Roof and crawl space vents are required by most building codes. The function of the vents is to remove excess moisture from those spaces. Moisture can enter the crawl space from the soil or through the foundation wall from the surrounding landscape. It can also enter the attic space if you have a roof leak. Water can also flow upward through your ceiling in the living space of the house. If too much moisture accumulates, then fungi can grow, resulting in mold or decay growth.
In crawl spaces, cross-ventilation is called for (meaning that, ideally, vents will be present on all sides of the crawl space), however, if your house is built on a concrete slab, or over a basement, you won't have crawl space vents.
Attics will usually have both inlet and outlet vents. Inlet vents, such as in soffits (eaves) are usually located on the lower portion of the roof.
Crawl space vents are positioned at different locations along the perimeter wall.
Several types of vents are used to ventilate attic spaces, including:
- Soffit vents (there are different kinds of soffit vents, with the common feature being their location along the eave of the house)
- Through-roof vents (also known as ‘eye-brow’ or 'dormer' vents)
- Gable-end vents
- Ridge vents
A 'strip vent,' which is commonly found in soffited eaves, such as those shown in this photograph.
A through-roof ( or 'eye-brow') vent, usually located near the ridge (or peak) of the roof, but can be located anywhere on the roof surface.
A gable-end vent, usually located just below the ridge of the roof.
A ridge vent (as seen from the side of the roof) is found along the entire ridge of the roof. Baffles along the front edges of the vent keep rain from entering and provide for a negative pressure region that helps pull air out of the attic.
Evidence from recent wildfires in the West has shown that vents are an entry point for embers and flames. Embers can 'rain' on and around homes for hours before and after the wildfire flame-front reaches and passes your house. Embers that enter your attic can ignite construction materials and other items you may have stored there. Flames can also enter if embers ignite near-home vegetation or debris that has accumulated on a deck.
Locally generated embers and flame can enter through vents. Examples are shown below.
This plant immediately under the a vent (and next to a single pane window) could be a problem if it ignites.
Needles (debris) from nearby pine trees have accumulated on the lower roof section and could easily ignite from embers. The resulting embers and flames would thereafter expose the gable-end vent to embers or flames.
If ignited, this trellis vegetation could also expose the gable-end vent to embers or flames.
Most vents incorporate a screen at the inlet. Most building codes stipulate a minimum mesh size of 1/8-inch to minimize plugging of vent holes and reduction in air movement. Smaller mesh screen is easier to plug up, whether by air borne debris, or as shown in the photograph below, being painted over during routine painting.
- Inspect and maintain vegetation in the vicinity of under-eave vents. Remove highly combustible vegetation.
- Clean vents on a regular basis to minimize build up of debris in the mesh.
- Remove debris that accumulates near roof vents. This includes grounds near crawlspace vents.
- Consider preparing vent covers that can be temporarily installed when a wildfire approaches your home. Vent covers can be manufactured from plywood or other solid substance that would provide short term protection from embers and flame.
In some new construction, under-eave vents have been eliminated. In those cases, the inlet vent function is being performed by using through-roof vents located in the lower region of the roof (i.e., near the roof edge). In California, some vents have been accepted for use in the under-eave area. See information about these vents on the Office of the State Fire Marshal website.
New homes that incorporate unvented attic spaces into the design are currently available, and are being built in some locations. This construction option may be more widely available in the future, but shouldn’t arbitrarily be implemented in existing homes because of moisture-related durability problems that could develop.