My question is what type of sealant or insulation should I put under the floor. I'm going to go back with 3/4" marine plywood. The house is about 18" off the ground and I'm on the Gulf coast. I'm only having problems with the floor where the linoleum is no problems where I have carpet.

I very just ran across this:

Retrofit Pamphlet

It's really not what you're looking for, but it's an interesting overall read for those of us with older homes.

Thanks. That was a good read.

I'm a little confused, what's the problem you're running into? Mold or something else?

The floor is completely rotting out under the linoleum. I'm trying to find out what I need to do to the underside of the floor that's open to the crawl space. Thanks for any advice.

I'm going to need a computer to type this one out... Using pressure treated subfloor or marine won't change the basic science behind why the problem is happening in the first place though. Will be back later today...

Doyle77 said:

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The floor is completely rotting out under the linoleum. I'm trying to find out what I need to do to the underside of the floor that's open to the crawl space. Thanks for any advice.

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The problem you're running into is caused by your subfloor being cooled to a temperature below the dew point of the air in your crawlspace. This tends to manifest itself earliest in areas with thin, non-permeable flooring like what you're seeing in areas under the linoleum. The problem will not go away with treated or marine plywood, and depending on the moisture content of the material you may run into problems with it warping or shrinking as it dries out.

The sources of moisture underneath your house besides the obvious (leaks) will be from bare earth and the ambient air entering the crawlspace. To cut down on moisture from the earth I'd recommend making sure that all gutters kick water out at least a few feet from the house and cover the ground in the crawlspace with plastic at least 10 mil thick, overlapping seams by 1' and sealing them with something other than fabric backed duct tape. To deal with moisture from humid air condensing on things below the house you first need to make sure that any ductwork is completely sealed. Beyond that, if you are not in a flood zone encapsulating (closing all vents and installing a dehumidifier) works very well. If you are in one and the vents are required by flood code it's a little more of a challenge. I've read that spray foam applied to the joists and subfloor works well though.

Hope this helps some!

Thanks for the help.