For a long time, advocates of reusing old buildings have been basing their case, at least partially, on the conventional wisdom that old buildings save energy.

A common-sense assumption has been that it takes more energy to tear down an old building to build new one than to renovate.

A major study, released recently by the Advisory Council on Historic Preservation, backs up that belief. Based on recent experience, the study showed that it does take more energy to construct a new building than to rehabilitate an existing one.

The consulting firm that carried out the study developed a series of mathematical formulas to measure the energy investment in various kinds of buildings.

The basis for the study was the concept of "embodied energy." That concept recognizes that each brick, block and 2x4 represents a specific amount of energy measured in BTUs (British Thermal Units).

It takes energy to fell the tree, turn it into lumber or plywood, ship it to the building site and put it in place. In fact, about 5 percent of the energy consumed in this country each year is used in making of materials and construction of new buildings.

Replacing all buildings in the United States would require the energy output of the world for one year - just for materials and construction.

The study looked at a number of dissimilar preservation projects to test the formulas. The conclusion was that rehabilitation can produce significant energy conservation benefits. Among the projects studied:

Lockfield Garden Apartments in Indianapolis, one of the first government-sponsored, low-income housing projects. It represents more than 550 billion BTUs of embodied energy.That is the equivalent of 4 1/2 million gallons of gasoline. Rehabilitating the complex could require only one-third as much energy for materials and construction. The energy savings would equal 2,250 billion BTUs, or 2 million gallons of gasoline.

The Grand Central Arcade, an old hotel in the Pioneer Square area of Seattle, which is filled with shops and offices. It was rehabilitated in 1972 and required 20 of the energy a new building would have taken. That saved 700,000 gallons of gasoline, more than 90 billion BTUs.

A carriage house owned by Nancy M. Austin on 11th Street SE Capitol Hill, the smallest structure in the study. A two-story brick building, it represented more than 1 billion BTUs, or 8,000 gallons of gasoline. Even with the amount of materials required to convert it to a three-story apartment building, it consumed half the energy a new building would have.

The advisory council found that these savings took place not only at the time of rehabilitation. Properly done, old buildings will consume about the same amount of energy as new buildings, the researchers said. The Austin house, for example, uses about 5 percent less energy than a new building because of its energy conversation design, double-glazed windows and extra insulation. Over a 30-year period it will save enough to heat and cool and equivalent building for 10 years.

And Grand Central Arcade has a net energy investment advantage over a new building for the next 200 years.

Of course, energy investment is not the only factor to be considered in a decision to retain or demolish as old building. The advisory council has had very few tools or techniques to use in determining the relative worth of old and new buildings. The formulas developed as part of this study provide just such a tool.

The advisory council, an independent federal office established under the National Historic Preservation Act, advises the President and Congress on historic preservation matters. It also has a special responsibility to review and comment on federal and federally assisted projects, such as dams, highways and office buildings, that threaten historic spots.