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Will closed-cell spray foam rot wood in a crawlspace over time?

ChristopherJR | Posted in Energy Efficiency and Durability on

I’ve been really going back and forth on spray foam in a vented crawlspace. After reading a case study done by LSU, closed cell foam is their recommended option for flood prone areas. They advised against encapsulation in areas like this. They tested closed cell spray foam, without covering the bottom of the joists, just spraying between the joists, and claim this is the best method.

When talking with encapsulation companies, they claim the wood will rot within a year and the entire house will cave in if you use spray foam in a crawlspace. They also want $15k to encapsulate. Why would engineers, wood experts and PHD building scientists do a case study, provide excellent reporting on the closed cell foam for vented crawlspaces, if it was as bad as encapsulation companies portray it to be?

The idea of having a liner and dehumidifiers that have to be managed under the house for $15k initial cost and $100-$200 per month on dehumidifiers vs. spraying closed cell and making a complete barrier; seems like a no brainer to me. Closed cell seems like the obvious choice and reading this case study from LSU seems to back that up substantially. Any opinions that would go against the LSU study would be greatly appreciated. I’m very curious what the best minds in the industry think about this.

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  1. JC72 | | #1

    The study said that also covering the joists would probably be beneficial but they didn't test for that.

    Closed Cell Spray Foam (ccSPF) is expensive but in warm flood prone climates it works well because you don't need to use a lot it in order to reach the required R-value and at only two inches in thickness it becomes a vapor barrier.

    IMO using ccSPF is ideal for flood prone areas because you want flood water to be able to move through the crawlspace vs becoming trapped or causing the space to collapse.

    Downsides: Plumbing penetrations might be difficult to seal and may require touch ups as the house settles. ccSPF is very expensive. Wood needs to be dry when you apply it and the technicians MUST be competent. Termite inspections become difficult.

    Now if you don't live in a flood prone area then going the other route might be better because you can bring the crawlspace into the housing envelope with transfer grills, remove the floor insulation (because you're now insulating the walls of your crawlspace), add a small supply air duct* from your HVAC system to the space so you can control humidity like you would in your living areas. Depending on where you live radon exposure could be an issue so mitigation would be an additional expense.

    *local codes allow?

    1. ChristopherJR | | #2

      Thanks for the reply, much appreciated. I've been reading about the encapsulation for sometime as well. I actually tested it out on one section of the home that is very easy to get to, as there's no piers. The issue I had with the encapsulation was, a rat came through under the exterior block wall when it was still dirt under the house. The rat chewed up the corners to get underneath, somehow dug his way out again. Then the odor inside the home was much stronger because the air below was stagnant. I ended up adding some air transfer grilles inside the hatches to get down into the crawl. I ended up tearing it out and adding a concrete rat slab. On the new side of the house that I'm speaking of, built in 2002, a new encapsulation with an Aprilaire dehumidification system would work well. But on the old side of the house there only about 19" of crawlspace and piers everywhere. It would be a nightmare to encapsulate with all the piers in the way. I would most likely have to hire a company to do it, which would be $10-$15k from the quotes I've received. Then the issue that comes into mind is, I am bringing black tar coated wood into the house envelope. Since it's below, I would get some of that vapor into the home with no fresh air exchange. I've discussed with someone else in this site the option of 1 cfm exhaust per 50 sqft to create a negative pressure. So I would be spending $10-$15k plus purchase April air dehumidifiers for $1.2k each. I would need 3 because there's 3 different crawlspaces. Then around $50-$100 per month in electricity to run all of the units. ORRRRR, I just spray closed cell to create an air and moisture barrier, and the crawlspace becomes an outside space similar to a shed; for $6,000. I could have them spray the underside of the joists for an extra $1,000 just to be safe. I really wish there was a clear choice, oh there is, don't ever buy a crawlspace again lol.

  2. Peter Yost | | #3

    Hi Christopher -

    Spaces, including crawlspaces, need to be clearly either "inside" the conditioned building or "outside." Completely. You joke about never building a crawlspace again: in flood-prone areas, you are compelled to design and operate the crawl for both "normal" and flood conditions. Regardless of whether or not the crawl is vented (insulated and airsealed at the first floor) or unvented (insulated and airsealed at the perimeter walls), it's nearly impossible to design for both with a crawl.

    Piers on the other hand, make it really clear what is inside and outside the building, in normal and flood conditions.

    I have tested air exchange in vented crawlspaces and even under windy conditions, you simply don't get good air exchange for the space. The ventilation rules in the code are laughable. And getting a good air barrier at the plane of the 1st floor is challenging to say the least.

    Long way of saying: crawlspaces are a great example of Joe Lstiburek's best line in my book: "there is never any free thermodynamic lunch." You pay your thermodynamic fee with either a vented or an unvented crawl.


  3. thegreatyogurt | | #4

    No opinion, just an anecdote. A friend got closed cell under her house, and it ended up rotting her joists and subfloor. Not sure if she did encapsulation or not.

    Supposedly, humidity got through between the foam and the floor somehow, and then it couldn’t get out since the cells aren’t permeable.

  4. CastleFL | | #5

    Hi all, this is an old post but fingers crossed. I have similar crawl space odors and have gone through many steps to try to eliminate the source (95 year old Creosote soaked joists) removing old batt insulation with poly on the bottom, two coats of BIN shellac to seal it in, installed two exhaust fans which are exhausting 480 CFM on constantly. Still we have smell. In process of a hybrid crawl space sealing, closing up the vents and lining floor and walls, spray foaming mechanical penetrations (trying to create negative pressure with the fans) made it worse (and obviously we boxed in the Creosote but this cleaning and lining with poly needs to happen for dust/moisture issues). Thinking that I haven't created enough negative pressure so looking into a negative pressure machine that would exhaust almost like a hospital (high CFM, low decibel) and duct that out through an existing vent that we are closing up. Also planning to add a dehumidifier with pumped out condensate line. Would I keep the two exhaust fans I've already installed? If this doesn't work, is it a good idea to then seal the subfloor/joists with spray foam to lock in the odor? Framing is in perfect condition probably because of the Creosote but concerns me for insurance, future resale if an inspector can't see the framing in the crawl. So concerning this post from 2019 and looking at the LSU study, seems like closed cell is the better option? Would that completely eliminate the Creosote smell?


  5. Peter Yost | | #6

    Hi CastleFL -

    if this were my building, before doing anything else, I would determine just how much, if at all, you are actually depressurizing the both your crawlspace and your conditioned space above. Including what the pressure regimes are for times when other exhaust fans are on in the conditioned space--bath fans, kitchen range hood, clothes dryer.

    I don't know what the chemical compatibility is w/r/t the creosote, the BIN shellac, and spray foam. Another important consideration.

    As with many real-world situations, it is difficult to advise from afar; just too many confounding conditions might exist.

    Hope this helps - Peter, Building-Wright

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