Crawlspace Ventilation Vs. Closed Crawlspaces

// // Pic 1 After (2)Pest management professionals (PMPs) first began installing moisture ground barriers and ventilation back in the mid-1960s, after the then-National Pest Control Association, the U.S. Department of Housing and Urban Development (HUD) and the U.S. Federal Housing Authority (FHA) jointly made the first recommendation of using ventilation to remove excess moisture from crawlspaces.

Later, HUD developed the recommendation of 1 sq. ft. of ventilation per each 1,000 sq. ft. of surface area in the crawlspace area. Still later, the recommendation was amended to 1 sq. ft. of ventilation to 1,500 sq. ft. of surface area in crawlspaces when 70 percent to 80 percent of the ground surface was covered with polyethylene and vents were located on at least three sides of the crawlspace, allowing for cross-ventilation.

In 2000 the now-International Residential Building Code (IRBC) again changed the code to not require ventilation as the sole recommendation in crawlspaces. Below is the actual code as it applies to single residences:

Ventilation. The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement or cellar) shall be provided with ventilation openings through foundation walls or exterior walls.  The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 m2 for each 100 m2) of under-floor space area.  One such ventilating opening shall be within 3 feet (914 mm) of each corner of said building.

Openings for under-floor ventilation. The minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m2) for each 150 square feet (100m2) of under-floor space area. One such ventilating opening shall be within 3 feet (914mm) of each corner of the building. Ventilation openings shall be covered for their height and width with any of the following materials provided that the least dimension of the covering shall not exceed ¼ inch (6.4mm):
1. Perforated sheet metal plates not less than 0.070 inch (1.8 mm) thick.
2. Expanded sheet metal plates not less than 0.047 inch (1.2 mm) thick.
3. Cast iron grills or grating.
4. Extruded load-bearing brick vents.
5. Hardware clothe of 0.035 inch (0.89 mm) wire or heavier.
6. Corrosion-resistant wire mesh, with the least dimension being 1/8 inch (3.2 mm).

1. Where warranted by climatic conditions, ventilation openings to the outdoors are not required if ventilation openings to the interior are provided.
2. The total area of ventilation openings may be reduced to 1/1,500 of the under-floor area where the ground surface is treated with an approved vapor retarder material and the required openings are placed so as to provide cross-ventilation of the space. The installation of operable louvers shall not be prohibited.
3. Under-floor spaces used as supply plenums for distribution of heated and cooled air shall comply with the requirements of section M1601.4.
4. Ventilation openings are not required where continuously operated mechanical ventilation is provided at a rate of 1.0cfm (10m2) for each 50 square feet (1.02L/s) of under floor space floor area and ground surface is covered with an approved vapor retarder material.
5. Ventilation openings are not required when the ground surface is covered with an approved vapor retardant material, the space is supplied with conditioned air and the perimeter walls are insulated in accordance with Section N1102.1.7.

The IRBC did not do away with ventilation requirements. It just set up the five exceptions listed above. In another section of the code it does say the crawlspace has to be accessible, or there must be a way to get into the crawlspace.

The real problem is that the Energy Commission is recommending crawlspaces be closed or sealed.

Research Findings

Studies over the past 20 years have shown that in areas of the country where the relative humidity reaches more than 64 percent during certain periods of the year, open vents can allow more warm air into the crawlspace, increasing the moisture in that area. Closing the vents, covering the ground 100 percent with 6 mil clear polyethylene and covering the inside foundation walls up to four in. below the sill plate with clear 6 mil polyethylene, results in less moisture.

Provisions need to be made at the foundation wall ground junction for removal, if treatment for subterranean termites should ever become necessary. Installation of a dehumidifier may be recommended during this closed crawl procedure to quickly reduce the amount of moisture that can accumulate in the crawl space.

This map offers a visual of the areas of the U.S. where closed crawlspace procedures may be beneficial in reducing moisture in crawlspaces:

climate map

This map is an example and does not mean to indicate the relative humidity in any specific area. Areas 1, 2, 3 and 4 have a moisture potential that could be addressed by the closed crawlspace procedure.

Several state regulations still require ventilation, and in some states there are provisions for exceptions to ventilation. In some states, counties, parishes and cities, building permits may also be required. Check with your state rules, regulations and officials prior to initiating the closed crawlspace program in your market.

For addition information, visit these online resources:

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