Take the edge off winter's cold - and your bills - with insulation

Consumers have a choice: pay higher utility bills in a poorly insulated house , or higher monthly mortgage payments in a well-insulated and (consequently) more costly home.

Cynics who say it doesn't much matter which route you take should consider again. Heating costs are likely to rise as the years go by. Good insulation, on the other hand, is a one-time charge.

There's another important reason to go with additional insulation: The Internal Revenue Service's compassion for the mortgage-saddled homeowner.

While the IRS is quite unconcerned with your heating and cooling bills, it happily allows you to deduct the mortgage interest when calculating your income tax. You might say good insulation helps you unload some of the high cost of energy onto Uncle Sam.

Bankers too may be sympathetic to energy costs. They like the way energy efficiency reduces the burden of running a home. As a result, a lower income frequently qualifies for a higher-priced home if the extra costs contribute to greater energy efficiency.

Knowing, then, that it is generally more cost-effective to retain our heat (or coolness) than to generate more, it helps to know which insulation is best for a particular application.

There are several important facts about good insulating materials:

* Resistance to heat transfer. This is referred to in the industry as the R-value. The more slowly heat moves through a material, the higher its R-value and the more effective it is as an insulation. The R-value listed on the package is always expressed at so much per inch. In other words, an insulation listed at R-3, when packed into a 6-inch wall cavity, would add R-18 of insulating value to that wall (the R value times the number of inches).

It follows, then, that a high R-factor becomes important when there is very little room for the insulation - a narrow 3 1/2-inch wall cavity, for example.

* High moisture permeability. Water vapor that moves in must be able to move out without hindrance.

* Low moisture absorption. You don't want the insulation to attract and hold moisture like a sponge. Wet insulation loses much of its resistance to heat transfer. In other words, it stops being a good insulator.

* Low combustibility and low smoke toxicity. Material shouldn't burn too readily and, in the event of fire, it must not give off highly toxic smoke. Just because a particular insulating material can burn, however, is no reason to discard it altogether. As long as it is separated from the home's interior by wallboard it is acceptable.

Some nonflammable materials, such as fiberglass, include a binder (glue) that can burn. Remember, too, that hollow walls support a fire far more readily than filled walls, even if the filling is flammable.

* Low specific weight. Any insulation that weighs more than 3 pounds to the square foot could cause a ceiling to sag. On the other hand, you would not need to consider weight if the insulation were being placed directly on the cellar floor.

Insulation comes in three basic categories: batts and blankets, loose fill, and rigid boards.

Batts and blankets differ in only one respect: The blanket has a foil and/or kraft facing; the batt has none. They are placed between the framing timbers of walls, ceilings, and floors, and are also used to wrap around water heaters and hot-air ducts.

The materials used in these products, fiberglass and rock wool, both have an R-value of 3.2 and are identical in every other respect except one: rock wool is 21/2 times as heavy.

Loose fill is poured or blown into existing wall cavities as well as between ceiling joists. The principal materials are cellulose, fiberglass, rock wool, vermiculite, perlite, and polystyrene beads. To this list might be added polyurethane, which is foamed in place; once it sets it becomes, in effect, a rigid-board application.

Rigid boards that are glued or fastened in place can also sometimes double as sheathing.

Type R-value Moisture Moisture permeability absorption BATT/BLANKET Fiberglass 3.2 high low Rock wool 3.2 high low LOOSE Cellulose 3.4 high varies Fiberglass 2.2 high binders Rock wool 2.9 high low Perlite 3.5-2.5 high low Polystyrene beads 3.8 medium medium Vermiculite 3.0-2.4 high low RIGID BOARD Polystrene extruded 5.0 low low Expanded 4.0 medium medium Polisocyanurate 7.0 medium low

Type Density (lb./cu.ft.) Combustibility Smoke toxicity BATT/BLANKET Fiberglass 1.0 binders/backing low Rock wool 2.5 binders/backing low LOOSE Cellulose 2.6 yes carbon monoxide Fiberglass 0.6 binders low Rock wool 1.5 low low Perlite 2-11 low none Polystyrene beads 0.8 medium carbon monoxide Vermiculite 4-10 no none RIGID BOARD Polystrene extruded 1.8 yes carbon monoxide Expanded 1.0 yes carbon monoxide Polisocyanurate 2.0 yes carbon monoxide Source: Dept. of Energy/Manufacturers' Associations