By Ben Schiller

If you knew how much heat was exiting your house, and where it was emerging from, you might do something to plug the gaps. That's the proposition of a startup that's heat-mapping big swathes of U.S. cities and suburbia.

While we're sleeping, Sagewell sends out a Ford Escape that roves streets, taking thermal pictures. To its equipment, your house looks like an fuzzy purple and yellow blob--the yellow bits showing where heat is escaping, and wasting your money.

At a rate of up to 20,000 homes a night, Sagewell already has about 500,000 homes, from 10 states, mapped so far. And the plan is to go much further. Chief operating officer Brad Harkavy says by the end of the winter, the S.U.V will have doubled its area-coverage.

Harkavy says the pictorial evidence helps convince people that insulation is worth it. Finding out that many states have generous subsidies for efficiency improvements also helps. In Massachusetts, for example, utilities pay 75% of the cost. He recently insulated his three-story home in Cambridge (built in 1895) for a personal cost of $700; his energy supplier chipped in with another $4,000, or so. It will take him three years to pay back the $700, he says; the whole amount might have taken 10.

State law forces utilities to set aside efficiency funds, but the money comes from customer's bills, Harkavy says. "Every householder pays into this fund. So, if you are not capitalizing on that, you're stupid."

Householders go to Sagewell's site, type in their address, and bring up their images. The company makes money by referring insulators or window suppliers, or by going through local community organizations. The most common work is to blow cellulose into gaps on the front of the house (Harkavy recommends doing it before painting).

Some people have been "weirded-out" by the surreptitious filming, Harkavy admits. But he says the equipment looks only at the outside--"it doesn't go through the walls." Also, the images are available only to owners, and not the general public.

So far, Sagewell has concentrated on colder states (e.g., in the Midwest), or the coastal states, where environmental policies, and incentives are stronger. Homes in the South tend to be younger, and better insulated, Harkavy says. Eventually, you could imagine much of America heat-mapped--much like it's been Google Street View-ed. If so, the maps could provide a strong catalyst for energy efficiency, with benefits to many.

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