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Should I put an A/C return in my encapsulated crawl that has a bit of a smell currently?

hifiaudio2 | Posted in General Questions on

I am having a new HVAC pancake system (Trane XR17 and sdv9 furnace) installed in my crawlspace this week. The installers have removed a lot of existing ductwork and I can smell the crawlspace in the house above right now, since the air just flows up. At one point I had considered installing a return in the crawlspace along with the supply registers to help keep the area dry without a dehumidifier that I use right now, and also to take advantage of the “free” cooler air in the crawl in the summer months, but the current smell is putting me off. The humidity is only around 50-55% down there now, and has been like that for weeks since we encapsulated. It is a musty smell, nothing else. Should I go ahead and put in a return and figure that with the new airflow and a Merv11 filter on the unit that I would quickly get rid of the smell, or is it likely to linger? I like the idea of treating the encapsulated crawl like I would any basement in another home, but it the smell is not likely to go away I wouldnt want to do that. Thoughts?

The encapsulation by the way is with a 12 mil poly on the ground and 1.5 in of closed cell on the walls and 3″ of open cell in the rim joist.

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  1. GBA Editor
    Martin Holladay | | #1

    There are two ways to condition a crawl space. You should use the method that depressurizes the crawl space (install an exhaust fan in the rim joist, and provide a passive grille that allows air from the home to enter the crawl space as makeup air).

    For full details, see this article: Building an Unvented Crawl Space.

  2. hifiaudio2 | | #2

    So it is unlikely that the smell simply goes away once adequate airflow is down there?

  3. GBA Editor
    Martin Holladay | | #3

    I would never hazard a guess on whether a smell will go away -- especially since some people have a very acute sense of smell. If the smell bothers you, you can either (a) wait and cross your fingers, or (b) use the conditioning method I suggest (so that the crawl space is depressurized with respect to the house).

  4. user-2310254 | | #4

    My sensitive nose (and lungs) hate that moldy, musty smell. It would have to go.

    My personal experience suggests that venting the crawlspace with a properly sized fan will have the most immediate impact on your problem. You could vent until the smell goes away and then shut down the fan to see what happens. I would monitor the humidity level and also inspect the crawlspace to see if there are any unsealed areas in your poly.

  5. hifiaudio2 | | #5

    What I have noticed so far with respect to the smell... when I had the encapsulation done, I put a dehumidifier on one side of the ~3300 sq ft crawl ("mostly" a big open square) and we cut two supply vents into the HVAC ductwork... but it turned out that they only cut supplies into the trunk of ONE of the two hvac units. The side that has the vents and the dehumidifier smells normal / fine. The side without smells musty. I am hoping that when I install this new 5 ton two zone hvac with four total supply ducts cut at the far four corners of the crawl and a return in the middle, ALL of the musty smell will go away. The humidity that I have been monitoring with a hygrometer in the middle of the crawl is 50-55%.

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