Managing attic moisture is a balancing act

Energy Q & A

Q. We are having a problem with moisture in our unfinished, unheated attic. The moisture will freeze on the inside of the attic roof and make a mess when it thaws and runs down into the house in the warmer weather. We have tried recommendations from a couple of local contractors, the local hardware supply business and an insulation specialty business without success. Among other measures, we have tried an attic fan, increased insulation, rafter vents and caulked openings to the attic, as well as making a bonnet for the attic entrance. There is a plastic vapor barrier between the second floor and the attic as well as paper-backed insulation on the knee-high wall on the sides of the second floor.

Are there other measures a person can take to eliminate the moisture in an attic?

Fred & Pat Urban

Sullivan, ME

Answer provided by Richard C. Hill of Old Town, emeritus professor of mechanical engineering at the University of Maine.

A. To understand moisture problems in attics, two experiments should be conducted. On a windless day, crack open a window in the basement or first floor and cover the opening with a paper towel. The towel will blow into the room indicating that outside air is coming into the building. If a similar test were conducted on an attic vent, the towel would blow to the outside. The dwelling acts like a chimney: pulls air from the lower levels of the building and discharges at higher levels.

A second test involves the use of a polished chrome coffeepot filled with ice and water. Does the outside of the pot “fog over,” indicating the condensation of moisture? Such condensation shows the inside relative humidity (if the inside temperature is about 70 degrees) to be above 22 percent.

We learn from the first experiment that room air tries to work its way up into the attic. From the second experiment we learn that if this air comes into contact with a surface below 32 degrees the water vapor associated with that air will condense. The initial sign of this condensation will be frost on the roof shingle nails; next the roof boards; then the roof rafters will show frost. Nearly every house in Maine will show some frost in the attic this time of year. (The shingle nails show frost, not ice or water, because the water phase change takes place below the triple point – the vapor phase moves to the solid phase without passing through the liquid phase.)

One would expect that moisture thus formed in the attic would melt and soak the wooden structure with subsequent mold and rot. This seldom happens for three reasons.

1) Most attics are ventilated with outside air. Slots in the soffit and in the ridge will allow outside air to sweep through the attic and evaporate the moisture formed.

2) Serious mold or rot requires extended time exposure of wet wood to temperatures above 50 degrees. The sun, along with the ventilation cited above, will dry the roof before serious damage can be done.

3) Most buildings have barriers that prevent moisture-laden air from entering the attic space.

A successful management of attic moisture becomes a balancing act. How much attention has been paid to limiting moisture-laden air from entering the attic versus how much ventilation air has been available to purge the attic moisture.

But, at times, the simplicities above may be complicated by one or more of the following:

1) Dryer vents, bathroom vents, kitchen exhaust systems must be vented to the outside – not vented into the soffit or attic space.

2) Humidifiers should not be used under any circumstances – to use them is just asking for trouble.

3) The exhaust of direct vent heating systems are often pulled into the soffit with subsequent condensation in the attic.

4) A dirt crawl space, or a concrete floor without vapor barrier can put much moisture into dwelling air.

5) Never use unvented combustion appliances.

6) Many attics are complicated with dormers and knee-walls. The management of barriers to air migration and ventilation is most difficult.

7) Cathedral ceilings present hard-to-manage constructions. The flow of humid interior air and the purging by outside air is awkward to arrange.

And, of course, the problems may be as simple as roof leaks or ice dams that cause wet attics.