If your attic or basement is unfinished, use a can of expanding foam insulation (such as Great Stuff Insulating Foam, $5 at deerso.com) to seal holes in the attic floorboards and basement ceilings.
That could reduce heating and cooling costs by about 10%. "Electricians and plumbers turn ceilings and floors into Swiss cheese when they run their pipes and wires," says Cormany.
You may also see gaps around the chimney or around recessed lighting. Ask at the home center for heat-resistant insulation for those spots.
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