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
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.
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
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.
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
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:
Here you can find some more information and alternatives:
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