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Community and Q&A

Spray foam for Florida crawlspace?

ChristopherJR | Posted in Energy Efficiency and Durability on

Good evening to all! I purchased a wood frame Spanish style house with a crawlspace in sunny (humid)  South Florida. The house has a rat slab underneath and is vented throughout.

The  batt insulation completely failed (still code here) and was removed. Hydrogen peroxide was sprayed on all wood members to clean. There are currently some ventilation fans removing air from underneath the home, not sure if this is a good idea, but the air that come out of the exhaust fans smells musty, I can imagine why it’s from a dark dark place lol.

The beams are from 1920 and have some type of black coating, looks like tar. I would not want to encapsulate because I do not want that air coming into the house without anywhere to go or the ability for fresh air exchange.

In South Florida it seems that every house with a crawlspace just has a musty smell, I would attribute it to the stack effect and the humidity level inside the house, because of the humidity level inside the crawl?

We currently have two dehumidifiers running 24/7 inside of the home to maintain 45% humidity. I’m seriously considering filling the hoist cavities with open cell foam, then spraying a 2″ top coat of closed cell. The only reason I’m considering that is because it would allow a full coating of closed cell on a near flat surface, rather than coating all the floor decking and the 2×8 floor joists.

The primary objective here is to provide a seal and to block humidity. I own several other units with concrete foundations and none of them have humidity issues and/or odor problems.

The reason I’m posting on this site is, after reading a lot of blogs and Q/A, there’s a lot of very professionals that understand building science on this site. The contractors down here have zero clue on crawlspaces. My concern is wood rot over the long term.

Any knowledge is greatly appreciated and respected. Thank you for your time.

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  1. Expert Member
    Dana Dorsett | | #1

    >"I would not want to encapsulate because I do not want that air coming into the house without anywhere to go or the ability for fresh air exchange."

    A primary source of moisture in a vented crawlspace in Florida is always the outdoor air. With a vapor barrier on the floor of the crawlspace and sealing off all leaks to the exterior puts the crawlspace air within the pressure and water vapor boundary of the house, where it can be dried by air conditioning.

    To prevent stagnation or outgassing of creosote from any treated beams, exhaust-only ventilation of the crawl space keeps the crawl space at negative pressure to the rest of the house- all the nasty stuff goes outdoors, not the house. The IRC calls out 1cfm of ventilation for every 50 square feet of crawlspace area (egL a 1000' crawlspace can make do with 1000'/50= 20cfm of continuous exhaust ventilation.) Since the air entering the crawlspace is fully conditioned air, it won't be adding humidity to the crawlspace air, and the wood stays dry. Since the direction of air flow is always from the house into the crawl, and never the reverse, any contaminants in the crawlspace are still isolated from the rooms above.

    1. ChristopherJR | | #2

      Thank you for the reply. One section of the house is from 1920 and is full of pilings, concrete blocks, jacks etc. it would make putting a plastic barrier on the floor nearly impossible since there's only 20" of room max, some areas lower. We poured concrete with a sealer mixed into the concrete to attempt the best floor covering possible.
      Knowing that is the site conditions, would you recommend spray foam to seal off the crawlspace from the home above?

  2. Expert Member
    Dana Dorsett | | #3

    An inch of closed cell foam on both the subfloor & joists/beams in the area with too many obstacles will protect both the wood and the house from crawlspace humidity. It's not cheap stuff, about a buck a square foot per inch of depth (board-foot) in my area for the climate damaging but cheaper HFC blown foams, about $1.35/board-foot for the more benign but slightly higher R foams blown with HFO1234ze.

    In south Florida there is zero energy savings benefit to installing cavity-fill insulation, or even 1" of closed cell foam. The primary goal here is air sealing and vapor retardency. At 1" closed cell foam runs about 1 perm (give or take) on water vapor diffusion, which is sufficient for protecting from the water vapor drives during maximum humidity & highest air conditioning loads when the floor is much colder than the outdoor dew point.

    1. ChristopherJR | | #5

      Thank you for your in depth knowledge. The best case study I could find from a reputable source that is in a similar climate zone, is by LSU that studied 12 New Orleans homes. Their recommendation was to stay away from encapsulated crawlspaces in flood prone areas, the article states the reasons. They do recommend closed cell inside the hoist cavities, but they go on to discuss how covering the joists with spray foam is not necessary in the Q/A section. I found it to be a very good resource.

      Based on your response, spraying 1" of closed cell on the cavities and the entire joists would provide the coverage necessary to make the surface impermeable. If you don't mind, would you reference this case study and see what you think? The spray foam contractors have suggested to just use 2" of closed cell in between the joists, but not actually spraying the bottom of the joists. This is also what LSU case study did and suggested in their Q/A section at the bottom of the study.

      As long as we can block the smell from the crawlspace from getting inside the house, that is the main objective. We do not care about R value because the crawlspace stays cool from the AC being on. When we do get a cold front once or twice per year, the house stays cold for days like a cooler because the crawlspace retains the cold like a cooler. It's the smell of the humidity down there that is bothersome.

      The encapsulation companies have stated that if you use spray foam on a vented crawlspace that the wood will disintegrate. They claim that the wood will sweat like a cooler when the AC is on above. Any opinion on that?

  3. Expert Member
    Peter Yost | | #4

    Hi Christopher -

    Bare dirt or unsealed porous concrete can be responsible for up to 0.05 lbs H2O per square foot per hour ( If your floor area is 1,000 square feet, that is up to 1200 pounds of water moving up through the surface of the bare dirt in one day.

    Making sure that you have continuous ground cover/moisture barrier is critical.


    1. ChristopherJR | | #6

      Thank you for your comment. I added a sealer to the concrete mixture that also fixes cracks if they occur, I cannot recall the name of the product.

      If closed cell spray foam is applied, will that alleviate the need to worry about what the humidity is underneath the house? The idea is that it essentially becomes part of the outdoors and the wood becomes part of the house.

      LSU did a case study in a flood prone area. It's the best source I could find. See the link below. Your opinion is very much appreciated, as this will be a decision I have to make that is proven both ways. In the end it will be a judgement call.

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