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SWS Detail: 4.1003.7 Ignition and Thermal Barriers—Spray Polyurethane Foam |
The purpose of insulation is to provide thermal resistance that reduces the rate of heat transmission through building assemblies. Characteristics such as R-value per inch, density, fire safety, vapor permeability, and airflow resistance help weatherization specialists choose the right insulation for the job.
Fibrous insulation materials are the most economical insulations for buildings. If blown at a high density, fibrous insulations aren’t air barriers themselves, but they may contribute to the airflow resistance of a building assembly that functions as an air barrier. The term mineral wool describes both fiberglass and rock wool. Rock wool is both a generic term and a trade name. We use rock wool in the generic sense as an insulating wool spun from rocks or slag. Fiberglass is wool spun from molten glass.
Cellulose was once made from virgin wood fiber under trade names like Balsam Wool. Now cellulose is manufactured primarily from recycled paper, treated with a fire retardant.
A vapor permeable air barrier should cover fibrous insulation installed vertically or horizontally in human-contact areas to limit exposure to fibers, which may cause respiratory distress.
Most fiberglass batts are either 15 inches wide or 23 inches wide to fit 16-inch or 24-inch spacing for wood studs or joists. However, manufacturers also provide 16-inch or 24-inch widths for metal stud construction.
The advertised R-values of batts vary from 3.1 per inch to 4.2 per inch depending on density. Installed fiberglass R-values may be 5% to 30% less than advertised depending on installation quality and the effectiveness of the assembly’s air barrier.
Installers must cut and fit batts very carefully. Batts achieve their advertised R-value only when they are touching all six sides of the cavity it inhabits.
See “Open-Cavity Wall Insulation” on page 173.
Fiberglass blankets are typically three to six feet wide. Blankets come in a variety of thicknesses from 1 to 6 inches. Fiberglass blankets are used to insulate metal buildings, to insulate crawl spaces from the inside, and to insulate water heaters.
Although fiberglass doesn’t absorb much moisture, the facings on blankets and batts can trap water in the batts, which can dampen building materials and provide a water source for pests.
Insulation manufacturers make batts and blankets with a number of facings, including the following.
• Unfaced: Vapor permeable and Class-A fire rating of ≤25 flame spread.
• Kraft paper: A Class II vapor retarder that is flammable (Class-C or Class 3) with a flame spread ≥150.
• Foil-kraft: foil bonded to kraft paper. A vapor barrier with a flame spread of >75 (Class-C or Class 3).
• Foil-skrim-kraft (FSK): Aluminum foil bonded to kraft paper with skrim netting in-between as reinforcement. A vapor barrier available as a Class-A material with a flame spread of ≤25.
• White poly-skrim-kraft (PSK): White polyvinylchloride bonded to kraft paper with skrim netting in-between as reinforcement. A vapor barrier available as a Class A fire-rated material with a flame spread of ≤25. The white surface maximizes light reflection.
Loose fiberglass is blown in attics from 0.4 to 0.6 pcf and at that density the R-value is around 3.2 per inch. Blown fiberglass is non-combustible as a virgin product. However, some blown fiberglass is made from chopped batt waste that contains a small amount of combustible binder.
Loose fiberglass is blown in attics from 0.3 to 0.8 pcf and at that density range, the R-value is around 2.9 per inch. Expect around 5% settling within five years after installation.
Fiberglass manufacturers now provide two blowing products, one for standard densities of up to about 1.4 pcf, and another for dense-packing to more than 2.0 pcf.
In closed cavities, installers blow fiberglass from 1.2 to 2.2 pcf, with the R-value per inch varying from 3.6 to 4.2. Higher density achieves a higher R-value. The high-density fiberglass is typically reserved for walls where the superior resistance to settling, airflow, and convection has extra value over lesser density.
Cellulose is the most inexpensive insulation and among the easiest insulations to install. Loose cellulose is blown in attics from 0.6 to 1.2 pcf and at that density range, the R-value is around 3.7 per inch. Expect around 15% settling within five years after installation.
In wall cavities, cellulose is blown at a higher density of between 3.5 to 4.0 pcf, to prevent settling and to maximize its airflow resistance. At that high density, cellulose’s R-value per inch is around 3.4. Evaluate the strength of wall cladding before blowing a wall with cellulose to prevent damage during installation.
Cellulose absorbs up to 130% of its own weight in water. Before anyone discovers a moisture problem, the cellulose could be soaked, shrunken, double its dry weight, and far less thermally resistant. Avoid using cellulose in regions with an annual average precipitation of more than 50 inches or an annual average relative humidity of more than 70%. We believe that cellulose shouldn’t be installed in the following places regardless of climate.
• Horizontal or sloped closed roof cavities
• Floor cavities above crawl spaces or unconditioned basements
• Crawl space walls or basement walls
Rock wool is a type of mineral wool like fiberglass. Rock wool has a small market share in North America. Rock wool batts have similar R-values per inch as fiberglass batts and contain flammable binders. Rock wool itself is non-combustible so blown rock wool doesn’t burn.
Rock wool is also the most moisture-resistant insulation discussed here. In rainy and humid climates, rock wool is the least likely insulation to harbor moisture or support pests.
Installers mix fibrous insulation with sprayed water and a small amount of adhesive in damp-spray applications either in open cavities or directly adhered to building surfaces. Sprayed cellulose contains a non-corrosive fire retardant to prevent metal corrosion when used in contact with metal building components.
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SWS Detail: 4.1001.5 Dense Pack Preparation, 4.1088.7 Insulating Inaccessible Attics |
Perform these important steps before and during insulation-blowing.
ü Verify that the electrical source can provide the ampere draw of the insulation machine.
ü Measure the pressure created by a blowing machine by connecting the hose to a fitting attached to a manometer. Close the feed gate and turn the air to the highest setting. For cellulose, the blowing machine should develop 2.9 pounds per square inch (psi) or 80 inches of water (IWC) For other types of fibrous insulation, check manufacturer specifications for blowing machine set up.
ü Verify that you’re blowing the correct density of fibrous insulation by using the bag’s weight or the manufacturer’s coverage tables.
Important Note: Dense-packed fibrous insulation can reduce air leakage and convection in closed building cavities. However, don’t use dense-packed fibrous as a substitute for the air-sealing techniques described throughout this guide.
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SWS Details: 4.1001.6 Unvented Roof Deck—Preparation for Spray Polyurethane Foam, 4.1001.7 Vented Roof Deck—Preparation for SPF, 2.0100.3 Global Worker Safety |
Spray Polyurethane Foam (SPF) is combustible and creates toxic smoke. Foam insulation usually requires covering with a thermal barrier or an ignition barrier, discussed in“Fire Protection for Foam Insulation” on page 103. SPF comes in two formulations: closed-cell and open-cell. Both are described below.
Spray foam is an insect-friendly material that can aid termites and carpenter ants in establishing a colony in wood structures. Mitigate all sources of ground water before installing foam near a foundation. When foam is installed on the outside of foundations, the surrounding soil should be treated with a termiticide if necessary. Inside a crawl space, foam must never provide a direct link from the ground to wood materials. The International Residential Code (IRC) forbids foam below grade in “very heavy” termite-colonized areas.
Caution: SPF is hazardous to installers. Installers must wear special personal protective equipment and ventilate spaces during installation. SPF can also harm occupants who breathe the toxic vapors during installation. SPF requires precise mixing of the two components at specific temperature ranges. Improperly mixed or installed spray foam can emit vapors for months or years resulting in long-term respiratory hazards.
Closed-cell polyurethane spray foam (SPF) is an air barrier and a vapor barrier and is the most expensive insulation discussed here. Closed-cell SPF is a good value when space is limited, where an air or vapor barrier is needed, or where its structural strength and durability are needed.
Spray foam professionals install closed-cell SPF from two 55-gallon containers through hoses and a nozzle that mix the material. The closed-cell foam installs at approximately 2 pcf density and achieves an R-value of 6 or more per inch. However, roofing applications call for a density near 3 pcf.
Closed-cell polyurethane foam is also packaged in smaller containers in the following products.
• One-part high-expanding foam for air sealing.
• One-part low-expanding foam for air sealing.
• Two-part high-expanding foam for air sealing and insulation of surfaces.
Polyurethane open-cell foam is installed at between 0.5 pcf to 1.0 pcf and achieves an R-value of around 3.7 per inch to 4.7 per inch depending on density.
These open-cell formulations are injected into a hole, one inch or smaller, through an injection nozzle and not a fill tube. (The plastic fill tube would clog and isn’t cleanable.) The open-cell foam can subject a wall cavity to some pressure, so evaluate wall-cladding strength before injecting it.
Open-cell foam can absorb both water vapor and liquid water. Open-cell foam can hold moisture and become a medium for mold growth. We recommend that contractors don’t install low-density spray foam in the following locations.
• Underside of roof decking
• Underside of floor decking above crawl spaces
• Crawl space walls
Lesson: Applying 2- and 1-Part Spray Foam
In this video from WxTV, you’ll see some application details for both 2-part foam canister kits, and 1-part canned foam. Click here to see the video.
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SWS Detail: 4.1003.7 Ignition and Thermal Barriers—Spray Polyurethane Foam |
Plastic foam is the generic term used by the IRC for both rigid and spray foams. Plastic foams are combustible, and create toxic smoke when they burn.
The following fire-safety and durability issues are particularly important to installing foam insulation.
• Foam insulation requires a thermal barrier covering of at least half-inch drywall when installed in a living space.
• Foam may require an ignition barrier when installed in attics or crawl spaces or it may not.
• A thermal barrier is a material, usually drywall, that protects combustible materials behind it from heat and flame creating a fire.
• An ignition barrier is designed to delay the ignition of the material it protects. Ignition barriers include plywood, galvanized steel, damp-spray fiberglass, and intumescent paint. Intumescent paint is a proprietary latex coating designed to delay the ignition of foam insulation in a fire.
The IRC requires a thermal barrier (half-inch drywall) for spray foam in all living areas and storage areas. Instead of a thermal barrier, installers may use an ignition barrier (1.5 inches of fibrous insulation or intumescent paint) to cover foam in attics and crawl spaces that aren’t used for storage.
Fire protection requirements vary among foam formulations, according to the amount and type of fire retardant. Foam insulations generally fit into one of two classifications.
• Class I or Class A; ≤25 flame spread
• Class II or Class B; flame spread 26-75
If a foam product has a flame spread of 25 or less (Class I), it may require no thermal barrier or ignition barrier. If a foam product has a flame spread of more than 25, further testing may qualify it for exemption from the thermal barrier or ignition barrier requirements of the IRC.
Code jurisdictions and individual building officials vary in their interpretation of the IRC depending on these three factors.
1. Foam manufacturer’s fire-testing reports.
2. The possibility that residents might use an attic or unoccupied basement for storage or even living space.
3. The possibility that no one may ever enter the space again except for maintenance.
Foam board is combustible and creates toxic smoke if it burns. Foam insulation usually requires covering with a thermal barrier or an ignition barrier, discussed in“Fire Protection for Foam Insulation” on page 103.
Foam board, although not an insect food, is an insect-friendly material that can aid termites in establishing colonies in wood structures. Mitigate all sources of ground water before installing foam near a foundation. When foam is installed on the outside of foundations, the surrounding soil should be treated with a termiticide. Inside a crawl space, foam must never provide a direct link from the ground to wood materials where termites or carpenter ants are common. The IRC forbids foam below grade in “very heavy” termite-colonized regions; the foam must be kept 6 inches above grade.
EPS foam board, sometimes called beadboard, is the most inexpensive of the foam insulations. EPS varies in density from 1 to 2 pcf with R-values per inch of 3.9 to 4.7, increasing with increasing density. EPS is packaged in a wide variety of products by local manufacturers. Products include structural insulated panels (SIPS), tapered flat-roof insulation, EPS bonded to drywall, and EPS embedded with fastening strips.
EPS is flammable and produces toxic smoke when burned. It has a low maximum operating temperature (160 degrees F) that is a concern for using EPS under dark-colored roofing or siding. EPS has shrunken in some installations.
EPS is very moisture resistant and its vapor permeability is similar to masonry materials, which makes EPS a good insulation for masonry walls.
Dense EPS (2 pcf) is appropriate for use on flat roofs and below grade. Dense EPS is also more dimensionally stable and less likely to shrink. Use weatherproof coverings to prevent degradation by ultraviolet light and freezing and thawing at ground level.
XPS is produced by only a few manufacturers and is popular for below-grade applications. XPS is more expensive than EPS and has an R-value of 5.0 per inch. XPS may be the most moisture-resistant of the foam boards.
XPS is flammable and produces toxic smoke when burned. XPS must be covered by a thermal barrier when installed in living spaces. XPS has a low maximum operating temperature (160 degrees F) that is a concern for using XPS under shingles or dark-colored siding. XPS has shrunken in some installations. Use weatherproof coverings to prevent degradation by ultraviolet light and freezing and thawing at ground level.
PIC board has the highest R-value per inch (R-6 or a little more) of any common foam board. PIC is packaged with a vapor permeable facing or an aluminum-foil (vapor barrier) facing. PIC is expensive but worth the cost when the thickness of insulation is limited.
PIC is combustible and produces toxic smoke during a fire. However some products have fire retardants that allow installation in attics and crawl spaces without a thermal barrier or ignition barrier.
PIC has a low maximum operating temperature (<200 degrees F) that may be a concern for using PIC under dark-colored roofing or siding. Use the high-density (3 pcf) PIC board for low-sloping roof insulation.
Polystyrene (EPS) beads can be poured or blown into cavities. The cavities must be airtight or the beads will escape, making an annoying mess. EPS beads have an R-value between 2.2 and 2.5 per inch. Beads work well for filling hollow masonry walls.
These expanded minerals are pour-able and used when a non-combustible insulation or high temperature insulation is needed. R-value per inch is between 2.0 and 2.7 per inch. These products are good for insulation around single-wall chimney liners to prevent condensation in the liner. Existing vermiculite may contain asbestos, and it must not be disturbed by anyone except a licensed asbestos abatement specialist.