Should I put plastic over insulation?

Fiberglass Insulation In most climates, it needs a vapor barrier. Some builders rely on batts with attached kraft-paper facing to do that job, but Tom recommends unfaced batts, covered in plastic with all the seams taped shut.

thisoldhouse.com - Insulation Education - This Old House

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Comfort Control

When it comes to insulation, says This Old House general contractor Tom Silva, money truly can buy happiness. "It's the wisest way to invest that I know of," he says. "A well-insulated house will make you more comfortable in every season. And it's quieter, too." Whether it's thick blankets of fiberglass batts, meringue like mounds of spray foam (Tom's favorite), or layers of seaweed (discovered under the floors of the first TOH project), all insulation works the same way: by trapping tiny air pockets that slow the movement of heat out of a house in winter and into a house in summer. Its effectiveness at resisting this movement is called its R-value; the higher the value, the lower your energy bills. But to achieve its rated R-value, insulation must be installed correctly. In fact, a bad job can actually make matters worse.

Foam Insulation

As far as Tom Silva is concerned, low-density polyurethane spray foam is the insulation technology to beat. Consider its advantages: It forms a lock-tight bond with studs and sheathing that blocks all air movement, it flexes enough to accommodate seasonal wood movement, and it retards (but doesn't halt) moisture passage. While spray foam is expensive, its installation costs are offset in the long run by lower heating and cooling costs.

Fiberglass Duct Wrap

Use at least R-5 insulation with a foil facing.

Low-Density Polyurethane Foam

Expands so much that it has to be trimmed flush with the studs before dry-wall can be installed.

Canned Spray Foam

(minimally expanding)

Seals and insulates around window and door jambs. But go easy on the trigger. Too much can distort the jambs and prevent sash or doors from working.

Extruded Polystyrene Foam Board

(2-in.) Insulates foundation and slab.

Polyethylene Foam Tube

(1-in.) Keeps heat in hot-water lines. Prevents condensation.

Insulating Cathedral Ceilings

When Tom insulates a cathedral ceiling or a finished attic, he also turns to foam. Sprayed against the underside of the roof deck, it seals against air movement, eliminating the need for a vapor barrier or ventilation. But its open-cell structure still allows moisture to escape.

Fiberglass Insulation

Fiberglass batts, the insulation found in most houses in the U.S., are inexpensive and quick to install. Like other batt-type insulation, fiberglass has a predictable R-value if not compressed but is difficult to fit around obstacles without leaving gaps. In most climates, it needs a vapor barrier. Some builders rely on batts with attached kraft-paper facing to do that job, but Tom recommends unfaced batts, covered in plastic with all the seams taped shut.

Baffle

Prevents insulation from blocking airflow on underside of roof.

Soffit Vent

(continuous) Lets fresh, cooler air into attic. The size of the openings added to that of the ridge vent should total at least 1 square foot for every 300 square feet of attic floor space.

Foil Tape

Seals seams on vapor barrier.

Polyethylene Vapor Barrier

(6-mil)

Prevents moisture from entering wall cavities. Seams overlap by 16 inches.

Fiberglass Batts

(unfaced)

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Batts in the Rafters

Simply jamming batts between the rafters of a cathedral ceiling or insulated attic will interfere with the roof's ability to breathe. The resulting buildup of moisture can soak the insulation (nullifying its R-value), foster the growth of mold, or even rot the framing. The illustration at left shows how Tom Silva maintains adequate ventilation and avoids those problems.

Common Mistakes With Batts

Loose Batts

Often the problem is sloppy framing. "If the studs are 16.5 inches on center and you're using batts made for 16 inches, you'll have a crack on each side that air can move through," says Tom. In this case, it's best to use cavity-filling foam or blown-in insulation.

Compressed Fiberglass Batts

Fiberglass insulation gets its R-value from the amount of air it traps between its fibers. If it's jammed too tightly into a cavity, it can't trap as much air and won't be as effective.

Doubled Vapor Barriers in Attic

Laying a second layer of fiberglass batts in the attic is an easy way to boost R-values. But if the new layer has a kraft-paper backing, it can trap moisture and turn the layers underneath into a soggy mess. "I see this mistake all the time," Tom says.

Heat Blockers

Not technically an insulation, radiant barriers keep houses cool by reflecting thermal radiation. These thin sheets of shiny aluminum bonded to foam board, bubble wrap, or sheathing are often installed in attics to block heat from the summer sun. To be effective, the barrier's reflective surface must always face an air space at least an inch thick and be installed shiny side up if laid on the attic floor, shiny side down if attached to the rafters. Tests show that a radiant barrier in an insulated attic can lower attic temperatures by as much as 30 degrees. While radiant barriers are a boon in warm climates, they're less useful in cold-weather regions because they prevent beneficial solar heat gain in winter. To see if it makes economic sense to put a radiant barrier in your house, visit the Department of Energy's Website.

Vapor Blockers

Once insulation gets wet, it's hard to dry it out. "It will sit there like a sponge, leading to mold problems and rot," says Tom. Vapor barriers—sheets of plastic or kraft paper—keep water vapor out of the wall cavity, so the insulation stays dry. Not every type of insulation needs a vapor barrier. But if it does, the barrier should face inside in northern, heating climates, and outside in humid southern climates.

Insulation Options

There are all kinds of materials to stuff in, nail on, spray on, or blow into walls and ceilings to lower your heating and cooling bills. Weigh your choices based on all the factors, including the abilities of the installer and the long-term energy savings.

Cotton Batts

No-itch batts of recycled denim from jeans factories are treated with borates to resist fire and insects.

Good for: New construction, attics

R-value per inch: 3.7

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Vapor barrier needed: Yes

Cost: 70¢ to 75¢ per sq. ft.

Loose-Fill Fiberglass

Fluffy bits of spun glass that are noncombustible and can't decay. Blown in dry. Tends to settle. R-value declines by as much as 50 percent at temps below 0 degrees F.

Good for: Attic insulation, new construction, or retrofit work

R-value per inch: 4

Vapor barrier needed: Yes

Cost: 21¢ per sq. ft.

Extruded Polystyrene (XPS)

Its closed-cell structure stops water and water vapor, resists compression, and holds its R-value over time. Must be protected from solvents and sunlight. Flammable and must be protected from fire with drywall or plaster.

Good for: In-ground foundation insulation, masonry structures

R-value per inch: 5

Vapor barrier needed: No

Cost: 50¢ per sq. ft. for a 2-in.-thick panel

Cellulose

Made from pulverized newspapers and treated with boron to resist fire and pests. Blown into place dry or wet. An adhesive reduces its tendency to settle.

Good for: Retrofit work, attic insulation

R-value per inch: 3.8

Vapor barrier needed: No, if packed to a density of at least 2.6 lb./cu. ft.

Cost: 17¢ per sq. ft.

Fiberglass Batts

Lightweight batts of spun glass offer a predictable R-value if not compressed, but the fibers offer little resistance to air movement and convective heat loss. The sample shown is held together with a nontoxic acrylic binder instead of the typical formaldehyde-based binder.