Before taking steps to improve crawl space ventilation, comply with these requirements.
ü Install a ground moisture barrier as specified in “Crawl Space Moisture and Safety Issues” on page 34.
ü The crawl space should have an access hatch or door that is 24 inches by 18 inches.
9.6.1 Naturally Ventilated Crawl Spaces
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SWS Detail: 2.0401.1 Air Sealing Moisture Precautions, 2.0403.3 Closed Crawl Spaces—Vapor Retarders on Walls |
When insulating the floor, the crawl space is usually ventilated naturally through passive vent openings in the foundation wall. A ground moisture barrier protects the floor insulation and other building materials from moisture. The vent openings can remove small amounts of moisture from the crawl space. Two specifications apply to ventilated crawl spaces.
1. A crawl-space with a ground-moisture barrier may have vent openings equal to 1 square foot of vent area to 300 square feet of crawl-space floor area. A minimum of two vents should be installed on opposite corners of the crawl space.
2. In a dry crawl space with a ground-moisture barrier, ventilation openings may be minimized to one square foot of net free ventilation area for every 1500 square feet of crawl-space floor area, according to the 2012 IRC.
9.6.2 Power-Ventilated Crawl Spaces
The 2012 International Residential Code (IRC) allows you to seal the crawl-space vents when you insulate the foundation walls. These three specifications apply to unventilated, power-ventilated, or conditioned crawl spaces.
1. If you removed moisture sources like standing water and installed a ground-moisture barrier, then you can seal the foundation vents completely.
2. The IRC requires foundation insulation installed from the subfloor to the ground in the crawl space. Then install the insulation 24 inches horizontally to lengthen the horizontal heat transmission path from the crawl space to outdoors.
3. The 2012 International Residential Code (IRC) requires 1 CFM per 50 square feet of crawl space floor area in powered exhaust ventilation. The IRC requires openings from the crawl space into the home so that make-up air comes from the living space.
9.6.3 Conditioned Crawl Spaces
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SWS Details: 2.0403.2 Closed Crawl Spaces—Ground Moisture Barriers, 2.0404.3 Closed Crawl Spaces—Crawl Space Conditioning |
The 2012 International Residential Code (IRC) requires 1 CFM per 50 square feet of crawl space floor area in conditioned supply air from a forced-air system. The IRC requires openings from the crawl space into the home for this option.
The conditioned crawl space, although allowed by the IRC, may be an ineffective moisture-and-energy solution for existing crawl spaces. Heating the ground in winter wastes energy. Refrigerating the ground in summer with an air-conditioning system wastes energy and may also cause moisture problems. We can’t recommend this practice.