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I'm trying to eliminate damp odor originating from a crawl space.

Here's the situation:

  • Half of the house has a basement. The other half is a crawl space accessible from the basement through an access opening. The access opening and another "vent" opening share the crawl space air with the basement.
  • The crawl space is surrounded by foundation walls with no windows. The floor is stones covered with overlapping heavy-duty plastic sheets (nothing is sealed).
  • There are two HVAC zones. The basement has one air handler for that half of the house, including air vets and a return in the basement. The crawl space has an air handler for its half of the house. It, too, has vents and a return in the crawl space to ventilate the crawl space.
  • The house is adjacent to a community water catchment area and the water table gets high during rainy periods. There is a sump pump in the basement that is fed from the gravel layer under the basement floor, and it runs continuously in periods of wet weather. There is a foundation wall and footer between the basement and crawl space, so I suspect the crawl space doesn't drain to the sump pump.
  • The house in in good shape in terms of exterior water (the rain gutters are good and discharge far from the house, and the grading is OK). There are no signs of water in the basement.

Here's the problem. During wet weather, the crawl space gets a "wet ground" smell that wafts out of the stone floor. The HVAC return picks it up and adds it to the household air on that side of the house. So half of the house gets the aroma.

I stuck a dehumidifier in the crawl space but it doesn't affect the smell. The air handler has an electrostatic air cleaner, but that doesn't filter smells. So I'm looking for a solution to keep the aroma out of the household area.

What I've considered:

  • I could seal the crawl space return, but that would starve the air handler of 1/3 of its air volume. Or I could duct it to the vent hole in the basement wall and pull better air. However, I would need to close the vents in the crawl space or it would just circulate the crawl space air by a longer route to the return. So again, there's an air volume problem.
  • If I isolate the HVAC from the crawl space air, the crawl space will no longer be ventilated. I assume the odor would become worse in the crawl space, possibly even a mold problem.
  • I'm not sure how practical it would be to seal the floor. There is no way to get concrete or any heavy material into the crawl space. Maybe adding another layer of plastic sheeting would help if there's a way to seal it and seal it to the walls (duct tape?). The floor area is on the order of 1,400 sq. ft., so it isn't a trivial area to seal.
  • I thought about putting a sump pump in the crawl space. However, There is probably 3 1/2' to 4' of gravel to get below the basement floor level to where the water actually is, and the sump pit would go below that. So it would be more of a well than a sump pit. I can pump the water out, but the ground will still be wet, so I'm not sure that would affect the odor.
  • I've seen various deodorizer products you stick in the ductwork. They get very mixed reviews.

Any recommendations on how to solve the problem?

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A similar problem in my home was resolved by installing a small muffin fan to exhaust the air outside. A 50 cfm was more than adequate, so I added a series inductor (ballast) to reduce electrical usage and noise. A timer was also tried, but running full-time at low speed was more effective in reducing odors. If you live in an area where radon is an issue (e.g. over granite), this also reduces the amount getting into the house.

Wire through a GFIC, since this would be a wet location!

Disadvantages:

  • A hole is needed for mounting the fan.
  • The fan uses a small amount of electricity continuously.
  • The fan may make noise.
  • A small amount of air may be pulled into the house, raising HVAC costs.

So far, this has eliminated the smell in the house and has reduced humidity in the enclosed area (storage room rather than a crawl space), without noticeable impact on utility bills.

  • An approach I hadn't thought about. There would be a few obstacles to deal with (hole through stone exterior, sealing it during the winter and summer, and a few other issues). 100 CFM would turn over the air once/hr, which could be enough to keep the odor down. I'll have to explore this. Thanks for the idea. – fixer1234 Jan 30 '18 at 2:11
  • Since it's in an unheated storage room, I've left the fan running summer and [cold!] winter. There are two layers of insect screen (both sides of fan). Cutting through stone, though, would be an issue. Can you dig underneath and put a fan in plastic pipe, well above ground level? – DrMoishe Pippik Jan 30 '18 at 5:12
  • Exhausting it to the outside would be quite a challenge. It could probably be ducted between floor joists to get it to the other end of the basement and then tie it into an existing exhaust duct, but it would be a very long run. A simpler version might be to not exhaust it outside, just blow it through something like a big activated charcoal filter and then recirculate it. But this gives me a bunch of possibilities to investigate. – fixer1234 Jan 30 '18 at 6:37

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