For more than 50 years, practice has held that the principal way to control moisture in attics has been to ventilate them.

There are other methods to effectively control moisture and incorporate attic ventilation into a broader strategy. These methods are outlined in an article titled "Venting of Attics and Cathedral Ceilings" that appears in the October issue of ASHRAE Journal, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

"While attic ventilation can be beneficial under some circumstances and climates, it should not be seen as the principal strategy to eliminate moisture and other problems in attics and rooms," said co-author William Rose, a research architect at the Building Research Council, University of Illinois, at Urbana-Champaign. "Rather, attic ventilation should be part of a broader range of control strategies."

Touted benefits of attic ventilation include reducing moisture problems, minimizing of ice dams, ensuring shingle service life, and reducing cooling load. Most building codes require attic ventilation to minimize condensation on the underside of roof sheathing, according to co-author Anton TenWolde, a research physicist at the Forest Products Laboratory, U.S. Department of Agriculture, Forest Service, Madison, Wis.

"Attic ventilation is firmly established as an important element in residential roof construction, and lack of ventilation is routinely blamed for a variety of problems and failures," TenWolde said.

Not always practical

The authors explain that sometimes adding attic vents may be impractical or undesirable. In addition, roof vents may have a role in forest fires in residential areas, allowing sparks to escape into the roof cavity and accelerating the spread of the fire due to draft in the ventilated space.

"Also, venting rules for attics have been extended to apply to cathedral ceilings but few studies have confirmed the validity of that," TenWolde said.

Rose said that other strategies have been shown to have a stronger and more direct influence on controlling moisture. Specifically, the authors recommend the following.

  • Indoor humidity control should be the primary means to limit moisture accumulation in attics in cold and mixed climates. While they recommend attic ventilation as an additional safeguard, they do not believe it should be a regulated practice.

  • To minimize the danger of ice dam formation, heat sources in the attic and warm air leakage into the attic from below should be minimized. The need for venting to avoid icing depends on the climate and the amount of insulation in the ceiling. However, ventilation is necessary in climates with a lot of snow to prevent icing at eaves, regardless of insulation level.

Venting of attics and cathedral ceilings in cold and mixed climates. However, if there are strong reasons why attic vents are undesirable, unvented roofs can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below. However, ventilation is necessary in climates with a lot of snow to prevent icing.

Ventilation should be treated as a design option in cold, wet coastal climates and hot climates. Current technical information does not support a universal requirement for ventilation of attics or cathedral ceilings in these climates.

Preventing ice dams

Although attic ventilation is now generally credited for minimizing ice dams, early requirements for attic ventilation were entirely based on minimizing condensation in cold climates. The 1949 publication Condensation Control in Dwelling Construction did not mention minimizing ice dams as a potential benefit of attic ventilation, but recommended installation of heavy roll roofing felt or sheet metal under the shingles over the eaves to protect against water leakage from ice dams.

By 1967, the "cold roof" concept had been introduced, but it was based on a combination of measures.

The importance of attic heat sources strongly emerged in a recent Canadian study of 33 houses in Ottawa. All 16 houses with ice dams had interior chimneys and their attics were about 7¿F warmer than attics of houses without ice dams. Houses with ice dams also tended to have less insulation in the ceiling and less eave ventilation, either due to fewer soffit vents or fewer insulation baffles at the eaves.

An important study of ice dams was conducted by Tobiasson et al., who observed that ice dams seldom occurred when outdoor temperatures were above 22¿F. Since ice dams did not occur when the attic air temperature was below freezing, the researchers arrived at a "window" of temperature conditions that lead to ice dams.

They concluded that chronic ice dams can be avoided with attic ventilation systems that keep the attic temperature below freezing when the outside temperature is 22¿F, so heat from below does not melt the snow on the roof.

(Researchers) concluded that the need for ventilation is related to the amount of snow to be expected in the area and the amount of insulation in the ceiling. In Philadelphia, Washington, D.C., and Chicago, roofs with at least R-20 need not be ventilated. In Madison, Wis., Boston, and Sioux Falls, Idaho, the minimum amount of insulation increases to R-30, and in Minneapolis, and Portland, Maine, it increases to R-40.

In Marquette, Mich., and Bangor, Maine, all roofs, no matter how well insulated, need ventilation to avoid ice dam problems.