Although adding insulation to the walls of an already-built house is no job for the do-it-yourselfer, there are a number of other places where the homeowner can add insulation without professional help - which will help to lower fuel bills during cold weather as well as air-conditioning costs during hot weather.

In a house where the attic floor is unfinished, insulation can be easily placed between floor joists on the attic floor. If some insulation is already in place, more can be added by placing it on top. Use either loose fill, which comes in bags and is poured out to the desired thickness, or batts or blankets that are laid out between the floor beams.

There should be a vapor barrier on the bottom, so if you have some insulation in place chances are that there is already one in place on the underside. Additional insulation should not have a second vapor barrier (or if it has, it should be slashed with a sharp knife every few inches).

The value of any insulation is determined by its R rating: the higher the number the more insulating value it has. Thickness is only an approximate indication because six inches of one type may have no more insulating value than five or five and a half inches of another type.

On batts or blankets the R value should be clearly imprinted on the side; on loose fill the R value for various thicknesses will be imprinted on the bag so you will know how thick a layer to pour.

As a rule, the attic floor should have insulation rated at least R-22, but if the house is in a colder than average area, or if the house is electrically heated, R-33 is recommended. If there is already some insulation in place and you do not know its R rating, add at least an R-11 if the old material is about three or four inches thick, and at least an R-19 if is less than three inches thick.

In any case, when installing insulation on an attic floor, make sure the insulating material does not cover any light fixtures that are recessed into the ceiling below (so that they project up into the attic), and do not allow the insulation to block air vents along the eaves.

If the attic floor is finished or covered so that the floor beams are no longer exposed, adding more insulation to the floor becomes more difficult. Either the floor boards will have to be pulled up in sections so that batts or loose fill can be pushed into place, or a professional will have to blow loose fill through holes which will be bored in the flooring.

When the floor is finished but the roof rafters overhead are exposed, it is often easier and more effective to add insulation to the underside of the roof instead of to the floor of the attic. For this job batts or blankets of mineral wool or fiber glass should be used.

They should be stapled up between the beams so that there is an air space between the underside of the roof sheathing and the upper side of the insulation, with the vapor barrier facing down (always toward the side that is heated in cold weather.)

The insulation should run from the floor up to the collar beams - the horizontal beams that go across near the top - then across these and down the other side.