Vapor barrier for patching rotten floor?

Hi all:
I'm attempting to replace a section of my kitchen floor all the way down to the crawlspace (which does get very wet at times) that was badly damaged (termites and water leak). The subfloor consisted of: 1" tar-covered particle board, diagonally set 1x8s, and 1" plywood. I intend to place a pre-finished hardwood layer on top of the fixed floor.
Now, I can easily replace the 1x8s and the top layer of plywood, but the tar-covered particle board was probably put down in 1942 when the house was built and doesn't seem to be used any more. My question is, 1) What should I use to replace this board and the 1" space and psuedo- vapor barrier that it represented, and 2) does a "vapor barrier" over 1/2 of the kitchen sub-floor make any difference? Or should I just put down 1" of good plywood directly on the floor beams.
Thanks in advance for the help.
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You have asked a number of questions. Some of your descriptions and measurements seem a little out. First of all, I am not aware of particle board used in the 1940s, BUT they used a tar impregnated fiber board, similar to Homosote, as wall sheathing. Your original builder may have used it under the floor to keep drafts from working their way through the gaps between the 1 x 8s. I am not aware of it being 1" thick, usually only 1/2" thick -- possibly two layers were used and it was NOT a vapor barrier, just a wall sheathing to keep the weather from working its way inside. It was intended to be covered to protect it from the weather, and was sold as having some insulating ability. One brand was called "Insul-board". The "R" factor was next to nil. I still see what looks like it, used in some cheaper construction projects.
Next, the plywood would have been added much later, as it was not readily available for construction in the 1940s. Also 1" thick plywood is not commonly used in construction, you may want to check the actual thickness.
The need for a vapor barrier over an unheated crawl space will depend on where you are. If you need or want it use 6 mil poly sheeting. I would replace the 1 x 8s with solid plywood, and shim the tops of the joists to compensate for the missing tar board. If you have a lot of dampness, you may want to use pressure treated plywood, but better to to eliminate the cause of the wetness and keep it dry.
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Thanks for the quick response. Sorry my inaccurate terms come from total lack of construction experience. Your advice was very helpful.
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.
vapor barrier should go on dirt, with overlaped and sealed edges.
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Here's the thing: You stated that your crawl space gets really wet at times. The problem with wet crawl spaces is, - They cause mold growth in every organic surface, and that includes wooden floor joists, sub floors and some types of insulation. - They harbor moisture loving pests, such as termites (and if did detect some termite damage too right?) -They ruin your indoor air quality and are a burden when it comes to heating and cooling costs.
So, without solving the moisture problem in your crawl space, no matter what you do in terms of vapor barrier in your kitchen floor, it will not keep the moisture from the crawl space from rotting anything that is underneath the vapor barrier. Does it make sense? So it is not even a matter of "if" you are going to do this kitchen floor replacement again, it is only a matter of when... And if it the crawl space gets wet as you say it does, it will be sooner than you'd like to think.
My recommendation is that you control moisture in the crawl space, by encapsulating and conditioning it. And you will not need to worry about placing any vapor barriers on your sub floors. There are a ton of independent studies about the benefits of crawl space encapsulation. You cane see some of them here: http://www.crawlspace.org
Here you can find some more information and alternatives: http://www.basementsystems.com/learning_center/crawlspace /
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