FIXING HIDDEN leaks in even heavy insulated attics will cost the homeowner less than $15 in materials but will save an average of 15 percent in space heating costs each year.

Princeton University researchers, in a recent series of experiments funded by the U.S. Department of Energy in occupied wood-frame houses, found that most houses have paths by which warm air can escape from the living space through the attic, bypassing attic insulation, and reducing the fuel savings expected from 30 to 70 percent.

House-to-attic warm-air leaks in wood-frame houses alone result in the waste of almost 20 percent of all the home-heating energy (2 percent of all energy) used in the United States. That is equivalent to wasting more than 600,000 barrels of oil a day.

Sealing these bypasses should be among the first steps taken to improve the energy efficiency of a house. The leaks should be sealed before you add attic insulation. Once a lot of insulation is in place, it becomes difficult to find these leaks, and the job of eliminating them can be tedious.

A number of specific pathways along which warm air can escape into the attic, bypassing insulation, cannot easily be detected from within the living area of a house. Gaping holes are rare. But from the attic, often a number of cracks and openings into the house below can be seen.

In many houses, plumbing vents and electrical wiring come up inside walls and pass into the attic through openings in the attic floor. Cracks can often be found where the ceiling panels of the room below meet interior walls and supporting frames.

Gaps also may exist between the edges of the attic floor and the exterior walls. In some older houses (usually built before 1940), these gaps extend all the way to the basement. Similar gaps can be found in recently built wood-frame townhouses where the frame joins the masonry firewalls between the houses in a row. In many houses a sheet-metal flue from the furnace passes from the basement into the attic through a shaft that is open at both ends. Masonry chimneys also usually have openings around them where they enter the attic. Through all of these cracks and gaps, warm air within walls or in the basement escapes into the attic, bypassing the insulation.

All of the heat leaks mentioned above are easy and cheap to fix. The bigger openings in the attic floor can be closed by stuffing them tightly with some bulky fireproof material, such as fiberglass. Narrower openings might be easier to caulk or tape.

CAUTION: In some localities, building codes prohibit stuffing any material into the opening around the flue because of a potential fire hazard. However, our measurements indicate that stuffing the opening with fiberglass at the attic floor level would not constitute a fire hazard. Fiberglass insulation chars at around 800 degrees, whereas the flue at the attic floor never reaches even 200 degrees.

Not all of the heat-loss paths around attic insulation are easy to find. Spaces above staircase ceilings and dropped ceilings connect the attic with warm air spaces within the walls of the rooms below. However, areas above staircase ceilings and dropped ceilings cannot always be seen from the attic, if fiberglass insulation has been stapled to the wooden frame at the same level everywhere on the attic floor. Once these areas have been found, they may be easily and effectively sealed off from the attic by stapling or taping a plastic vapor barrier (e.g., a polyethylene sheet) over the opening (but underneath any insulation on the attic floor).

Another house-to-attic warm-air leak that is simple and cheap to fix is a poorly sealed attic trapdoor or entry door. A little weatherstripping plugs this leak. If the attic is insulated, the trapdoor or entry door should be insulated, too.

If heating or cooling ducts run through the attic, there should be no openings around the ducts where they pass through the attic floor. If ducts are not insulated, their joints should be checked for tightness (tape loose joints with duct tape), and then they should be wrapped with insulation. R-11 or R-19 fiberglass batts may be used for this purpose.

A whole-house fan mounted in the ceiling also may be letting warm air escape during the winter. A tight fitting insulation cover should be installed under the fan opening during the heating season. Rigid foam insulation (1- to 2-inches thick) sandwiched between layers of plywood or masonite is good for this job. Another type of insulating cover can be built by stapling fiberglass insulation above a plywood sheet cut to fit the opening. At least R-19 insulation should be used if space permits.

CAUTION: When sealing heat-loss paths into the attic, do not seal the attic vents to the outside. They are there to let moisture escape.

There are some less obvious bypasses that do not involve the attic. For example, warm air often leaks from the house through electrical switches and outlets located inside the house on exterior walls. Air leaking through these outlets can travel within the wall cavity and pass to the outside through openings in the outer surface of the wall. Depending on wind conditions, cold air from the outside can enter a house along the same path. These leaks can be stopped by foam rubber gaskets available in some hardware stores. Unscrew the rectangular plate that covers the electric outlet or switch, place the foam gasket behind the plate, and screw it back on. Incidentally, dirt behind the plate is one sure sign of a leaky outlet. Cracks where the frame sits on the foundation can be either caulked or stuffed with fiberglass insulation.

The researchers didn't look at all housing styles in all parts of the country, so there may be other bypasses not included in the list above. Even when a complete catalogue has been compiled, the homeowner will have to be on the lookout for warm air leaks unique to a particular house.