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Crawlspace ventilation and whole house ventilation?

aaroncg72 | Posted in General Questions on


We’re planning a sealed crawlspace in our upcoming house, for several reasons, not the least of which is the excellent science and articles about crawl spaces, on this site. It’s my understanding that to meet the code for sealed crawl spaces, one must either;  provide a continuously operated exhaust fan which draws air out of the crawl space at a rate of 1 CFM for every 50 square feet of crawl space floor – would be 26 CFM for our 1296 SF floor area – or; provide a heating duct blowing into the crawl space from a central ducted heating system. As we won’t have a central ducted heating system, I think the first option is our only option. There will need to be an opening in the floor connecting the crawl space to the first floor, naturally.

I’ve also just learned that we are required to have a whole house ventilation system, which also draws air out, and continuous operation is much preferred, for several reasons. For our size home and number of bedrooms, it looks like we need 60 CFM of continuous exhaust.

So my question is this. Can these two continuous exhaust fans be the same unit? So in other words, is there any reason we couldn’t simply have one fan of approximately 85 CFM, in the crawl space?

The few diagrams I’ve seen for the simple exhaust system tend to show the exhaust fan and duct on the top of a second floor house, exhausting high out the gable end, soffit, or roof. However, I couldn’t find anything in the actual WA VIAQ code which specifically says exhaust has to be in those locations. But then, very few people in WA use sealed crawl spaces….

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  1. aaroncg72 | | #1

    On a much related note, I'm having difficulty understanding exactly what is required when building an unvented crawl space, when it comes to radon. We're building in Lewis county, WA, which is yellow on the radon maps (lowest levels). Reading section AF103.5 of Appendix F of the IRC, I read it as NOT requiring radon mitigation (Passive Submembrane Depressurization System), IF you have an approved mechanical crawl space ventilation system. Sure seems like a continuously operated exhaust fan in the crawl space would be just that?

    But then, I read my state (WA) amendment to the IRC, Section R408 of WAC 51-51-0408, specifically section R408.3 dealing with unvented crawl spaces. It lists two criteria, first being covering exposed earth with a class 1 vapor retarder and sealing and taping all joints/penetrations, running it up the stem wall, AND installing a radon mitigation system consistent with appendix F. The second criteria is a continuously operated mechanical exhaust fan, at the rate I mentioned before. What I don't get, is if you can do just ONE of these two, or if you have to do both?

    The IRC seems to indicate you can do one or the other, and I don't find a reference to vented vs unvented crawl spaces.

    Has anyone here actually built an unvented crawl space in WA state? :)

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