WHEN Deb Venn of Vergennes, Vt., bought her house in 1989, she was able to purchase it with a unique kind of mortgage that enables her to save big bucks in thermostat-breaking energy bills.Ms. Venn qualified for the house through an energy-efficient mortgage loan - a loan that allows a buyer to add the cost of energy upgrades to the mortgage. For Venn, the process worked like this: After she decided to buy her 100-year-old house, her lender ordered an energy rating along with the appraisal. Energy Rated Homes of Vermont, a nonprofit group that has done home energy ratings since 1987, gave the house a one-star rating, revealing that it would be very expensive to heat. The home would have to be upgraded to a four-star ranking for Venn to qualify for the energy-efficient mortgage. Costs for the improvements totaled $8,000. Venn, a carpenter who did some of the work herself, had to insulate the walls and ceiling, install an efficient central heating system, and weatherstrip the windows and doors. Though Venn's monthly mortgage payments increased, she cut her energy bill in half. What she saves in energy costs actually exceeds the additional amount she pays on her mortgage. "It's a real conscientious thing to be doing," says Venn, who says much of the housing in the United States is becoming more unaffordable to a growing number of people because heating costs are so expensive. Energy-efficient mortgages (EEMs) have existed since President Carter ordered the program in 1979. Four federal lending agencies - the Federal National Mortgage Association, the Federal Home Loan Mortgage Corporation, the Federal Housing Administration, and the Veteran's Administration - all have guidelines for underwriting such mortgages. Yet out of 6 million loans each year, only 2,000 are EEMs, says Jim Curtis, president of Bay Area Energy Consultants in San Francisco. Mr. Curtis has helped push throu gh 3,000 EEMs since 1982. Curtis and others championing EEMs offer several reasons why so few have been written: * Many lenders, unaware of EEMs, don't inform buyers. * Lenders and realtors have few incentives to promote them. * Assessing what improvements are needed slows down the loan process. Realtors generally want to close on a house quickly. * The majority of states have no home energy rating system, such as Energy Rated Homes of Vermont, that gauges energy efficiency and determines what upgrades are required. These barriers may soon crumble. The Department of Energy, the Department of Housing and Urban Development, mortgage associations, special-interest groups, and building agencies have been collaborating this year on how to integrate energy-efficient mortgages and home energy rating systems on a national level. The group is expected to announce a strategy in January. Right now, the four federal lending agencies all have different guidelines for underwriting EEMs, and "none of them are user-friendly," says Richard Faesy, director of Energy Rated Homes of Vermont and a participant in the collaborative effort. EEMs have been particularly successful in Vermont because many lenders, realtors, and builders are members of Energy Rated Homes of Vermont. In other states rating agencies are not closely connected with EEMs. About 10 states have energy rating systems, and 19 others plan to develop them, says Ron Hughes, president of Energy Rated Homes of America, the umbrella organization for these rating systems in Little Rock, Ark. The group's goal is to start them in every state. "You're setting it up to fail if you 're try to do an energy-efficient mortgage without a rating system because it is totally confusing about what 'efficient' means," Mr. Hughes says. With EEMs, someone with a lower income can afford a larger mortgage than they normally could because lower-than-average utility bills leave them with more money each month. Venn, for example, was able to qualify for her home by paying 40 percent of her income, instead of the standard 33 percent.