Insulating walls

Sections of this story: Overview: Essential Knowledge | Techniques, materials and tools | Taking it further

Overview: Essential Knowledge

Installing batt insulation in an attic Photo: Karen Doherty

Insulation is one element in a tightly knit construction system intended to improve indoor comfort and reduce energy consumption. In rehab work, installing insulation or improving existing insulation levels will be critical in providing comfort.

Insulation should never be applied without considering its effect on other aspects of construction. Some factors to consider when evaluating wall insulation are density and compressibility, air leakage, moisture control, fire safety, and wall construction in existing homes.

R-value

Each type of insulation has a density at which its R-value per inch is greatest, but reaching this density is not always cost-effective.

For 3 1 /2-inch thick fiberglass batts, an R-13 batt contains 40 percent more material. An R-15 batt contains 180 percent more material, than an R-11 batt (Fig. 1).

FIGURE 1: TYPICAL FIBERGLASS BATT DENSITIES AND VALUES

To achieve a desired overall R-value for dry blown-in insulation, and to prevent settlement, the installed density must be above a recommended minimum. For convenience in comparing estimates or monitoring the installation, have the bidder or installer calculate the number of bags required to achieve the required density.

Unless insulation completely fills all the wall cavities, air leakage can bypass the insulation and create a risk of condensation. Reducing air leakage is an inseparable part of insulating: you should not do one without doing the other. Typical locations for air leakage through walls are at the sill, the wall plates, vertical corners, around openings, and at electrical devices.

Before filling stud cavities of older homes with blown-in or foamed-in-place retrofit insulation, explore the construction. Stud cavities are often interrupted by blocking half-way up the wall, or in girt-frame construction, by full-depth diagonal corner braces. The outside walls may be "back-plastered," where a hidden layer of plaster creates two parallel cavities within each stud space, neither deep enough to receive loose-fill insulation.

After insulating, an infrared camera scan of the wall will show cavities that have not been fully insulated. Avoid deliberately ventilating walls, since any convective airflow within an outside wall risks condensation within the wall and compromises its R-value. Ventilation passages behind the exterior finish are called for when extreme interior humidity is expected and no vapor retarder can be applied, or where wood siding is applied directly over exterior foam insulation.

Techniques, materials and tools

Of the innumerable possible combinations of insulating materials and wall configuration, the following list covers those in common use and uncommon systems that are recommended.

1. INSTALL BATT INSULATION

FIGURE 2: BATT INSULATION CAN BE FACE OR INSET STAPLED

Fiberglass insulation is available in batt form, typically sized 93 inches long to fit within the stud-space of an 8-foot wall, or in continuous rolls. Both forms are here referred to as "batt insulation." It is available in many thicknesses, densities and in widths to fit framing at 16-inch and 24-inch centers.

Unfaced batts can easily be cut to fit into odd-sized spaces, and are preferred where a continuous membrane vapor retarder is installed. Residential batts are available faced with kraftpaper and aluminum foil, and commercial batts with a flame-resistant foil facing are available.

All have extended tabs on the facings to secure them in place. When properly applied, the facings create a partial vapor retarder. Only unfaced or fire-retardant-faced batts can be left exposed in attics or occupied spaces. If not accurately cut around wiring and other obstacles, faced batts create large air cavities that compromise their effectiveness. Tabs can be "inset stapled" to the sides of the studs, or "face stapled" to the inner face (Fig. 2).

Face stapling is preferred because it creates a better vapor retarder and avoids the air cavity left between the facing and the wall finish when inset stapling. Unless this cavity is carefully sealed at the top and bottom, it can compromise the wall's air-tightness and R-value.

Staples into stud faces must be fully set to avoid interfering with drywall installation.

In a three-sided wall cavity, friction-fit unfaced batts, covered with a separate vapor retarder, will typically result in a more effective installation than will stapled faced batts. In an open wall, the facings are usually necessary for attachment.

• ADVANTAGES: An economical, flexible, and well-known product. Provides a dependable thickness of uniform density and does not settle, if properly installed. Faced batts can insulate an open stud wall.
• DISADVANTAGES: Effectiveness requires careful installation to avoid gaps and consequent convective losses. Glass fibers can be irritating if touched or inhaled.

2. INSTALL ENCAPSULATED FIBERGLASS INSULATION

Fiberglass insulation is available in rolls or batts, encapsulated with kraft paper or plastic to reduce mechanical irritation to installers. These can be used in any installation where batts are appropriate. One face is extended to form attachment tabs. The faces on sound control batts do not have a vapor retarder; and some exterior wall batts have a polyethylene vapor retarder on the flange side.

Some encapsulated batts have a Class A fire rating and can be left exposed if allowed by local codes. Owens Corning markets Miraflex, made from loose, virtually itch-free glass fibers with no binder, contained in a plastic sleeve. Cutting encapsulated batts around obstructions is possible, but exposes the fiberglass. Cutting Miraflex releases the fibers; the product is is therefore intended primarily as attic floor insulation.

• ADVANTAGES: Ideal for rehab contractors. Can be left exposed (check local codes). Flange-attached with or without a vapor retarder on some products.
• DISADVANTAGES: More costly than regular batt insulation. Cutting encapsulated batts exposes the fiberglass. Cutting Miraflex releases the fibers.

3. INSTALL BLOWN-IN, LOOSE-FILL INSULATION INTO CLOSED STUD SPACES

Loose-fill insulation (fiberglass, cellulose, or mineral wool) can be blown into closed stud spaces through openings formed by drilling holes through the interior or exterior finish, or by removing strips of interior or exterior finish, at the top and bottom of each rafter space.

Careful installation is required because material can bridge over electric lines and other obstructions, causing voids and later settlement. Beware of blocking; blow into cavities above and below it. At least a moderate amount of pressure is required to produce sufficient density to inhibit settlement.

Fiberglass blown into a 2 by 4 stud cavity at a density of about 1.5 pcf produces R-13 without excess pressure on finishes. In a "dense-pack" installation of cellulose, dry material is applied at high velocity through a narrow tube inserted through a single hole at the top and extended to the bottom of the cavity. The tube is gradually withdrawn, compacting the material to a density of 3 to 3 1 /2 pcf.

"Stabilized" cellulose includes an adhesive, and "fiberized" cellulose is made in strands instead of chunks; both processes are claimed to inhibit or prevent settlement. Insulation packed into and filling wall cavities suppresses air movement within the cavity, does not create a vapor retarder, but may substantially improve fire safety.

• ADVANTAGES: If the cavities are completely filled under sufficient pressure, provides superior insulating performance without settlement, greatly reducing air circulation within the walls. May improve fire safety.
• DISADVANTAGES: Some types of installation may leave voids and/or settle after installation. Blown-in materials do not form a vapor retarder, and form only a partial air barrier.

4. INSTALL BLOWN-IN OR SPRAYED-ON INSULATION INTO OPEN STUD SPACES

Figure 3: Installing Ark-Seal's BIBS system

An inexpensive material can be applied as a membrane over open studs to form "see-through" cavities, within which any blown-in insulation can be applied under moderate pressure. In Ark-Seal's "Blown-in-Blanket (BIBS)" system, fiberglass mixed with some water and adhesive is blown through slits cut in a tightly stretched nylon netting. In Par-Pac's "Dry-Pac Wall System," dry cellulose is blown at 3 pcf density into a cavity closed by a reinforced polyethylene vapor retarder (Fig. 3).

The material is installed from the bottom up to minimize voids and settlement. All such installations will cause the membrane to bulge out; make sure this bellying does not interfere with drywall installation. Various types of polyurethane-and polyisocyanurate-based insulations, such as Icynene, can also be spray-applied into stud cavities. A thin layer of such material can form an air-barrier skin, over which cheaper material can be placed.

Excess sprayed on insulation must be scraped off. Photo: Paul Torcellini

Excess material must be scraped off, and windows and electrical devices protected or cleaned. Water is mixed with cellulose in a "wet-spray" application. The water combines with the starch in the cellulose to form a natural adhesive, which holds the material in place. Excess material must be scraped off, but can be reused. The material must dry out before a finish is applied; excessive water can prevent drying and generate rot or mildew.

High-density insulation filling cavities may improve fire safety.

• ADVANTAGES: Fills the cavities without settling. Visual inspection is possible to ensure filled cavities. Greatly reduces air circulation within walls. May improve fire safety.
• DISADVANTAGES: Bellying of the interior membrane may interfere with drywall installation. Sprayed-on foam products are more expensive than batt or loose-fill installations, and are messy processes, requiring cleanup and protection. Excess water in wet-spray applications may lead to rot and mildew.

5. INSTALL RIGID WALL INSULATION

A 3 /4-inch to 1-inch layer of rigid insulation, typically polyisocyanurate (ISO), molded expanded polystyrene (EPS), or extruded polystyrene (EPS), is a widely used adjunct to cavity insulation. Where cavity insulation cannot be installed, rigid foam may be the only way to insulate a wall.

It is preferably applied on the outside of the framing, keeping the framing warm enough in cold weather to prevent condensation within the walls, and inhibiting thermal short-circuits through the studs. It is also useful on the outside as a base for cement stucco or exterior insulation and finish systems (EIFS), although the latter should be part of an engineered system that provides interior drainage.

A layer of foam is essential in conjunction with conventional steel framing to prevent surface condensation. In Gulf-coast climates, a layer of foam behind the interior finish is preferred over an exterior layer or a layer on both sides of the studs, especially with steel framing.

Since foam cannot be relied upon to resist racking, it must be applied over structural sheathing or in parallel with a system of wall shear bracing. Structural sheathing separated from the framing by an outside layer of foam may not meet code racking requirements (consult with a structural engineer). Celotex makes a structural polyisocyanurate foam sheathing that, when glued and nailed to the framing, acts as racking bracing. Wood siding should not be applied directly to foam insulation.

• ADVANTAGES: Isolates framing to minimize or eliminate internal condensation and reduce cold bridging through framing. Can add more than its rated R-value to a wall assembly.
• DISADVANTAGES: More costly per R than fiberglass insulation. If substituted for exterior sheathing, requires other measures to create racking resistance in the structure. Should not be used with EIFS except as part of an engineered system with interior drainage. EPS cannot support one-coat cement stucco over more than 16" stud spacing.

6. INSTALL A RADIANT BARRIER

While radiant barriers and coatings are commonplace in high-temperature industrial applications (typically 500 F or more), they are marginally effective at ordinary temperatures. To be cost-effective in building applications, they must have a very low incremental cost (from 2 to 10 cents/square foot, depending on the application).

Clean, shiny aluminum foil facing a 3 /4 inch or deeper air space can create a radiant barrier. Foil-faced insulation held back and inset-stapled creates only a marginally effective radiant barrier because the insulation bulges into the air space, and because the cavity can create heat loss through convection. It is always more effective to fill the cavity with insulation.

If an air space is present for other reasons (for example, the cavity between sheathing and brick veneer), facing the air space with foil-faced sheathing will add thermal resistance if the material remains clean. A new form of radiant barrier is Radiance paint, which contains aluminum dust.

• ADVANTAGES: An easy way to add insulating value at brick cavity walls.
• DISADVANTAGES: Marginally cost-effective; unlikely to be effective if exposed to dirt or condensation. Not cost-effective in cold climates, except possibly Radiance paint. Seldom or never cost-effective if the air space is deliberately "stolen" from conventional insulation. Foil may create a vapor barrier where one is not desired.

7. INSTALL A STRUCTURAL INSULATED WALL PANEL

Rigid foam insulation adhered to structural skins can create a structural insulated panel (SIP). SIPs provide a combination of structure and insulation. Depending upon the design, the panels may be self-supporting or may be a non-load-bearing exterior skin applied over a post and beam frame.

Very tight joints are crucial, since a small amount of air leakage through a joint is guaranteed to create destructive condensation at the most critical structural point. Panels are typically fabricated to order and delivered to the site for quick erection. Acoustical tightness is readily noticeable.

• ADVANTAGES: A high-R wall that can be load bearing and resists racking. Provides excellent acoustical resistance. Allows a high level of prefabrication and fast on-site erection.
• DISADVANTAGES: Not yet in common use, and therefore more expensive than ordinary framing. Requires careful air-sealing at all joints.

Taking it further: Reading list & Supplier information

READING LIST

• ASHRAE, 1997 Handbook of Fundamentals, Inch-Pound Edition, Chapter 22: Thermal and Moisture Control in Insulated Assemblies--Fundamentals; Chapter 23: Thermal and Moisture Control in Insulated Assemblies-- Applications; Chapter 24: Thermal and Water Vapor Transmission Data, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA; 404-636-8400; www.ashrae.org.
• "Fiberglass vs. Cellulose: Making the Choice," Ted Cushman, Journal of Light Construction, Sept. 1995, pp 27-31.
• Energy Source Catalog; Iris Communications, Inc.; 800-346-0104.
• Energy Star Fact Sheets; Builder Guides; U. S. Environmental Protection Agency (EPA), Atmospheric Pollution Prevention Division; 888-STAR-YES.
• Home Energy's Consumer Guide to Insulation; www.homeenergy.org/
• Residential Energy Efficiency Database (REED); http://139.142.36.88/reed/index.htm.
• Sawnee Energy Library; www.energydepot.com.
• U. S. Department of Energy (DOE) Energy Efficiency and Renewable Energy Clearing House (EREC); 800-363-3732; e-mail: doe.erec@nclinc.com.
• "Insulation Fact Sheet," August 1997, Document DOE/CE-0180.
• "Loose-Fill Insulations," May, 1995, Document DOE/GO-10095-060.
• "Insulation Materials: Environmental Comparisons," Alex Wilson, Environmental Building News, Volume 4, Number 1, January/ February 1995.
• "Insulation Comes of Age," Alex Wilson, Fine Homebuilding, February/ March 1996, No. 100.

BATT, SPRAY-ON, AND LOOSE-FILL INSULATION

• Cellulose Insulation Manufacturers Association, 136 South Keowee Street, Dayton, OH 45402; 937-222-2462; www.cellulose.org.
• American Rockwool, Inc., P. O. Box 880, Spring Hope, NC 27882; 919-478-5111.
• Ark-Seal International, 2190 South Kalamath, Denver, CO 80223; 800-525-8992.
• Building Products Division, The Celotex Corporation, P. O. Box 31602, Tampa, FL 33631; 813-873-4000.
• CertainTeed Corporation, 750 East Swedesford Road, Valley Forge, PA 19482; 800-523-7844.
• GreenStone Industries Inc., 6500 Rock Spring Drive, Suite 400, Bethesda, MD 20817; 888-592-7684.
• Icynene Inc., 376 Watline Avenue, Mississauga, ON L4Z 1X2, Canada; 800-946-7325.
• International Cellulose Corp., 12315 Robin Boulevard, Houston, TX 77245-0006; 800-444-1252.
• Johns Manville Corporation (formerly Schuller International Inc.), 717 17th Street, Denver, CO 80202; or P. O. Box 5108, Denver, CO 80217-5108; 800-654-3103.
• Knauf Fiber Glass, 240 Elizabeth Street, Shelbyville, IN 46176; 800-200-0802.
• Owens Corning, One Owens Corning Parkway, Toledo, OH 43659; 800-354-PINK or 800-GET-PINK.
• Par-Pac, 4545 East 52nd Avenue, Commerce City, CO 80022; 800-850-8505.
• Rock Wool Manufacturing Co., 203 North Seventh Street, Leeds, AL 35094; 205-699-6121.
• Sloss Industries Corporation, 3500 35th Avenue North, Birmingham, AL 35207; 205-808-7803.
• U. S. Fiber, Inc., 905 East Martin Luther King Dr., Suite 400, Tarpon Springs, FL 34689; 800-666-4824.
• Western Fiberglass Group, 6955 Union Park Center, Suite 580, Midvale, UT 84047; 801-562-9558.

STRUCTURAL INSULATED PANELS

• Structural Insulated Panel Association, 1331 H Street NW, Suite 1000, Washington D. C. 20005; 202-347-7800; e-mail: sipadc@aol.com.

RIGID FOAM INSULATION

• Expanded Polystyrene Molders Association (EPSMA), 1926 Waukegan Road, Suite 1, Glenview, IL 60025-1770; 800-607-3772.
• Polyisocyanurate Insulation Manufacturer's Association (PIMA), 1001 Pennsylvania Avenue, N. W., 5th Floor, Washington, DC 20004; 202-624-2709; www.pima.org.
• Celotex Building Products, P. O. Box 31602, Tampa, FL 33631-3602; 813-873-4230.
• Dow Chemical Company, Styrofoam Brand Products, 2020 Willard H. Dow Center, Midland, MI 48674; 800-258-2436.
• Johns Manville Corporation (formerly Schuller International Inc.), 717 17th Street, Denver, CO 80202; or P. O. Box 5108, Denver, CO 80217-5108; 800-654-3103.
• Owens Corning, One Owens Corning Parkway, Toledo, OH 43659; 800-354-PINK or 800-GET-PINK.
• Tenneco Building Products, 2907 Log Cabin Dr., Smyrna, GA 30080; 800-241-4402.

RADIANT BARRIER PRODUCTS

• Energy-Brace reflective sheathing, Fiber-Lam, Inc., P. O. Box 2002, Doswell, VA 23047; 804-876-3135.
• Radiance Low-e interior paint, ChemRex, 889 Valley Park Drive, Shakopee, MN 55379; 800-433-9517.
• Thermo-ply reflective sheathing, Simplex Products Division, P. O. Box 10, Adrian, MI 49221; 517-263-8881.

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This story is excerpted from The Rehab Guide: Roofs , one in a series of guidebooks produced by the U.S. Department of Housing and Urban Development (HUD) to keep the design and construction industry abreast of innovations and state-of-the-art materials and practices in home rehabilitation.