Attic and crawl space vents are incorporated into buildings to help remove excess moisture that can move into these spaces, either from the soil (in the case of a crawl space) or from the living space (in the case of attics and cathedral ceilings). Depending on detailing used, air from the crawl space can also communicate directly with the attic space. Movement of moisture can be via diffusion through materials, or convection through air leakage. Moisture movement via diffusion is a slow process. Moisture-related problems are more likely related to air leakage.

Leaks in the building enclosure (roof or exterior wall) can also result in elevated moisture conditions. Leaks are the cause of most moisture-related problems in buildings. Depending on the size of the leak, vents may or may not be able to provide sufficient drying capacity to avoid moisture-related damage.

There are several kinds of vents. In crawl spaces two or more vents, to allow for cross ventilation, are used and should be located on at least two opposing walls. In attics and cathedral ceilings, air inlet and air outlet vents are used. Inlet vents are usually in the eave and soffit area. Outlet vents are placed at or near the ridge line of the roof. A ridge vent and through-roof vents can be seen on the roof of the building shown here.

A strip vent in a soffited eave and frieze block vents in an open framed eave are shown here.

Vents are vulnerable to wildfires when embers and flames enter into the enclosed attic or crawl spaces, and ignite combustible materials in those areas.

This is a photograph from a test conducted in a fire research laboratory. It shows fire penetrating a frieze-block vent with flames entering into what would be the attic. Vent screens don’t impede flame entry. Wind-blown embers can also enter through vents, and depending on the ember size and quantity, and the amount of combustible materials in the attic or crawl space, ignitions can occur.

Example of fine fuels in attics. Embers will ignite finer combustible materials which, if sustained, can in turn ignite larger structural wood members and other combustibles in these spaces.

Laboratory studies have shown that embers passing through ¼” mesh screen have sufficient energy to ignite fine fuels (cotton and shredded paper). Screening was originally used in vents to prevent rodents from entering these enclosed spaces. Vulnerability to wildfire exposures wasn’t a consideration at the time.

Whereas finer mesh screening would minimize the chance of embers resulting in ignitions (smaller embers would have less energy), building officials are concerned that finer mesh screens can become plugged with debris, or paint, thereby having a negative effect on the ‘moisture management’ function of vents. The 1/16” mesh screen shown here has been covered up by paint.

Because of the wildfire threat, smaller mesh screens are starting to be allowed. This will require the home or building owner to be more careful, and not allow paint or debris to plug up vent screens. Other vents, specifically designed to resist the entry of embers and flames, have been developed. More information on these vents is provided in another section.