Three years ago two nearly identical one-story houses were built side-by-side at Mounty Airy, Md.

The houses -- built by David Smith as part of a study conducted by the National Association of Home Builders Research Foundation under a contract with the Department of Housing and Urban Development -- differed only in one aspect: modifications made for energy efficiency. Sophisticated monitoring devices were installed in both houses, each of which was occupied by two adults and two children.

The results of the study, contained in a booklet recently released by HUD, show that -- as compared with the conventional house -- the largest energy-saver in what was dubbed the energy efficient residence or EER was produced by turning down the thermostat on the water heater to 120 degrees. This undramatic action accounted for 28 percent of all the energy saved in the house. (The thermostat on the heater in the conventional home was kept at the more typical level of 160 degrees.)

A 22 percent saving in the EER house was achieved by using a heat pump for heating and air conditioning. The comparison dwelling had an electric furnace with separatey installed electric air conditioning.Both houses had only electricity as a utility source of energy. In the EER house, energy-saving appliances produced a saving of 2 percent of the total.

The families in both test houses kept their thermostats at 70 in the winter and 78 in summer when air conditioning was used.

More insulation was used and special measures were taken to plug cracks and holes in the EER house, which used 49 percent less energy than the comparison house in a one-year period. The difference, 12,981 kilowatt hours at the rate of $90424, produced a saving of $545.

Overall, the study showed that energy consumption in a typical new house can be trimmed 50 percent by taking actions for which the owner will recoup costs within five to six years.

Seventy energy-saving devices in the EER house were monitored. The additional cost of installing them was $5,382, which theoretically would have been repaid by savings in energy use within nine years.

However, the cost could have been cut by $2,927 to just $2,455 by eliminating two relatively expensive features that proved to be less effective than expected -- a vestibule designed as an "air lock" and a kitchen-family room heated by a recirculating fireplace unit with the help of solar gain from south-facing windows.

Ralph Johnson, president of the NAHB Research Foundation, said that some energy-saving actions of lesser importance were deliberately included in the study in order to test their practical cost effectiveness. He said elimination of those less effective features would reduce the payback period for the energy conservation features to about 5 years.

Johnson said that what he termed a short payback period, such as five years, "makes very good sense."

Adding that the needed products and material for energy-saving features are readily available, Johnson conceded that cost of such products and materials have indeed been rising but not as fast as the cost of energy.

Other findings about energy efficient residences reported by HUD:

Windows should be minimized on all but southern exposures.

South-facing glass provides passive solar gain and cuts heating bills. However, when the sun is not shining, any glass draws energy out of a home. To cut the loss, the EER house used thermally insulated draperies available in department stores.

South-facing windows should have double panes in most climates and be shielded by an overhang to avoid the high angle of the summer sun. In cold climates, other windows should be triple-glazed; east and west windows should be shielded by overhangs or deciduous trees.

Heating and cooling equipment is more efficient if undersized rather than oversized.

A heat pump is recommended for moderate to moderately cold climates if electricity is used for heating and cooling.

High-efficiency separate air conditioning equipment will save money.

In cold climates, ceiling insulation should be R-30 to R-38; frame walls should have an R-value of 17 to 19. (The R-value is a standard label used by manufacturers of insulating products, of which mineral wool in batts is typically used for ceilings and walls.)

Sealing and caulking of all construction joints is highly important.

Fluorescent lights are four times more energy-efficient than incandescent lights.

While a good source of emergency heat, fireplaces are energy losers. The EER house's fireplace had many energy-efficient features but still contributed nothing to energy saving. The conventional fireplace in the comparison house was a major source of energy loss.

Rectangular, one-story houses are the most efficient in saving energy. Reducing 8-foot ceilings by six inches also saved energy.

The research foundation is currently completing a second EER house near Damascus. The contemporary, two-level house will maximize passive solar heating, use a rock bin to store excess heat and have a roof shaft for natural summer ventilation and a heat pump that will obtain heating and cooling through a closed loop inserted in a well.

In this test house, dubbed "EER-2," large south-facing windows will admit winter sunlight into an enclosed two-story solarium that will act as a solar collector. When the glass doors of the adjacent family and living areas are open, the heat will enter the house directy. When they are closed, heat will rise to the ceiling through an opening in a sun deck and will be drawn into a duct leading to a rock bin on a lower level. Heat from the storage bin will be circulated through the house at night and on sunless days.

The EER-2 house also will have heavily insulated thermal shutters that can slide out of walls to close tightly and provide an insulating level of R-14. Triple-glazed windows provide only R-2 1/2 insulating value. A well-insulated opaque wall is rated at R-19.

A copy of the EER-Energy Efficent Resident study, which is fairly technical in nature for the average homeowner, can be obtained by writing to HUD User, Box 2080, Germantown, Md., 20767.