QUESTION: I am remodeling my house and don't want moisture in the attic. I plan to have 15-inch insulation in the attic.

Last year, I installed a new power roof fan and followed the recommendation from the manufacturer that the thermostat should be set at 104 degrees. I have done research on whole-house fans but don't like that they require open windows.

My point is that they do not say much about reducing heat in the attic in the summertime, and I want to know why they recommended that the fan be set at 104 degrees instead of 90 degrees as you have mentioned.

ANSWER: The purpose of the attic fan is to move air through the attic to remove both hot air and moisture.

Warm air can hold more moisture than cold air, so the fan is doing double duty in the summer.

If the fan is on a thermostat set at 90 degrees, it will work longer and remove more air. In the winter the warm, moist air inside the home will rise, passing through the ceiling coverings into a cold attic space where the moisture is released.

If the fan has a humidistat or is simply left to operate all the time, then a majority of the moisture will be pulled outside of the attic space. Moisture left in a cold attic space can damage the wood roof framing and the attic insulation. Once the insulation is damp it will be forever.

I have seen rain-like droplets on the surface of attic insulation below the cracks in each piece of roof decking where moist attic air had condensed back to liquid.

A gable vent fan is preferable over a roof-mounted fan simply because any penetration to the roof is a potential source for a leak and one fan should be sufficient for most attic spaces.

The fan should be sized for the attic space it serves. A typical attic fan will pull anywhere from 1,050 CFM (cubic feet or air per minute) for a 1,500-square-foot attic, up to 1,600 CFM for a 2,280-square-foot attic.

No attic should have more than one fan, which would pull too much air from the attic space. If the attic has a negative pressure, it will pull conditioned air from the living space below.

All attics need intake ports either at the opposite gable end or at the soffits (the overhang of the roof), but not from the interior of the home. To prevent conditioned air loss, the attic access should be sealed and insulated, ceiling light fixtures should be caulked, and recessed light fixtures should be insulated only if they are designed to be covered.

Also consider a ridge vent system only if you have adequate soffit vents. A ridge vent provides airflow via thermodynamics, that is warm air rises.

Each rafter space will be cool and dry with a ridge vent.

The airflow through a ridge vent is free whereas an attic fan draws electricity and the motor will fail sooner or later, usually sooner, and will have to be replaced.

(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 286, Evansville, IN 47702.)