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 6 inches (152 mm) minimum at the joints and shall extend to the
foundation wall.
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EXCEPTION:
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The ground cover may be omitted in crawl spaces if the
crawl space has a concrete slab floor with a minimum thickness of 2 inches (51
mm).
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R408.2
Openings for under-floor ventilation. The minimum net area of
ventilation openings shall not be less than 1 square foot (0.0929
m
2) for each 300 square feet (28
m
2) 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), and operational louvers are permitted:
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 grill or grating.
4. Extruded load-bearing brick
vents.
5. Hardware cloth 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).
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EXCEPTION:
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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 AF (Radon) of this code.
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R408.3
Unvented crawl space.
Ventilation openings in under-floor spaces specified in Section R408.2 shall
not be required where:
1. 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 AF (Radon) of this code.
2. 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 m
2)
of crawlspace floor area. Exhaust ventilation shall terminate to the exterior.
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EXCEPTION:
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Plenum in existing structures complying with Section
M1601.5, if under-floor space is used as a plenum.
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R408.8
Under-floor vapor
retarder. This section is not adopted.
Notes
Wash. Admin. Code
§
51-51-0408
Amended by
WSR
14-24-055, Filed 11/25/2014, effective
5/1/2015
Amended by
WSR
23-02-058, Filed 1/3/2023, effective
7/1/2023
Amended by
WSR
23-23-104, Filed 11/15/2023, effective
3/16/2024
Statutory Authority:
RCW
19.27.031 and chapters 19.27 and 34.05 RCW.
13-04-068, § 51-51-0408, filed 2/1/13, effective 7/1/13. Statutory Authority:
Chapter 19.27 RCW. 10-18-036, § 51-51-0408, filed 8/25/10, effective 9/25/10.
Statutory Authority:
RCW
19.27.031 and
19.27.074. 10-03-098, §
51-51-0408, filed 1/20/10, effective 7/1/10. Statutory Authority:
RCW
19.27.074,
19.27.020, and chapters 19.27 and
34.05 RCW. 07-01-090, § 51-51-0408, filed 12/19/06, effective
7/1/07.
