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Conditioned Crawlspace Options

Bluegoose68 | Posted in General Questions on

Conditioned Crawlspace Options

I am building a 24’x36’ shop with a conditioned interior and crawlspace – climate zone 4a (middle TN).  I know that crawlspaces aren’t the first choice for most on this forum but the lot is sloped enough to make it difficult to pour a flat slab.  I also like the flexibility for future revisions that a crawlspace provides, and I find a wooden floor much more comfortable than a slab to stand on for long periods.

Since the building doesn’t have interior walls (just one big room), I am planning on heating/cooling with a ductless minisplit.

Is there a general consensus on the “best” approach for creating a conditioned crawlspace?  I’m not subject to a building code where I live, but the 2018 IRC (R408.3) lists 4 options for unvented crawlspaces:

1)    Continuously operating exhaust fan with a transfer grill to the main floor
2)    Conditioned air supply and return path to the main floor
3)    Plenum in existing structures
4)    Dehumidification

For Option (1), I’m sure a small fan is inexpensive to purchase and operate but since it’s pulling conditioned air from the main floor, does it get expensive to replace that conditioned air? Seems like we go through a lot of trouble to build an airtight structure to run an exhaust fan continuously and suck air out of it.
Pro: keeps the crawlspace air separate from the main level
Con: cost to replace the conditioned air?

For Option (2), a ductless minisplit doesn’t have ductwork to directly supply conditioned air to the crawlspace but I think I could accomplish the same goal by putting a transfer grill with a small fan in it (and another transfer grill on the other end of the building) to circulate the air.
Pro: should be inexpensive to operate
Con: connects the crawlspace air to the main level – but it should be “clean” air since this is a new build (and I keep excess moisture out of the crawlspace) 

For Option (3), I don’t understand this option.

For Option (4), if I run a dehumidifier in the crawlspace, do I need to connect the crawlspace to the main level with transfer grills?  The building code does not elaborate on that detail.
Pro: should do a good job of controlling humidity
Con: cost to run a dehumidifier?  How do you run the condensate line to the exterior without it freezing in the winter?

Which option do most people gravitate towards?  Any suggestions or advice would be appreciated.  Thank you.

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  1. Expert Member
    Akos | | #1

    Go for option 4. This also puts some extra heat into the crawlsapce which help with relative humidity. A couple of transfer grills will won't hurt as the minisplit can also assist.

    Around me crawlspaces require a floor drain, so the dehumidifier can drain to there.

    If you have plumbing down there, a baseboard heater with a built in thermostat is also a good idea. It might never need to run, good if it is there in case of those polar vortex days.

  2. Expert Member
    Michael Maines | | #2

    I've been through this exercise many times and have tried all kinds of ways to save energy and equipment costs, but they all have drawbacks. I agree with Akos that today I'd use a dehumidifier, preferably a high-quality one (Santa Fe or similar) vs. a disposable, freestanding one. Up here in CZ6 I'd also add a strip of electric baseboard heat but that may not be necessary in CZ4.

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