Homebuilders will soon be able to use a new set of Department of Energy guidelines to calculate the monthly energy savings to homeowners of various conservation devices.

The voluntary standards will allow builders to determine whether the savings produced by options such as space heating and cooling, water heating and energy conserving appliances will be greater than the additional mortgage payments borne by the home buyer to cover their costs.

These residential building energy performance guidelines, recently revealed by the Department of Energy in early draft form, are the second in a series of aids for builders. Originally conceived as mandatory standards, the guidelines for residences follow similar rules for manufactured housing, published in the May 9 Federal Register and still open for comment.

The third element in the set, which is yet to be released, is a series of recommendations to the American Society of Heating, Refrigeration and Air Conditioning engineers for changes in the so-called ASHRAE-90 standard for commercial buildings.

Only federal office buildings and four-plex and eight-plex federal housing will be required by law to abide by the sets of guidelines.

The guides will enable a builder to estimate, in different geographic regions of the country, the comparative values of different energy-saving options in virtually any combination. The accompanying worksheets will then permit a builder to calculate the relative savings in dollars for any combination of these design aspects.

The heart of the guidelines will be a heating and cooling "calculator" with several adjustable slide rule tabs that show how one option affects all others. No electronic calculators or computers will be needed.

The guidelines will come with different sets of these sliding tabs for various residential building types--ranch house, two-story bi-level, split-level, end unit town house and center unit town house--and separate sets of tabs for each of 45 locations around the country. Even greater precision will be possible by using the guides' tables of "location multipliers," which pinpoint climatic conditions for a total of some 1,100 cities and towns.

Despite the seeming complexity of this arrangement, a full calculation of energy options should not take very long, its developers say. The heating slide rule calculator allows a builder sequentially to adjust tabs representing ceiling insulation, wall insulation, floor and foundation insulation, infiltration levels, floor area, window area and equipment efficiency to obtain a single reading of energy savings. By turning the slide rule sheet over, the same calculations can be made for cooling.

With a very rudimentary understanding of the energy guide labels that come with major household appliances, calculations can also be made for the use of different refrigerators, dishwashers, and clothes washers and dryers. Then with the help of the accompanying worksheets, estimations of savings will be possible for solar domestic water heaters, movable insulation, nighttime setback thermostats, roof and wall coloring, whole-house fans, and such passive solar options as direct gain and attached sunspaces or greenhouses.

Emanuel Levy of Steven Winter and Associates, who helped prepare the guidelines under contract to the Energy Department, explains that the slide rule calculator is used a first time to find the energy efficiency of a conventional set of design options. Then, the procedure is repeated for each change sought, because the tab settings are cumulative--that is, a change in the first tab produces changes throughout all others to follow.

Levy notes this enables a builder to prepare a package of special options for conservation, one at a time, for both maximum savings and higher profits. This way, he says, some highly favorable items can be grouped with more marginal energy savers so that the monthly energy bill can still be reduced by more than the increase in the monthly mortgage payments to cover the extra expenses.

For example, a builder will be able to trade off one energy efficiency step for another, to find the ones that save the most energy for the dollar or to present a beneficial package centering around a buyer's preference, such as solar water heaters or a greenhouse.

As good as this sounds in theory--and it may well yet prove to be as accurate and helpful as claimed by Levy, DOE and the National Institute for Building Sciences, which also participated in the guidelines--some building industry representatives are clearly unhappy with the way the work was prepared.

Stanley L. Matthews, vice president for technical services and governmental affairs with Rockwool Industries Inc., of Denver, Colo., was the most outspoken critic when DOE and NIBS discussed the guidelines at a recent briefing for the housing trade. Matthews contended that there are errors in the guidelines' descriptions and diagrams of how to install insulation in a home.

Some procedures show floor joists and wiring in the wrong place, he said, while others show insulation methods for types of convenience outlets that are no longer made. Still other parts of the guidelines offer advice on the use of 2 by 6 studs that are not readily available, he contended.

Matthews acidly commented that information on proper procedures can be obtained from the National Association of Home Builders research foundation, and the agency could save 37 pages of paper in the document.

Moreover, he argued, when extra insulation is installed in a house, a builder who is as conscious of dollar savings as he is of energy savings might be able to substitute smaller ductwork and down-size the equipment without short-changing the house's performance--a choice not offered by the guidelines.

Matthews further disputed the value of having the guides include calculations for putting moveable insulating shades and shutters over windows because their effectiveness relies on the diligence of the homeowner in positioning them on a daily basis. (The findings of a recent survey on owners of passive solar homes in Virginia that was presented to the 1983 annual meeting of American Solar Energy Society would tend to allay those fears considerably).

Another critic at the briefing, held at NIBS' Washington headquarters, noted that the guidelines make no allowances for using different levels of insulation in the same wall or ceiling, such as R-19 insulation in a cathedral ceiling and R-38 in a level ceiling in the same house.

Levy admitted that errors in the guides were expected to come to light at the briefing and during the upcoming 60-day comment period once the guides are published in the Federal Register in September.

However, he defended the overall structure, extent and flexibility of the guidelines. Allowing for calculations of different ceiling insulation in the same house would make them too complex, he said. Builders should be able to make their own determinations of equipment sizing relative to the conservation options, he added.