Crawl Space Ventilation Code
Effective July 1, 2020
Chapter 51-51 WAC
R408.1 Ventilation. The under-floor space between the bottom of the floor joists and the earth under any
building (except space occupied by a basement) shall have ventilation openings through foundation walls
or exterior walls. A ground cover of six mil (0.006 inch thick black polyethylene or approved equal shall be
laid over the ground within crawl spaces. The ground cover shall be overlapped six inches minimum at the
joints and shall extend to the foundation wall.
Exception: The ground cover may be omitted in crawl spaces if the crawl space has a concrete slab
floor with a minimum thickness of two inches.
R408.2 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 300 square feet (28 m2) of under-floor area. Required openings shall be evenly placed to provide cross ventilation of the space except one side of the building shall be permitted to have no ventilation openings. 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 1/4 inch (6.4 mm):
- Perforated sheet metal plates not less than 0.070 inch (1.8 mm) thick.
- Expanded sheet metal plates not less than 0.047 inch (1.2 mm) thick.
- Cast-iron grill or grating.
- Extruded load-bearing brick vents.
- Hardware cloth of 0.035 inch (0.89 mm) wire or heavier.
- Corrosion-resistant wire mesh, with the least dimension being 1/8 inch (3.2 mm).
Exception: The total area of ventilation openings shall be permitted to be reduced to 1/1,500 of the
under-floor area where the ground surface is covered with an approved Class I vapor retarder material
and the required openings are placed to provide cross ventilation of the space. The installation of
operable louvers shall not be prohibited. If the installed ventilation is less than 1/300, or if operable
louvers are installed, a radon vent shall be installed to originate from a point between the ground cover
and soil. The radon vent shall be installed in accordance with the requirements of Appendix F (Radon) of
R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections R408.1
and R408.2 shall not be required where:
- Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall
overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall
extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem
wall; and a radon system shall be installed that meets the requirements of Appendix F (Radon) of this
- Continuously operated mechanical exhaust ventilation is provided at a rate equal to 1 cubic foot per
minute (0.47 L/s) for each 50 square feet (4.7 m2) of crawlspace floor area. Exhaust ventilation shall
terminate to the exterior.
Exception: Plenum in existing structures complying with Section M1601.5, if under-floor space is
used as a plenum.
A105.2.2 Mechanically ventilated crawlspaces.
Assume to have 1.5 air changes per hour, with less than 1.0 ft2 of net-free ventilation in the foundation for every 300 ft2 of crawlspace floor area. The crawlspace is not actively heated. Floors over unheated basements may only use those values which have R-0 perimeter insulation.
A105.2.3 Heated plenum crawlspaces.
Assumed to have 0.25 air changes per hour, with no foundation vents. Heated supply air from central furnace is blown into a crawlspace and allowed to enter the living space unducted via holes cut into the floor.
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