CONSTRUCTION : Filling in Gaps on Varieties of Home Insulation

Modern home insulation works because myriad tiny closed pockets trap air, which acts as an insulator.

Insulation comes in several forms: batts and blankets that fit between studs and joists, loose fill that is poured or blown in, rigid foam sheets, and liquid foam that is injected into wall cavities.

All are rated by their ability to resist heat loss (R-value). The higher the R-value, the more an insulation will prevent heat loss and cut fuel bills. In general, an existing home will benefit most from the addition of batts or loose fill between joists in the attic. This where the most heat is lost, and in an unfinished attic, it’s a job that the homeowner can do easily for moderate cost.

Blowing loose fill or injecting liquid foam into finished walls is a complex process and requires a contractor.

Here are some guidelines:

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Fiberglass

Fluffy fiberglass batts and blankets are by far the most popular insulation and are standard for most new wood-frame houses. They are economical and easy to install. Blankets are long rolls. Batts are simply blankets cut into 4- or 8-foot lengths. They come in widths to fit between joists and studs spaced at standard 16- or 24-inch intervals. Thicknesses commonly correspond to stud and joist widths and range from R-11 to R-30 in insulating value.

Fiberglass is available with or without a vapor retarder. Install any insulation so the vapor retarder faces the wall’s heated side, which is the interior side in all but the hottest parts of the Sun Belt. When installing more insulation over existing insulation, use the kind without a vapor retarder so moisture won’t get trapped between the old and new layers.

Fiberglass particles can be harmful. Wear a breathing mask, gloves, a long-sleeve shirt and a hat when handling fiberglass.

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Mineral Wool

Mineral wool is similar to fiberglass. It’s available in batts and blankets but is most commonly sold in bags as loose fill. It’s easy to install. In an unfinished attic with no insulation, just lay a sheet of polyethylene plastic as a vapor retarder, pour the fill between joists and spread it with a rake.

Loose fill pours easily into cavities that can’t be reached with other insulations. But it settles in time, particularly in vertical wall cavities, so care must be taken to install it properly.

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Cellulose

The advantages of cellulose are its low cost and the ease with which it is blown into otherwise inaccessible areas. Made of recycled paper, it’s treated by the manufacturer with a fire-retardant chemical.

The main disadvantage of cellulose is that it is messy to install. Because it is organic, it’s also susceptible to moisture retention, which can cause it to shrink and lose its insulation value.

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Urethane Foam

Urethane foam is a very effective insulator. It is pumped as a liquid into wall cavities, where it solidifies and forms its own vapor retarder. But it is expensive and must be installed by a contractor. It also emits a toxic gas when it burns and has been banned in some states.

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Plastic Foam Sheets

Three types of rigid plastic foam sheets are commonly used for insulation:

* Extruded polystyrene is the best choice for below-grade insulation of exterior walls and floors because it resists moisture. But it’s costly.

* Expanded polystyrene is the least expensive of the three, but it’s not moisture resistant. It’s commonly used beneath exterior siding as the core of foam-core panels.

* Polyurethane sheets are also used primarily under siding. They give a high R-value per inch but are costly.

All three are flammable. If installed indoors, they must be covered with fireproof gypsum wallboard.