Some Thoughts on Ventilation of Unoccupied Spaces


Unoccupied spaces such as attics, basements, and crawl spaces must be ventilated to remove moisture and water vapor that has entered from surrounding air or soil, or has been created by human activity. Humidity above about 65% supports the growth of molds. Insects thrive in damp environments. If vapor is not allowed to escape, and the temperature drops below the dew point, vapor will condense on building materials, causing deterioration in insulation, framing, interior and exterior paint, etc. The quantity of water vapor depends on building type (e.g., residence, school, etc.), activity (e.g., bathroom, kitchen, etc.), air temperature, and relative humidity. Generally, crawl spaces and basements require greater ventilation than an equivalent area of attic. In new buildings, minimum required ventilation is specified in the Residential Code or Building Code. Achieving these minimums is a good practice in older buildings, whether or not required by codes.1


During winter, attic ventilation cools the attic and decreases chances for condensation and damaging ice dams. During summer, attic ventilation prolongs the life of asphalt shingles by moderating excessive summer heat build-up; shingle manufacturers require a minimum amount of ventilation in order for them to honor their warrantees. Year-round, attic ventilation expels humid air that rises up through the building from the basement, reducing condensation in exterior wall cavities and helping to preserve exterior paint. Attics are usually vented through a combination of soffit vents at eaves allowing cool air to enter the attic, and ridge or gable vents at the peak allowing hot air to escape. In some historic buildings it is difficult to install soffit vents because rafters terminate on a heavy timber plate that blocks the flow of dry, cooling air; in these cases alternate provisions should be made.


Basement ventilation allows the escape of humid, musty air that promotes rot, mold, and mildew and attracts wood eating insects. In most cases, adequate ventilation can be achieved through foundation vents or screened basement windows installed to create cross ventilation. Basements are usually vented by leaving basement windows open spring, summer, and fall to promote air and vapor movement out of a building. Screens in these windows help protect against insect and animal intrusion. For safety, consider installing burglar bars as well. These can be obtained at

hardware stores and are simple to install. Wood basement windows close to grade are protected by installing window wells. Some owners use dehumidifiers to reduce humidity in basements. When dehumidifiers are used, basement vents or windows should be closed. Effectiveness of basement dehumidifiers is usually marginal and costs to operate, high; Bero Architecture does not recommend their use except as a last resort. Neither ventilation nor dehumidifiers will ameliorate standing water; the source of the water should be investigated and, when possible, eliminated. Properties located on level sites and in areas with a high water table sometimes require perimeter drains leading to a sump pump but exterior moisture sources should always be controlled 2 before resorting to a drain and sump pump system.


Crawlspaces typically have dirt floors and lack windows and so tend to be damp. There are two methods of introducing air for ventilation: from the out-of-doors via openings in the foundation wall; or from the adjacent basement by openings in the shared basement wall. Because of modern concerns for energy conservation, owners often wish to add insulation to reduce heat loss through the floor above. If ventilation is introduced through the exterior perimeter wall, insulation can be added between the floor joists, providing a cool crawl space. If ventilation is introduced from the basement, insulation is usually added to the crawlspace perimeter foundation wall, providing a warm crawl space. A cool crawlspace is more efficient but is not suitable where water pipes excluded from the heated envelope may freeze.

1. Please refer to Bero Architecture’s “Some Thoughts on New York Building Codes

2. Please refer to Bero Architecture’s “Some Thoughts on Stormwater Management


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