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Crawl space moisture control with open vents...help!

Posted by cheri127 (My Page) on
Sat, May 5, 12 at 10:52

Hello all. This is my first post here and I hope someone can help me. The building and insurance code in our beach town insists that we maintain open flood vents in our crawl space. We have a damp, moldy smell in the house coming from the crawl space and I have no idea how to dehumidify the space if I must maintain open vents. Will a circulating fan help to at least bring in fresher if still moist air? I have no idea what to do. Any advice would really be appreciated. Thanks.


Follow-Up Postings:

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RE: Crawl space moisture control with open vents...help!

I forgot to add that the floor is concrete and the walls cinder block. Water got into the space from the irrigation system and we've had the smell ever since.


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RE: Crawl space moisture control with open vents...help!

"If it is not possible to treat the crawlspace as a part of the house such as in flood zones in coastal areas...it is important to construct the house such that the crawlspace is isolated from the house--outside of the building boundary. These situations should follow recommendations for homes built on piers." Building Science Corp. (see link)

Here is a link that might be useful: Building Science Corp.: Crawlspace Insulation


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RE: Crawl space moisture control with open vents...help!

I'm not sure how this helps as the crawlspace and house are already built. Am I missing something?


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RE: Crawl space moisture control with open vents...help!

Then you retrofit as if the house were built on piers.

See linked document from Photograph 5 forward for the alternatives.

Essentially, you're trying to keep the wood framing warmer than the air below so it is not a condensing surface. The spray foam achieves that goal and serves as an air seal to prevent the movement of odour from the crawl to the conditioned living space.

Here is a link that might be useful: Building Science Corp. : New Light in Crawlspaces


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RE: Crawl space moisture control with open vents...help!

Thanks, Worthy. I actually am quite familiar with the paper you linked and couldn't agree more with the author. I'm selling the house mentioned above so don't want to invest a lot of money to address this problem. All of the houses in the area are the same.

But while I have your attention, perhaps you can give your opinion on the house we currently use. It, too, has a vented, unconditioned crawl space but has a sand floor and the A/C ducts run through it bringing the bad smells into the living space. However, we can close the louvers on the vents on this house because they are SmartVents, so the whole vent pivots open in the event of a storm surge. We closed the vents and installed a SaniDry dehumidifier, which has helped but not completely fixed the problem. We are going to have the floor covered in the next week.

I would love to remove the batting under the floor above and insulate the walls and condition the space but I don't think the vents are sealed enough for this not to be a huge waste in energy. Opening the space more to the outside and treating it as pier construction is not an option due to the nature of the house construction. So, what are my options? I have an HVAC guy coming out next week for a full evaluation, replace any dirty vents and SEAL them well. This will improve indoor air quality but not moisture control in the space. Do you think it's possible to achieve RH<50% just by covering the floor, closing the vents and running the SaniDry? If not, what can I do?


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RE: Crawl space moisture control with open vents...help!

Why not go all the way and do a conditioned crawl by sealing the basement floor, adding perimeter insulation and removing the fibrous insulation? Plus using a mechanical dehumidifier and sealing/insulating the ducts with mastic and then sprayed foam insulation?

Smart Vent makes insulated units that look like they may address your air leakage concerns.


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RE: Crawl space moisture control with open vents...help!

In most cases, flood vents offer enough air movement to keep the average crawlspace dry and mold free. However, under some circumstances, such as high humidity, enclosing topography, and poor building construction, moisture can gather and mold can grow leaving a home with that dank, wet smell. Here are some solutions: an interior drain tile system will take care of intruding water and a circulation fan on a timer will take care of the high humidity and potentially stagnant air.


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