Moisture control in conditioned crawl space

[Bcarlson78248]

I'm building an addition that has a conditioned crawl space of about 160 sq ft. We already have plans for one 32"x24" access from the basement to the new crawl space, and a 2nd 12"x12" opening to the basement at the other end so there will be cross ventilation. Now the city wants us to add power ventilation to ensure good air exchange. The crawl space will have rigid foam insulation under it and on the walls, followed by a moisture barrier, then 4" of stone on the floor. I don't expect to have much moisture infiltration, but you never know.

Since I need to add a vent fan, could I use a humidity control for that fan? Is there one that will work effectively in this application, where it might be running 24/7 during humid months? It does not need to be a high flow fan, so maybe there is one designed for lower speed continuous operation.

Thanks,

Bruce

 

[Dana]

The IRC 2018 requires 1 cfm of continuous ventilation for every 50 square feet of crawlspace floor area. That ventilation can either be exhaust-only, or it can be circulating air from the conditioned space above, OR simply adding a 70 pint dehumidifier. But a humidistat operated fan isn't in the prescriptive code.

See section R408.3 Unvented crawl space (near the bottom of the page.)

The very low power Panasonic FV-04WS2 set to 20cfm exchanging air between the crawlspace and basement only uses abut 4 watts (~35 kwh per year), and though overkill relative to minimum flow for a 160 square foot crawlspace it would fill the bill. Running it at a 25% duty cycle would cut the average power down to about 1 watt, but unless your electricity is ridiculously expensive it probably isn't really worth setting up a duty-cycler. It's about $17o at box store pricing, but can be had for under $140 at multiple web-stores.

 

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[Bcarlson78248]

Thanks for the information and the pointer to the actual code. My permit is under the 2015 Virginia residential building code, which has the same requirements as the R408.3 paragraph you referenced.

I've used Panasonic fans for my bathrooms and found them very reliable and quiet. The one you linked to may be a good choice for the crawl space. Lowes carries them, and they also have a 10 CFM model (FV-01WS2) that is about $40 cheaper

Bruce

 

[Dana]

Sounds like you found the even MORE appropriate fan then!

I've seen the other one used frequently in this application for bigger crawlspaces than yours.

You only need 3.25cfm to meet the minimum letter of the code, but 3x that much isn't extreme overkill.