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ramairws601 10-10-2009 05:25 PM

spray foam under floor - who has done it?
was thinking that sprayed foam between all the original joists and filling the joists with can foam would provide 3" or so of insulation and sound deading. who has done it before and any problems you have seen?

ramairws601 10-12-2009 10:59 PM

Re: spray foam under floor - who has done it?

Originally Posted by Smitty
You might try the search function. I know it's been discussed before, but not in much detail, and I can't remember anyone having actually done it here.


tried search, found alot of maybe,what ifs, type of thing.............looking for someone that has actually done it and their experience with it or any thoughts on this topic are appreciated as well.

baadpuppy 10-12-2009 11:22 PM

Re: spray foam under floor - who has done it?
Before I came to my senses and decided converting a skoolie was smarter than trying to make a conventional RV full time livable on a budget, I bought a 1989 cobra malibu motorhome.

Underneath this motorhome is some kind of sprayed on foam, that has black paint sprayed on top(bottom technically) of it.

This foam covered many of the surfaces poorly, trapping salty slush from the roads against the metal. It also covered several maintenance items, preventing them from being maintained without cutting foam away. The foam is crumbly in places, spongy in places, and rubbery in places and rock like in other places.

Personally, I won't be using foam underneath of my bus. The only exception *might* be some kind of rhino-liner type of coating, but even then, I'd have to think hard about it first. It would be one thing if I were tearing it down to the frame and could verify proper application everywhere. It is another thing entirely when the bus is already in one piece, and spraying foam in might make nice places for water/salt/dirt/grease/bugs/critters to hide.

If you decide to go this route, please report back about your experience with it.


ramairws601 10-14-2009 01:49 PM

Re: spray foam under floor - who has done it?
between the joist is one thing but there is airspace in the joist itself that must be filled also. your idea of covering the joist area and spraying in through access holes may work provided your holes are close enough to fill the entire area but if the foam doesnt then you have a dead spot that you cant even see in there and could bow out the panel as well. I figured sprayed on and let expand past joist, then shave down to joist level and THEN cover with galvanized sheetmetal riveted/screwed onto joists. can use pour in foam from the top thru access holes or spray in thru holes as well into the joist themselves.

ramairws601 10-14-2009 03:54 PM

Re: spray foam under floor - who has done it?

Originally Posted by Smitty
If you're sheeting the interior of the floor with plywood, why couldn't you cut some polystyrene (or whatever you choose) to fit between the ribs that support the floor, then use some fender washers & sheetmetal screws to screw it to the bottom of the floor? Just be sure the screws don't come through the topside of the floor sheeting.


thought of that but there are a gazillion nails stuck down thru the floor metal from the old plywood that would be in the way. even though I cut off the tops after removing the flooring the pieces left over are really stuck to the metal. I tried pulling some out from the bottom and what a job it'd be to do all of them. just figured spray foam over all of it letting them be and have like 3" of insulation. If I were to completely fill the spaces between joists it would most likely cost more for the sheets vs spray in plus all that cutting of sheets to fit just right and having to slide them in over the frame rails (flooring nails in the way again) doesnt sound fun to me at all.

Sojakai 10-15-2009 01:46 AM

Re: spray foam under floor - who has done it?
Get a Moto-Zip tool and just cut the nails from below. Try and leave them a little long so they can help hold the foam in place if you use foamboard, and help hold up spray foam as it grabs onto them.

Otherwise, Call in a Pro. From what I have seen, they can spray foam into a finished house wall without causing a blowout, I think they should be able to handle filling a metal space through small holes.

sportyrick 10-16-2009 06:29 PM

Re: spray foam under floor - who has done it?
We found that 1.5 or 2" foam sheets cut a little wider then the floor joists will jam up from the underside pretty well, and you can slide them over a little to get above the frame and other things. It does leave the ribs open internally but maybe that is all that needs to be foamed, the floor ribs internally. You know that spray foam will hold styrofoam sheets in place also, that way would be a lot easier than putting steel under there and trying to fill the voids. I have found that the spray foam you are talking about expands slightly for months or years if applied in a thick layers. sportyrick

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