Too airtight, and mechanical ventilation needs be introduced to a house. Too loose, and the sources of heat loss need to be corrected.
The vast majority of houses? Too loose. And if yours is one of them, 40 to 60 percent of the heat your furnace produces will go through the roof and out walls and windows.
Not a good thing, considering what it will cost to heat a house this winter because of significantly higher prices for heating oil and natural gas. As all that warm air flows out of a house, cold air is pulled in to replace it, since the air pressure inside always has to equal the air pressure outside.
What's a homeowner to do?
Take a whole-house approach.
"Generalizations can't be applied to every home," said Andrew Rudin, director of the Interfaith Coalition on Energy, which helps churches and synagogues cut costs. "Homes not only have unique designers and builders when they are built, they have been remodeled and renovated over the years. Plus, the attitudes and behaviors and number of people living in them vary with each home."
Turning up your furnace won't do much good if the heat it produces goes up the chimney. You've got to figure out how the heat escapes, and block the exits.
Because heat rises, the most likely suspect is your roof. "That's why most of us wear hats when we go outside on a winter day," said Liz Robinson, executive director of the nonprofit Energy Coordinating Agency (ECA) of Philadelphia.
In a sense, when you seal and insulate an attic, you are putting a wool hat on it. But often attics are insulated without being air-sealed first, Robinson said. Without sealing, insulation doesn't do its job.
"Before you insulate, you need to use foams and sealants to close any penetration to the outside, such as at the ends of joists at the front and back of the house," she said.
