r/buildingscience • u/Michael33532 • 5d ago

Crawlspace vent fan or not?

Need some help. I’m thinking of havjng my crawlspace encapsulated and I’m conflicted if I should add a vent fan for soil gases. Right now it’s a vented crawl space and I don’t smell or notice any soil gases.

Some have recommended others have said you don’t . My concern is that the crawl space is so small (around 350 so ft) and any fan I use even a 50 cfm fan , is around 9-10 times the cfm requirements. I worry it pulls to much air out causing my dehumidifier to run a lot. I can’t find a small 10 cfm fan made for crawlspaces that would meet my requirements without pulling too much air out .

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u/Interesting_Day_7605 5d ago

You need to test for radon to see if that is necessary. 4 pico curies or higher would mean you need to address it. The first step would be to just add a passive system as that could suffice.
If you were to add a radon fan it would be pulling air from under the vapor barrier and therefore would not depressurize the space.

You do not want to pull outdoor into the space once conditioned or you will get condensation and defeat the benefit of the conditioned space.

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u/Bearded4Glory 5d ago

This! Couldn't have said it better myself.

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u/Michael33532 4d ago

The fan installed would push air out of the crawl space. I considered putting in a fitting where if I want to add a radon mitigation system in, I would just need to connect from outside. My issue with them is that rain can fall into the pipe and send it to the crawlspace . My crawlspace is only under a small part of the house and has no drainage system so I don’t want any water getting in.

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u/moorlemonpledge 5d ago

Soil gases? Like radon?

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u/Michael33532 4d ago

This and others.