Fixit: What should attic temperature be?

Q I have a remote thermometer in the attic. What should the attic temperature be, relative to the outside and house temperatures, assuming a new, supposedly well insulated house?

A Check the temperature an hour before sunrise.

In an ideal world, the attic temperature would be the same as the outside temperature. In reality, however, variables get in the way. For example:

• Snow acts as an insulator. As snow accumulates, it alters the temperature profile of an attic and roof surface.

• Snowpack on a roof, covering vents of any type, alters the airflow from ventilation, thus altering the insulating value of attic air and the temperature of attic air.

• The roof assembly: shingles, roofing papers, framing and deck have thermal resistance and possess thermal mass.

• The roof is a solar collector. Evidence is the sun melting snow on very cold winter days.

All of these -- roof structure, snow and air -- have "thermal mass," the specific heat capacity of the materials, the amount of energy absorbed or released, per pound, per degree of temperature rise or fall of the material. This thermal energy flow through the materials acts as a flywheel, slowing the rise and fall of temperatures.

On top of all this, add air leakage: Small air leaks, even in the tightest of attics, move lots of thermal energy. This warm air leakage increases as outdoor air temperature drops.

Because of all the variables, getting attic temperatures as close to the outdoor temperature an hour before sunrise is a great goal.

Keep a journal of outdoor and attic temperatures with notes on snow cover, snow blockage of roof vents and wind speed.

If you ever have an opportunity to have a blower door test on your home (often done as part of a home energy audit) ask for an "attic series leakage test." It's a measure of the leakage from within the home to the attic space.

The Minnesota Office of Energy Security offers free publications that provide good information on this subject and others regarding housing in a cold climate. Look for "House Diagnostics," "Home Insulation" and "Attic Air Leaks and Ice Dams" at www.energy.mn.gov. Or call 651-296-5175 or statewide toll-free at 1-800-657-3710.

Phil Smith, Energy Specialist, Minnesota Office of Energy Security

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-7032, or e-mail fixit@startribune.com. Past columns are available at www.startribune.com/fixit.