Spray Foam under floor of house?

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My neighbor (closed cell) had a toilet leak that rotted a 4’ x 6’ section of the sub floor. He noticed his tile floor getting spongey.

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Yikes- thanks

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I have no insulation on the sub-floor and vapor barrier on the dirt.

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Your floor doesn't get chilly? Do you have wood floors?

Our floor does get cold, we have adjusted. I also had fungus rot on the sub floor in 2 of 3 front rooms and needed the sub floor and joist replaced. I put in more vents in the chain walls and water drainage. Our house was made before AC was used. The floor no longer sweats in the summer.

i was strongly considering doing this but it is true, if water gets in the floors it acts as a barrier to prevent it from drying out and causes huge rot issues.

what i decided to do was to use regular bat insulation under the floor between the joists and then add a layer of foam board insulation across the underside of the floor rafters to hold the bat insulation in place as well as add an extra insulation barrier. you want to leave the foam board joints with about 1/8" gap between them so its not totally air tight which causes moisture issues.

by doing this you are fully insulated but it allows any moisture to leak through and dry out naturally. the cost is going to be about the same as spray foam, maybe slightly more if you pay someone to do it with the added labor involved in doing it.

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Dude on WWL says this is worse.

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its only an issue if you dont leave air gaps to allow moisture to escape as i edited in.

the foam board is used primarily as a means of support to prevent the bat insulation from falling out over time.

if its of that much of a concern to you then just use some 1 1/2" strips of wood every 6" or so to support the bat insulation

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He basically says you have to use closed cell foam or use foam board and tape and spray and SEAL EVERY LAST LITTLE CRACK THERE IS

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Yes he does, and IIRC, moisture of the floor boards need to checked with a moisture meter, and in cases if the moisture is too high, dehumidifiers must be placed un the house to dry the floor boards & joists before using spray foam. Also seem to recall from memory if sprayfoam (closed cell which is recommended) is used, he recommends the dry fall/winter installation period where everything underneath could dry naturally as opposed to spring/summer when humidity is excessively high.

But that is what I like about the guy, he will give the pros and cons of each method to be considered. You can also send him an email through his website Paul’s House on Facebook, and he’ll get back with you usually within a couple days with answers/suggestions. He likely has this question addressed on his blog of his website.

ETA LINK
He doesn’t give a great deal of detail with this post, but if you call his show and get through he will provide many more details, as he is not only answering the question for you, but prob a couple dozen other listeners that have the same question. And there is a link to the LSU AgCenter article on the subject in this blog link which might provide you most of the answers you need.

Close those vents, use batt under sub floor if you don’t want to use spray, run that vapor barrier all the way up to the block.

Closed conditioned crawlspace is the only way to go.

i had my subfloor sprayed with closed cell foam. this was in 2010. I've had no issues.

In.

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I fairly certain that at that time he recomme nded OPEN cell spray foam. Maybe he changed his tune,

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Probably did change his recommendation based on the results of the LSUAgCenter study that I linked earlier. He almost always cites that study when discussing his recommendation. I think he may have participated in it.

I iistened to his radio show from last week (May 11) on podcast this morning on a drive to Opelousas and the second caller on the show was a question of using closed cell foam underneath a crawl space. He answered it thoroughly. Seems like just about every week he has a call on this subject. Here’s a link to the show.
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