UW-Extension Cooperative Extension
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Contact: John Merrill
Entry Date: April, 2000
File Under: Housing
HOW IMPORTANT IS ATTIC VENTILATION?
Madison - John Merrill, a University of Wisconsin-Extension housing specialist, says a recent article in ASHRAE TRANSACTIONS, the research publication of the American Society of Heating, Refrigeration, and Air Conditioning Engineers, discusses whether ventilation actually provides all these benefits.
Authors Anton Ten Wolde and William Rose discuss the research on each of these benefits. The most common reason for ventilation is to prevent ice formation in the attic during the winter, when water vapor escapes from the house into the attic, condenses, and then freezes against the roof structure.
AIt turns out that ventilation can play a role in keeping attics dry in winter, but keeping indoor humidity in check is much more important, Merrill says.
For example, a 50-year-old study done in Madison found that the amount of moisture in an attic was greatest where the relative humidity in the home was highest, while the house with the least attic ventilation and the lowest relative humidity was the driest. The best ways to keep relative humidity low (around 35 percent) is to ventilate the living space, turn off humidifiers and correct basement water problems.
Using vapor barriers to keep moisture out of attics isn't that effective, since most moisture moves with air through leaks, rather than with vapor diffusion through the plaster. Merrill says this means that careful air sealing is an important step in preventing moisture movement to the attic from the house.
Cathedral ceilings can have special moisture problems, since effective ventilation is often difficult. Ten Wolde and Rose explain how partially vented cathedral ceilings can act like chimneys, sucking in humid air.
In other cases, ventilation may cause wind washing, which can reduce insulation effectiveness. Foam sprayed against the inside of the roof sheathing may be a good alternative. If this is done correctly, there is no moisture advantage to adding ventilation, Merrill says.
The research review by Ten Wolde and Rose shows that ice dams are unlikely with or without ventilation unless there is significant leakage of heat into the attic from below, either because of inadequate insulation or air leakage. Sources of heat leakage could include warm chimneys, poorly insulated duct work, leaky exhaust fans and ducts, plumbing stacks and poorly sealed and insulated attic hatches.
A third reason for ventilation is that roofs will get hotter and damper without ventilation, shortening the life of shingles. This argument has convinced shingle manufacturers who warranty their products only on ventilated roofs.
However, a study by Rose found that ventilation had minimum impact on shingle temperature. In ventilated attics, the temperature of the sheathing directly beneath the shingles was roughly 10 degrees Fahrenheit lower than it was for the sheathing above unventilated attics. Sunlight and wind were the primary predictors of shingle temperature.
The final argument for ventilation is that it keeps a house cooler in summer. However, the research that Ten Wolde and Rose reviewed make it clear that such ventilation does not have much impact on the temperature of the house below, particularly if the attic floor is well insulated. In one study that compared cooling costs for houses with various types of ventilation, a house with a powered attic vent fan actually cost more to cool.
I'm not recommending that ventilation requirements be ignored, but want to emphasize that other design and household management practices can be at least as important, Merrill says.
Attic ventilation does not eliminate the need for winter humidity control, for attic insulation and for careful blocking of air leaks.
The full article by Ten Wolde and Rose can be found on Roses home page on the world wide web at