (foamed-in-place) product. Both types expand and harden as the chemical mixture cures. It also conforms to the shape of the cavity to fill and seal it thoroughly. This type is often used in new construction.
There are also slow-curing liquid foams that are designed to flow over obstructions before it expands and cures. This type is often used for empty wall cavities in existing buildings. There are also liquid foam materials that are poured from a container.
Both are generally urethane foams. Latex and organic based foams are available, too. Be aware that these alternatives do not have as high an R-value as a urethane-based products.
Molded Expanded Polystyrene (MEPS) Foam Board
MEPS is a closed-cell material that can be molded into many everyday items, such as coffee cups and shipping materials, or into large sheets as construction insulation. This material is commonly known as "beadboard," and it has an R-value of about four per inch of thickness (2.54 cm).
To make beadboard, loose, unexpanded polystyrene beads containing liquid pentane are mixed with a blowing agent and poured into an enclosed container. The mixture is then heated to expand the beads to many times their original size. The beads are then injected into a mold and under more heat and pressure expand to become foam blocks that are then shaped as required.
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). Beadboard is manufactured at various densities, depending on the application. Beadboard for roofing materials has to be dense enough to walk on without damage. Wall insulation boards are several times less dense than roof boards because they don't have to hold the same weight. R-values range from 3.8 to 4.4 per inch (2.54 cm) of thickness. Since spaces between the foam beads can absorb water, a vapor diffusion retarder is necessary if water transmission through the insulation might present a problem for the user.
MEPS foams are also available as small beads of foam, too. This type is often used as a pouring insulation for concrete blocks or other hollow wall cavities. However, be aware that poured beads are extremely lightweight and take a static electric charge very easily. They are notoriously difficult to control and any wind at all often results in the beads flying all over the place. Also, if there is ever a hole in the wall the foam beads will continue to fall out of the hole until the wall is almost empty of beads.
Extruded Expanded Polystyrene (XEPS) Foam Board
Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation similar to MEPS. To make it, the polystyrene pellets are mixed with various chemicals to liquefy them. A blowing agent is then injected into the mixture, forming gas bubbles. The foaming, thick liquid is then forced through a shaping die. When cooled, the panel is cut as required. Foam densities are typically 1.5 pounds per cubic foot (0.21 kilograms per cubic meter).
XEPS is more expensive than MEPS, and like MEPS the R-value depends upon the density of the material. Generally, it's about R-5 per inch. It is also much more consistent in density and has a higher compressive strength than MEPS, making it better suited for use on roofs or for wall panels. Extruded polystyrene also has excellent resistance to moisture absorption.
Both MEPS and XEPS are often used as the insulation for Structural Insulating Panels (SIPs) and as Insulating Concrete Forms (ICFs).
Polyurethane and Polyisocyanurate
Polyurethane and polyisocyanurate are both closed-cell foams that contain a low-conductivity gas in the cells (usually one of the HCFC or CFC gases). The high thermal resistance of the gas gives these foams an R-value of between R-7 and R-8 per inch.
Both types are available as a liquid spray, poured foam, and also as rigid boards. They can also be made into laminated panels with a variety of facings. Foamed-in-place applications are usually cheaper than installing foam boards and perform better since it molds itself to all of the surfaces perfectly. However, be sure you use a contractor with plenty of experience with spray foam installations.
Over time, the R-value of the foam drops as some of the gas escapes and air replaces it. This phenomenon is known as thermal drift. When manufactured, the initial R-value is roughly R-9 per inch. Experimental data on this type of foam indicates that most thermal drift occurs within the first two years after manufacture and slowly decreases until it stabilizes at about R-7 per inch. It then remains unchanged unless the foam is damaged.
Foil and plastic facings on these foam panels help to slow the escape of gas from the cell structure. Testing suggests that the stabilized R-value of rigid foam with metal foil facings remains unchanged after 10 years. The reflective foil, if installed correctly, can also act as a radiant barrier (another type of insulation) that adds about R-2 to the insulating assembly. Panels with foil facings have stabilized R-values of 7.1 to 8.7 per inch.
Common Applications of Foam Insulation
Spray foam and foam boards can be used to insulate almost anything including roofs, walls, foundations, entry and overhead garage doors, pipes and tanks, under basement slabs, or over a slab-on-grade floor. Foam insulation sprayed or placed in wall and floor cavities both insulates and offers some degree of soundproofing.
Protect all types of foam insulation from direct sunlight. Over time, the sun's ultraviolet rays can damage them. For roofs, this is generally done by applying a coating such as tar, acrylic, silicone or rubberized paint. You can also cover the foam with a rubber or plastic membrane or a layer of asphalt and roofing felt. Make certain you are using compatible products, as the solvents in some coatings dissolve certain plastics.
There are several ways to incorporate foam insulation in concrete or masonry walls: pouring loose foam beads into masonry blocks, injecting/pouring liquid foam into the hollow block cores, manufacturing concrete blocks to accommodate rigid foam inserts, as lightweight concrete blocks that have polystyrene beads in the concrete mixture, and as rigid foam insulation inside a cast-in-place wall. There are also interlocking rigid foam panels and blocks that serve as permanent forms for concrete walls and foundations. These are commonly known as Insulating Concrete Forms (ICFs).
Potential Moisture Problems
In cold weather, warm, interior air containing water vapor can get past the wall finish and insulation and condense inside the colder wall cavity. In hot, humid climates the same thing can happen, just in the reverse direction with humid outdoor air in the summer condensing inside cool or air conditioned wall cavities. If enough of this happens, and the water cannot escape, wood rot, mold, and other moisture-related problems are likely to occur. For this reason, building codes often require installing a vapor diffusion retarder on the warmest side of the wall cavity.
Foam board insulation is commonly placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulk. However, this practice may worry some builders in cold climates since the foam board may act as a second vapor diffusion retarder. Studies have shown, however, that condensation rarely occurs in these areas unless something else is seriously wrong with the wall assembly (i.e., massive uncontrolled air leakage into the walls from the house). If the assembly is constructed correctly, the inside surface of the foam board stays warm enough to keep water vapor in its gaseous state long enough for it to escape.
When insulating a foundation, foam board provides the potential for easy insect tunneling, although foam insulation offers no actual food value. Insect burrows reduce the R-value and structural integrity of the insulation. For these reasons, some manufacturers treat their foam products with an insecticide, usually a borate compound. Many building jurisdictions also mandate treating the earth around the building with insecticides and keeping an area bare of insulation board, several inches wide, and all the way around the foundation of a house as an inspection area.
A better solution for below-grade walls in need of insulation is to install the foam board over the interior of the basement walls rather than on the exterior as is more commonly done. Interior applications prevent ground-dwelling insects from finding the foam board at all, and it eliminates the need for the inspection area where no insulation is allowed. However, most jurisdictions require installing a fire-barrier over the foam board. While this adds extra cost, the thermal performance of this method is superior in most cases to the more common foam board application to the exterior of the foundation. This equates with a dollar savings in energy that can repay you many times over for the additional cost that an interior application requires. Also, if you plan to convert a basement into a living space, there is almost no additional cost.
Foam insulation is relatively hard to ignite but when ignited, it burns readily and emits a dense, black smoke containing many toxic gases. The combustion characteristics of foam insulation products vary with the combustion temperatures, chemical formulation, and available air.
Because of the dangers described above, foams used for construction require a covering as a fire barrier. One half-inch thick (1.27 cm) gypsum wallboard is one of the most common fire barriers. Some building codes, however, do not require an additional fire barrier for certain metal-faced laminated foam products. Check with your local building code/fire officials, and insurers for specific information on what is permitted in your area.
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