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Residential Crawl Space

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					                                                                                             BLD ______________
Residential Crawl Space
Mechanical Ventilation Alternate Design
Lot: ________ Block: ________ Subdivision: _____________________________________________

Applicant Name: ________________________________________________________________________

Performance Based Engineered Alternate Design Criteria Policy
•   Crawl space ventilation rate shall be 1 cfm per 50 sq.ft. (0.02 cfm per sq.ft.) of crawl space.
•   Locate exhaust fan(s) and intake grille(s) to provide cross ventilation of entire crawl space area.
•   Exhaust fan shall be sized at .25” static pressure (minimum) at the required cfm.
•   Exhaust fan shall be provided with a back draft damper, and shall be ducted to an exterior exhaust grille.
•   Outside air intake grille(s) shall be provided with a barometric back draft damper.
•   Standard foundation vents (16” x 8” without manual damper) shall be used for exhaust and intake grilles. Each
    vent can serve a maximum crawl space area of 3500 sq. ft.
•   Exhaust fan shall be controlled by a thermostat and humidistat wired in series. The fan(s) shall be enabled to
    run if the temperature in the crawl space rises above 70 degrees F. The fan will then automatically cycle on and
    off dependent on the relative humidity in the crawl space. For the Boise area, any humidity reading above 60%
    RH shall cause the fan to be energized.
•   Provide a labeled, readily accessible switch for manual operation of the fan(s). This switch shall not deactivate
    the system.
    Note:  Any alterations or changes to this criteria must be performed by an Idaho licensed engineer.
    Any engineering must be submitted along with the plans and permit application.

Calculation Example
•   Crawlspace area: 2,000 sq.ft.
•   Minimum required fan size (cfm): Crawlspace area x 0.02 = 40 cfm
•   Number of (16” x 8”) intake air vents with backdraft dampers to be installed = 1 .
•   Number of (16” x 8”) exhaust air vents connected to exhaust fan to be installed = 1 .
    (Note: Single foundation vents on both intake and exhaust can ventilate a maximum of 3500 sq. ft. Larger
    crawl space areas require multiple openings or engineering).

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                                      Residential Crawl Space Mechanical Ventilation Alternate Design    2


                                                                                 BLD ___________________
Specifics For This Permit Application
1. Crawl space area is _____________________ sq.ft.
2. Minimum required fan size is ________________cfm.
3. Number of (16” x 8”) intake air vents with backdraft dampers to be installed: ________________________
4. Number of (16” x 8”) exhaust air vents connected to exhaust fan to be installed: ______________________

5. I am providing the labeled switch at the following location: ___________________________________________

    _______________________________________________________________________________________

    _______________________________________________________________________________________

    _______________________________________________________________________________________




___________________________________________              __________________________________________
Applicant’s Signature                                    Date

				
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