Urethane foams can make a major contribution to improving the energy efficiency of buildings when they are used as an air leakage control material or as a component of an air barrier system. Early test programs conducted in Canada showed that urethane foam can reduce total air leakage in existing homes by 30%-40%.

As an air barrier, urethane foams

• Offer continuity throughout the building envelope.
• May be fastened to supporting structures (self-adhesive).
• Resist peak wind loads, sustained stack effect, and pressurization from ventilation equipment.
• *Provide virtual air impermeability (see Table 1).
• Offer durability and long service life.

Rigid urethane foams are available in one-component (single) and two-component formulations. Both formulations contain two main ingredients, polyisocyanate and polyglycol. In single-component urethane, these are stored together as a blended mix, accompanied by a blowing agent in liquid form, and catalyzed to cure when exposed to moisture in the air. On release from their pressurized container, the two main ingredients react chemically, and the heat from this reaction causes the blowing agent to convert into a gas and expand to form the cellular structure of the foam. When the reaction is complete, the gas is trapped within the material.

In two-component urethane, the same two main ingredients and appropriate catalysts are kept apart until application. The chemical reaction when they are combined is much faster than with one-component foam. Curing is chemical, requires no air or moisture, and is independent of the surrounding environment.

Single-component urethanes are dispensed as a bead for gap and crack filling, while two-component urethanes are dispensed as a spray or a stream to fill larger holes and voids. A "gap" is generally between 0.25 in and 3 in wide, and a "hole" is anything larger. A "crack" is less than .25 in wide, and may be sealed with one-component foam or caulk. Caulk is generally used in exposed areas where appearance is critical, but foam can also be trimmed and painted.

Table 2 shows which foam to use where. If either one- or two-component foam is suitable, both are checked. In crawlspaces, a two-component kit is convenient for both insulating and sealing because use of machines in such confined spaces is messy and uneconomical.

One sealable area often missed is the stud space above a cold air return. It must be sealed because the cold-air return system will not work properly if it is not a closed loop. Builders often omit the wood blocking between the studs and above the grille. It is difficult to use one-component foam here, so a two-component kit is recommended. These kits are also useful in attic spaces where restricted movement and awkward access make locations difficult to seal. Masonry block party walls, which form "chimneys" because of their porosity and open cores, can only be sealed effectively with two-component foam. Another important use for the two-component kit is sealing between the stud spaces at the perimeters of drop ceilings.

Urethane foams are also used as a retrofit measure to reduce sound penetration and odor transfer; to reduce ingress of dust, dirt, insects, and airborne pests; to reduce condensation and other moisture-related problems; and to decrease relative humidity levels.

One-component foam is an insulating sealant consisting of a single mix of chemicals in one pressurized can or tank and formulated so that it will cure when exposed to the moisture present in air. The material mix is ejected from the container as a sticky bead. Because the foam needs moisture to cure, care must be taken to avoid problem areas such as confined, nonporous cavities; which may inhibit cure. Sometimes, to facilitate cure, a misting device may be used to spray a cavity and/or the foam itself. If the cavity is too big, two applications may be needed to make sure there is enough moisture available to cure the foam. While air sealing is the primary use for these materials, they also have insulating capability through R-values (aged) in the 3.6 to 3.9 per inch range.

Fast-cure systems are available, which may be tack free in just ten minutes. However, shelf life may be as little as six months.

One-component urethane foams cost one-tenth as much as the cheapest caulk available, based on a 0.5-inch bead-per-linear-foot comparison that we recently conducted. Packages of one-component urethane sold to consumers still come with a plastic screw-on cap and trigger that allow the can to be used as an aerosol. These products cure in one to two hours and have a shelf life of up to two years.

This is an inefficient delivery system, however. Since the mid-1980s, a gun with a self-purging nozzle has been available from Switzerland. The guns typically start at $100. They have become a tool of choice for those who apply foam for a living. The gun must be kept on the can at all times so that air cannot get into the barrel. If this is not done, the gun will gum up and be very difficult to clean. The popularity of guns has led to the production of inexpensive, disposable models.

In addition to cans, single-component urethane foam can be obtained in 10- and 16-lb cylinders, complete with a hose and hand-held valve assembly.

As the name suggests, two-component urethane foams come in separate containers, one for each component. Bulk two-component foams are expelled through special equipment and guns, while portable kits have special guns with a clip-in nozzle where the chemicals mix and react. Depending on the formulation, this type of foam can cure in as little as 3 seconds. Foams from portable kits generally cure in thirty to forty seconds. Fast chemical curing results in a higher expansion ratio for two-component foams. This makes them suitable for spray-on applications and for filling larger holes and cavities. They also have an R-value (aged) of 6.0 per inch.

For air sealing, the two-component urethane foams are usually used in kit form with densities ranging from 1.0 to 1.75 lb per ft³. There are different sizes for a wide variety of applications. Large refillable cylinders are also available. Portable kits are generally packaged with their own gun and hose assemblies. And because nozzles tend to plug with reacted foam if not used continuously, kits usually come with a supply of spares. Various nozzles are available for spray, pour, and caulking applications.

Large-scale spray applications call for sophisticated equipment complete with a self-purging gun capable of various outputs. Such equipment includes heated hoses, a control system, and large drums for chemical supply (available from three or four different manufacturers). In new construction, bulk sprayed foam is increasingly being used as an insulating air barrier system in cavity wall designs.

Training is essential for applicators using machine-applied two-component foam. The U.S. Society of Plastics Industry and Canada's Urethane Foam Contractors Association have both developed training and quality control programs to improve skills and ensure uniform quality of installed product. We advise training users of both two-component kits and single-component foam with guns.

The window industry is particularly sensitive to the quality of foam application, because of concerns about size distortion of the window opening. In Canada, foam is applied to most windows when they are installed, while in the United States acceptance has been slower because of the distortion problems. Installation of too much foam in large gaps by inexperienced applicators can warp vinyl or insubstantial metal frames. Quality assurance programs are being developed to overcome installation problems.

Gloves, protective eyewear, and long-sleeved clothing should always be used when applying any kind of foam. Uncured urethane foam should be cleaned immediately with paint thinner, acetone or nail polish remover. Cured foam cannot be removed with solvents. If foam hardens on skin, the affected area must be soaked in warm, soapy water; then a layer of petroleum jelly should be applied. The foam will eventually wear away.

When using one-component urethane from a can, always dispense the foam with the can upside down. Otherwise the propellant will escape, making the can unusable.

If using one-component foam to seal below the baseboard in a house with fitted carpets, simply roll back the carpet and inject a fast-curing foam between the baseboard and the subfloor. Wait until the foam is tack-free (about ten minutes) and then roll the carpet back into position, using the back of it to compress the foam bead into the gap.

Additional precautions should be taken when applying two-component foams. Fumes given off by the chemical reaction necessitate good ventilation within the work area. If a great deal of foam is being used, it may be necessary to wear masks and to provide a separate fresh-air supply during installation.

In cold weather, if using two-component foam in unheated areas, ensure that the chemicals are kept warm. With kits, they can be kept in an insulated box heated by a light bulb. Bulk foams specially formulated for use in cold weather are also available.

Be very careful about overspraying two-component urethane, because it sticks to everything. Be sure to protect all surrounding areas. Most two-component foams are ejected so fast that first-time users tend to use too much. Experiment on a small area first.

In some spray applications for two-component urethane foams, superior fire resistance is necessary. Systems with flame spread ratings below 25 are available for this purpose.

Although two-component urethane sets very quickly, its full bond strength is not achieved until two to three hours after application. Do not disturb the foam during this time, since it may become detached from the substrate. The full bond strength will actually be higher than the tensile strength of the foam itself. Tensile strength of fully cured 1.75-lb density foam is about 30 lb per in².

Finally, when injecting foam into hidden cavities, always ensure that there is a breather hole to allow trapped air and excess material to escape.

Foams are flammable, so care is essential during both application and normal use. Codes in the United States are not concerned with the flammability of sealants. The Canadian Building Code allows the use of combustible sealants in noncombustible construction, and foams used as sealants are considered "acceptable minor components."

Canada will soon introduce new standards of permeability for component materials in an air barrier system. Materials will have to have a permeability rating of less than 0.2 litres per second per square meter at 75 Pascals of pressure (see Table 2).

Canada's Saskatchewan Research Council tested for off-gassing of toxic gases during and after application of two-component foams. Results confirmed that they are not a source of volatile organic compounds and do not decompose like urea formaldehyde foam insulation.

To meet environmental requirements, chlorofluorocarbons (CFCs), which were traditionally used as blowing agents for one-component foams, have now been discontinued. They have been replaced by hydrochlorofluorocarbons (HCFCs), which have an ozone-depleting potential 97% lower than that of CFCs. Even these are now being slowly replaced with hydrofluorocarbons (HFCs), which contain no chlorine. Hydrocarbon (HC)-blown foams now available also do not damage the upper ozone layer.

Bob Braun is research and development manager and Janet Hansen is marketing manager of InstaFoam Products Incorporated, a division of Flexible Products Company of Joliet, Illinois. Tony Woods is president of CanAm Building Envelope Specialists Incorporated in Mississauga, Ontario, Canada, and is also a consultant to InstaFoam.