Our customer wants to install an unfinished, thin, hardwood floor over
pressure treated plywood.  The moisture difference between the two
materials needs to be within 2% to 4%.  The plywood tests at almost
20%, the hardwood between 4-6%.  We need a vapor barrier that will
close up around the nails.  One material suggested is "bitchathane"
but the flooring contractor checked with the manufacturer of one type
used by roofing contractors in "ice" situations.  The manufacturer
didn't think it would work but thought there might be another product
that would, but needed the list of materials in the pressure treated
lumber.  What should we use to fix this problem? We are already using
fans to try to dry out the plywood but it will undoubtedly take a very
long time to dry out enough.  We think the vapor barrier will be the
answer...

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Request for Question Clarification by ericynot-ga on 18 Oct 2002 15:07 PDT

I may have an answer for you, but it will have to wait until Monday
for confirmation probably.

The nails you need to "close up around" are where exactly? Through the
plywood into concrete or some other substrate?

You are not nailing the woold flooring to the plywood are you?

If you can provide a little more detail this weekend, I'll get right
back to your question

Thanks,
ericynot

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Request for Question Clarification by alienintelligence-ga on 19 Oct 2002 03:23 PDT

Hi carter9

Have you considered a "painted on" product like a 
latex paint or Thompson's water seal? Maybe with a
6-mil polyethylene layer in between?

-AI

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Request for Question Clarification by mrlathwell-ga on 20 Oct 2002 09:59 PDT

Just a couple of questions. 

Is this a basement development? If so, is the pressure treated plywood
on top of the concrete or suspended via a joist system? If the floor
is not associated with a basement development, is it part of a
sub-floor development? If so, what is the purpose of having a pressure
treated plywood sub-floor?

Where are you working? Environmental conditions can affect the
effectiveness of any solution. Give some idea of the region this
project is taking place, with consideration given towards the moisture
level of the land on which the house or structure is built.

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Clarification of Question by carter9-ga on 20 Oct 2002 11:21 PDT

The nails used will probably be galvanized finish nails, shot from a
nailgun thru 5/16" thick oak hardwood strips.  They will attach thru
the vapor barrier into the 3/4" tongue and groove, pressure-treated
plywood which was nailed into pressure-treated 2x8 douglas fir floor
joists.

We don't think we can use a painted-on product (latex paint or
Thompson's water seal) because it wouldn't "close up" around the
nails.  The nail holes create the avenue for moisture to get thru to
hardwood, causing it to "buckle" later.

The pressure-treated lumber (both joists and plywood) was used because
the area will no longer be accessible and there is no other
ventilation (nor could we have installed additional venting)!  The
code requires that there be a 12" space from the bottom of the joists
to grade (dirt) in normal situations.  As this area has a 4" concrete
pad existing, but only 8" from top of concrete to top of the joists
(raising the floor to equal other floors in the rest of the house). 
In addition, we use pressure-treated lumber in these situations
because normal plywood might eventually be attacked by mold, mildew,
and/or white fungus.  This might buckle the plywood, causing damage to
the flooring above.  Currently, we have fans running in this area
whenever someone is home to monitor the fans, in an effort to dry out
the pressure treated plywood.

The only possible answer we have come across is "bitchathane" but the
materials we found is not recommended for this application my the
manufacturer, according to the Owner's Flooring Contractor.

The property is located in San Mateo, California, approx. 23 miles
south of San Francisco on the peninsula.

The project is a remodel of an existing Family Room (single story) on
the back of a two-story structure.  It is approx 10 feet wide, 32 feet
long.  We removed the two exterior walls (the "L"), kept the existing
roof structure, formed, framed and poured a 6" tall curb, raised the
floors to meet the floors in the rest of the house, reframed the
outside walls to include two Marvin sliding french door sets and one
window.  Unhappily, at this point we found out about the moisture
problem.  We are trying to find out the easiest remediation.  We don't
want to remove the pressure treated plywood as it was glued down per
spec and nailed with ring-shank gun nails (very hard to remove and the
framing underneath might also need to be replace if damaged.  A
tremendous job, though not impossible.... This is why we are asking
for help.

Hi carter9,

Thanks for your very well-presented question. You obviously are
dedicated to finding the best solution for your customer, and I
believe I have come up with it. W.R. Grace & Co. makes a product
called Perm-A-Barrier System 4000 Wall Membrane that seems to satisfy
all your requirements.

I chased your problem around the Internet for a couple of days, then
found myself in conversation with the support people at Grace. They
assure me this product will do the job for you, and my experience with
their company's Construction Products Division gives me confidence in
their expertise although they obviously have an ax to grind in
answering your question.

Perm-A-Barrier is described on their website as "a tough,
self-adhering, self-healing composite membrane. It consists of 0.9 mm
(.036 in) of rubberized asphalt and an integrally bonded 0.1mm (.004
in), high density cross laminated polyethylene film. When combined
with a structural substrate, Perm-A-Barrier wall membrane produces a
virtually impermeable air and water barrier on concrete, masonry,
gypsum board and wood walls. It is designed to be a cost-effective
system which acts as both an air and vapor barrier. Perm-A-Barrier
Wall Membrane retards the diffusion of moisture through the building
enclosure. It provides a continuous seal against air and vapor leakage
around the entire building."

The product is sold as a vapor barrier for vertical surfaces, but I am
assured it will work well in your application as well. Here are its
most salient features:
 
    1)  virtually impermeable to moisture
    2)  cross-laminated film for dimensional stability and tear,
puncture, and impact resistance
    3)  cold-applied, so no flame hazard
    4)  self-adhesive
    5)  flexible to accommodate minor settling and shrinkage
    6)  self-healing around mechanical fasteners (one of your main
concerns)
    7)  comes in 3' x 83.4' rolls, each weighing about 75 lbs.

If you go to this page, you will find a thorough data sheet detailing
topics such as handling, safety, application, and so on:
http://www.na.graceconstruction.com/vaporret/download/permem.pdf

W.R. Grace representatives for your part of the country are:

North (Area Codes 209, 341, 408, 415, 510, 530, 559, 650, 707, 831,
916, 925)
Scott Tyson
Florprufe, Perm-A-Barrier, Procor
866-333-3726, select option 3, then option 2, extension 5675
scott.tyson@grace.com 

Bob Ogletree
Florprufe, Perm-A-Barrier, Procor
866-333-3726, select option 3, then option 2, extension 5670
Bob.Ogletree@grace.com 

The W.R. Grace office nearest to San Mateo is just across the Bay from
you at:
2140 Davis St
San Leandro, CA 94577-2292
Phone (510)568-3427

Again, I think the solution above will very nicely do the trick, and
Grace, as you probably know, is a reputable company with an extensive
network of support representatives and distribution points.

If you have any concerns about this answer, just ask for clarification
and I'll be happy to get right back to you. Thanks again for your
question, and best of success with your project.

Regards,
ericynot-ga