Q: We have a problem with condensation in our attic each winter. All vents seem to be open and free of obstructions. I have double insulation in about half the attic. During winter, water drips from the bath vents on the upper level even when they are not used for long periods. The attic also has ice buildup on the wood under the shingles. Help!
A: With proper venting, moisture should vent to the outside and not collect in the attic. Estimate the total square footage of the attic floor space. Divide by 150, and this will give you the total square footage of free venting the attic should have.
If the insulation on the attic floor has a vapor barrier that resists moisture migration (the vapor barrier has to face the heated side of the home, so you will have to dig down in the insulation to find it) you can divide the square footage of the attic by 300. Half of the venting should be at the eaves or overhang of the roof, and the other half should be at the gables or near the peak (ridge) of the roof.
Free venting means the area open to vent moisture or air. Most vents have a screen or other barriers to prevent pest and water entry. These block airflow and have to be taken into account when figuring the square footage of venting. For instance, a typical square roof vent covers a one-square-foot opening, but the vent is rated at one-half a square foot of free venting. Likewise, a foot-long piece of ridge vent has only 13.5 square inches of free venting, while a 12-by-12-inch piece of vented soffit material has around nine square inches of free venting.
Assuming your attic is vented properly, the next step is to find and reduce the sources of moisture inside the home.
Major contributors to high moisture levels are bathing, laundry, cooking, cleaning, flooding, water leaks and blocked flues on gas- or oil-fired appliances. Check the basement or crawl space for signs of flooding.
Evaporated water migrates through the home, condensing on the cold attic decking in winter. Improperly vented fans force moisture into the attic. A typical dryer load produces 38 pounds of moisture.
Vent hoses in the attic should be under the insulation and vented to the overhang area. Hoses that extend to the roof's vents are cold and will allow condensed vapors to fall back into the fan, where a stain will be visible on the ceiling. Calculate the venting needed, make sure the foundation is dry and make sure all vent hoses and flues vent to the exterior.
Have the furnace, water heater and fireplace flues checked annually for possible blockage or back-drafting problems. Also make sure the plumbing vent pipes vent through the roof to the exterior.
(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at PO Box 268, Evansville, IN 47702 or send him e-mail at d.barnett@insightbb.com.)
Resources American Society of Home Inspectors (ASHI)
To locate an American Society of Home Inspectors inspector near you, call or visit the website of ASHI.
American Society of Home Inspectors
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