Batt and Blanket Insulation

Inexpensive and Easy to Handle, Batts are The Most Common Insulation

UPDATED 8/21/2012

Bird's-Eye View

Batts are easy to install, but hard to install well

Because fiberglass batts are inexpensive and can be installed without special equipment, they are the most common type of residential insulation. But installing batts well is more difficult than people realize, and small installation mistakes have large penalties.

That said, batts can be effective if air leaks are eliminated. In addition to fiberglass, batts and blanket insulation are available in cotton, wool, and mineral wool.

See below for:
ABOUT BATT AND BLANKET INSULATION
COTTON BATTS
MINERAL WOOL BATTS AND BLANKETS
FURTHER RESOURCES


Key Materials

Many materials are available for batts and blankets

Among the materials used to make batt and blanket insulation are fiberglass, cotton, wool, and mineral wool.

Fiberglass insulation is made primarily from silica spun into glass fibers and held together by a phenol-formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen." glue or binder—though formaldehyde-free batts are available from Johns Manville and Knauf.

Fiberglass batts are available unfaced or faced. Facing materials vary; they include kraft paper, foil-faced paper, MemBrain, and vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). (either perforated or unperforated vinyl).


Design Notes

Facings do more than just hold the batts in place

It's important to consider the permeability of batt facings when specifying types -- some, like kraft paper, allow drying and inhibit wetting. Foil facings are impermeable and unfaced batts are often coupled with a plastic vapor barrier.


Builder Tips

Believe it or not, there's a saw for batts

Utility knife blades dull quickly when cutting fiberglass all day long. A better way is to use a specialy designed saw, such as the 14-in. Insul-Knife. The Insul-Knife is available for about $35 from Cepco Tool.


The Code

Make the R-value obvious, and post it near the electrical panel

Section N1101.4 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. requires installed insulation to include "an R-value identification mark" that has been "applied by the manufacturer to each piece of building thermal envelope insulation 12 inches or more wide. Alternatively, the insulation installers shall provide a certification listing the type, manufacturer and R-value of insulation installed in each element of the building thermal envelope."

Section N1101.4.1 of the IRC requires: "Insulating materials shall be installed such that the manufacturer's R-value mark is readily observable upon inspection."

Section N1101.8 of the IRC requires: "A permanent certificate shall be posted on or in the electrical distribution panel. The certificate shall be completed by the builder or registered design professional. The certificate shall list the predominant R-values of insulation installed in or on ceiling/roof, walls, foundation (slab, basement wall, crawlspace wall and/or floor) and ducts outside conditioned spaces..."

OTHER CONSIDERATIONS

Is fiberglass a health concern?
Controversies surround the question of whether potential health problems arise from the shedding of glass insulation fibers. Loose-fill insulation may pose a greater risk than batts in this regard. While some worry that airborne fibers might be carcinogenic, those concerns have been allayed to some extent in recent years.

Those concerned with the presence of phenol formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen." binders can choose batts with alternative binders or no binder at all.

How green is fiberglass?
Most fiberglass manufacturers now incorporate at least 30% recycled material, and some products are third-party certified for their recycled-glass content. To put these claims in context, it's worth noting that cotton batt manufacturers and mineral wool manufacturers claim their products have at least 75% recycled content.

Can I put a 12-in. batt in an 8-in. cavity?
Compressing a fiberglass batt reduces the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the batt but raises the R-value per inch.

Consider a 2x8 rafter cavity with a depth of 7 1/4 in. If the bay is filled with an 8-in. batt rated at R-24, the R-value of the cavity will be R-24. However, compressing a 12-in. batt (rated at R-38) into the 7 1/4-in. cavity will yield a higher R-value (R-27) than can be obtained with the 8-in. batt.

ABOUT BATT AND BLANKET INSULATION

Insulation batts and blankets are sold in bundles or rolls. When the insulation is cut into 4-ft. or 8-ft. lengths and sold in a bundle, each piece is called a batt. When the insulation is sold in a long roll, it is properly called blanket insulation.

In most cases, the width of the batt or blanket corresponds to the size of a framing bay—about 15 in. or 23 in. wide.

Fiberglass batts
Fiberglass batts have R-values ranging from 3.2 to 3.8 per inch, depending on their density. Denser batts have higher R-values because the ratio between the glass fibers and the air pockets has been optimized.

Examples of sloppy fiberglass batt installation, with compressed areas and voids, are all too common. For fiberglass batts to perform well, they must be installed perfectly. It’s essential that the batts be split to accommodate wiring (click photo to enlarge) and cut exactly to fit the framing bay where they are installed. Small voids can significantly impair the performance of fiberglass batts.

Fiberglass batts are air-permeable—they do a poor job of resisting airflow—so it is essential that they be installed in continuous contact with an impeccable barrier to air movement. For the very best performance, fiberglass batts should be installed in a sealed cavity (for example, a stud or joist bay) with an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. on all six sides. For more information on fiberglass batt installation, see Installing Fiberglass Right.

FOUR SIGNS OF A GOOD FIBERGLASS JOB:
Continuous, snug, and full: Insulation should be installed without gaps, including at corners. Batts should be trimmed a little oversize so that they fit the cavity snugly. The entire framing cavity should be filled, without any air gaps between the insulation and the drywall. Attic insulation should be installed over wall top plates to overlap wall insulation.

Not folded or excessively compressed: Batts should be delaminated where necessary to fit around wiring. There should be no gaps behind wiring, electrical boxes, or pipes.

Not susceptible to airflow: Batts installed in kneewalls and skylight shafts should be protected on all sides by a rigid air-barrier material like drywall, OSB, or Thermoply sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . If the roof assembly includes ventilation channels, insulation must not block airflow.

Anywhere but the basement. Fiberglass in basements is a bad idea for two reasons: (1) Fiberglass is air-permeable and vapor-permeable, so there is a danger that warm, moist interior air will pass through the fiberglass insulation and contact a cold surface—cold concrete or a cold rim joist. When that happens, condensation can collect on the cold surface. (2) Fiberglass performs poorly when wet. In fact, many builders have reported mold growth on wet fiberglass insulation.

Different facing materials do different things
Fiberglass batts can be bought as unfaced batts (also called friction-fit batts) or with an attached facing. By far the most common facing is kraft paper, which has a thin asphalt coating.

The kraft-paper facing that accompanies some fiberglass batts is a vapor retarder with a permeance of about 0.30 perm (although this value varies from manufacturer to manufacturer, and even from batt to batt). Like the permeance of asphalt felt or MemBrain, the permeance of kraft-paper facing varies with its moisture content. As the paper becomes damper, its permeance increases.

As typically installed, kraft-paper facing is not an air barrier.

Cold-climate builders in search of a single product that could act as both an air barrier and a vapor retarder began installing 6-mil plastic sheeting over unfaced fiberglass batts in the 1970s. However, as building scientists began to understand the need for walls to dry to the interior during summer months, plastic vapor barriers fell out of favor in all but the coldest climates.

"Our walls get wet in the winter and rot in the summer."
—Mark Bomberg

Because kraft facing allows some drying to a home's interior to occur during the summer, it doesn't have the disadvantages of interior plastic. Builders who prefer to install unfaced fiberglass batts may want to use a vapor-retarder paint or a product such as MemBrain.

Because foil-faced batts have a very low perm rating, they share some of the same disadvantages of plastic sheeting.

VinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate).-faced fiberglass insulation has been promoted for use in steel warehouse buildings or crawlspace walls. This product is available with either a perforated or an unperforated vinyl facing. However, building scientists now recommend against fiberglass insulation on crawlspace walls or basement walls.

Homeowners or contractors who are irritated by the itchiness that occurs when installing fiberglass batts may want to try using encapsulated fiberglass batts; brands include ComfortTherm from Johns Manville and Miraflex from Owens Corning. These batts encapsulate the fiberglass with polyethylene, so fewer fiberglass fibers are able to escape and irritate the installer's skin. (Of course, cutting the batts inevitably exposes some of the fibers.) Most manufacturers of encapsulated batts offer two types of poly encapsulation: one acts as a vapor retarder, and the other is perforated to provide more vapor permeance.

High-density fiberglass batts
For years, manufacturers of fiberglass batts sold products with a similar density. All 3 1/2-in. batts were rated R-11, while all 6-in. batts were rated R-19. As energy codes have become stricter, however, batt manufacturers have begun to offer a better-performing product: high-density batts.

High-density batts have additional glass fibers that do a better job of trapping small pockets of air. Because they contain more fibers, they cost more than low-density batts.

Most fiberglass manufacturers offer 3 1/2-in. batts in three densities, rated R-11, R-13, and R-15. In addition to traditional R-19 batts, manufacturers now offer a 5 1/2-in. batt rated R-21.

Denser batts perform better for the same reason that compressed batts provide a higher R-value per inch than uncompressed batts (see sidebar, "Can I put a 12-in. batt in an 8-in. cavity?"). For any insulation material to obtain the highest possible R-value, the ratio between the insulation material and trapped air spaces must be optimized. For years, fiberglass batt manufacturers have preferred to emphasize low initial cost rather than high R-values, so their products were rarely optimized for R-value. With recent changes in building code requirements for minimum R-values, however, manufacturers are now responding to a market demand for higher-performing batts.

What about formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen."?
Phenol formaldehyde (PF) binderGlue used in manufactured wood products, such as medium-density fiberboard (MDF), particleboard, and engineered lumber. Some binders are made with formaldehyde. See urea-formaldehyde binder and methyl diisocyanate (MDI) binder. has long been used for binding glass fibers in fiberglass insulation. During standard fiberglass insulation manufacture, a fairly small quantity of this PF binder is sprayed onto the glass fibers, and most of the formaldehyde is baked off during high-temperature curing. Some residual formaldehyde remains in the insulation.

In place of the phenol formaldehyde binder, two fiberglass manufacturers, Knauf and Johns Manville, now produce fiberglass batts with an acrylic binder. Johns Manville asserts that the new formaldehyde-free insulation eliminates concern over fiberglass insulation as a potential source of formaldehyde.

“While there is no evidence to suggest that the level of formaldehyde released by traditionally bonded insulation is at all harmful, concern about indoor air quality has continued to be expressed by architects, specifiers, builders, and consumers,” according to information on the Johns Manville Web site. Formaldehyde is classified by the International Agency for Research on Cancer (IARC) as a “probable human carcinogen” and is listed by the U.S. National Toxicology Program as “reasonably anticipated to cause cancer.”

COTTON BATTS

Cotton insulation batts are unfaced batts with 75% recycled content. The bulk of the raw materials used to make cotton insulation comes from fabric scraps left over from the manufacture of blue jeans, with synthetic fibers added to maintain loft. Nontoxic flame retardants similar to those used in clothing are added. The best-known brand of cotton batts is UltraTouch from Bonded Logic.

The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of cotton insulation ranges from 3.5 to 3.7 per inch.

Unlike fiberglass and mineral wool, cotton batts contain no mineral microfibers as a potential cause of respiratory problems.

Several installers have noted that cotton batts are difficult to install because they are manufactured in unusual widths (16 1/4 in. instead of 15 1/2 in.), and because batts compressed for shipping do not spring back to the thickness listed on the label. For more information on installing cotton batts, see "Problems Installing Cotton Insulation."

MINERAL WOOL BATTS AND BLANKETS

Mineral wool insulation is made from either molten slag—a waste product of steel production—or natural rock, such as basalt and diabase.

Mineral wool has a higher density than fiberglass, so it has better sound-blocking properties. It's also more fire-resistant than fiberglass.

Mineral wool batts are sold as unfaced batts or foil-faced batts. Manufacturers include Roxul and Thermafiber.

The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of mineral wool insulation is about 3.7 per inch.

Mineral wool insulation is rarely used in residential construction, having been mostly displaced by fiberglass.

FURTHER RESOURCES

“Insulation Inspections for Home Energy Ratings” by Bruce Harley, Home Energy magazine, January/February 2005

“Assessing the Quality of Insulation Installed in New York Energy Star Labeled Homes”

"Installing Fiberglass Right" by Martin Holladay


Image Credits:

  1. Chuck Bickford / Fine Homebuilding # 119
  2. CertainTeed
  3. Cepco Tool
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