When it comes to lowering heating bills, perhaps there's no home improvement as cost effective as adding insulation.

Homeowners who live in insulated houses pay 40 percent less in energy costs than those in uninsulated houses, according to a 1996 study by two groups: the Alliance to Save Energy and Energy Conservation Management, Inc.

The principle behind insulation is straightforward: it helps keep heat from escaping in the winter and it helps keep heat from coming in during the summer.

As unbelievable as it sounds, many houses have no insulation at all, according to Ralph Groves, who heads Rhode Island's weatherization program for low-income households.

The reason, he said, is that building codes didn't require insulation before the mid-1970s.

Only 20 percent of houses built before 1980 in the United States are considered well-insulated, according to the U.S. Department of Energy.

Almost all houses, even newer ones, can usually benefit from adding insulation.

It should be installed in the attic, in exterior walls, on basement walls, below floors and in crawlspaces.

Insulation is rated by an R, or resistance, value. The higher the number, the better it insulates.

For houses in Rhode Island, the federal Energy Department recommends R-49 in the attic and R-18 in the walls.

The three most common types of insulation are blankets or batting that come in pink, white or yellow rolls; loose fill that's blown into place with a special machine; and rigid foam boards.

Blanket insulation is usually made with fiberglass strands and can be installed in between studs in walls or between joists below floors or above ceilings. A 15-inch wide roll of paper-backed fiberglass insulation that covers 106 sq. feet, rated at R-19, might cost about $35.

Loose-fill insulation, such as cellulose, is made by grinding or shredding newspaper into tiny bits. The material is then treated with a chemical to make it fire resistant. It is often used to fill building cavities or in attics. A package of cellulose fill that covers 40 sq. ft. for an R-19 rating is about $8.

Foam-board insulation has a higher density and is often used in cathedral ceilings, doors and exterior walls. A 4-foot by 50 foot package might cost about $36.

All three types are available from home-improvement stores, such as Home Depot and Lowe's.

Which kind to use depends on what you are insulating.

For example, insulating the walls of an existing house that has no insulation is best accomplished with blown-in cellulose, said George Cross, owner of Cross Insulation of Smithfield. His company specializes in that type of insulation.

Last week, his company was blowing insulation into the walls of a one-story Cranston house, built in 1952, that had no insulation. Because there was no access to the cavities in the walls, a worker drilled 2-inch holes along the outside of the house every 16 inches.

Cellulose insulation, which comes in 30-pound bags, was dumped into a large barrel that has a machine to break up the clumps and shoot it out of a long hose. The end of the hose was inserted into each of the holes in the house, a valve was opened and the insulation was blown into the walls. Each section took about 30 seconds to fill. Afterward, the holes were plugged with rigid foam circles and the house's vinyl siding was replaced, hiding the plugged holes.

A typical house with no insulation might take 50 bags of cellulose, Cross said, adding that a typical job runs $2,000 to $2,500.

Another way insulation can save heating costs is by installing it on exposed heating-system and hot-water pipes.

Doing so can raise the water temperature by 2 degrees to 4 degrees, according to the Energy Department.

Pipe sleeves are commonly made from polyethylene or neoprene foam, and are sold at home-improvement stores. A package of 4 three-foot lengths can run from $4 to $7, depending on the diameter.

Match the diameter of the inside of the pipe sleeve to the outside diameter of the pipe you are covering. The pipes have a pre-cut slit that makes it easy to slide them onto the pipes and can be cut with scissors to accommodate turns. The sleeves can be secured in place with tape, wire or cable ties.

Cross said he recommends insulating all the exposed heating-system pipes in a basement, and insulating the first 6 feet of both hot and cold water pipes that connect to a water heater.

For more information about insulating heating pipes, see the Energy Department's Guide to Energy Efficiency and Renewable Energy at

http://www.eere.energy.gov/consumer/your(underscore)home/water(underscore)heating/index.cfm/mytopic=13060

And for more information about insulation in general, see the Oak Ridge National Laboratory's Insulation Fact Sheet at

http://www.ornl.gov/sci/roofs+walls/insulation/ins(underscore)01.html

Timothy C. Barmann covers energy issues, utilities and technology. He can be reached at tbarmann [at] projo.com