From: sasa3d@morisot.unx.sas.com (Ali Dogrusoz)
Subject: Moisture barrier on top of faced insulation or NOT
Originator: sasa3d@morisot.unx.sas.com
Date: Mon, 14 Dec 1992 20:55:22 GMT
Distribution: usa
Organization: SAS Institute Inc.
Followup-To: misc.consumers.house

Reguarding the putting plastic over the backing of the insulation, it
probably isn't necessary, but it probably won't hurt either -
most important is to make sure the vapor barrier is toward the
warm side (inside) space.  As for the ceiling, IMHO putting up a
plastic vapor barrier won't hurt (and in fact some sort of vapor
should be put up).  For the most part you can insulate a ceiling
by either putting in a faced (vapor barrier attached) insulation
on top of the sheetrock, or you can put up a platic vapor barrier
between the sheetrock and an unfaced insulation (I'm saying unfaced
since it tends to be cheaper than faced and having 2 vapor barriers
is probably not necessary).  Always keep in mind that you want the
vapor barrier toward the warm (generally interior) spaces.

Another point on insulating the ceiling.  I suspect that you are
insulating between the studs that form the roof of the garage?
If so this is something like insulating a cathedral ceiling, and
you will probably want to install baffels against the roof boards
and install some sort of sofit and ridge ventilation to help
ventilate the roof and take away any moisture that does find its
way into the insulation.

IMHO, you're not going to trap much moisture between a sheet of
plastic tacked to the inside of the studs and a piece of sheetrock
put up immediately over the top of it (sheetrock passes some moisture
anyway - unless painted with a vapor barrier paint).  However,
typically you probably need twice as much ventilation if you don't
have a vapor barrier to insure that all the moisture is carried away.

A few other points.  The purpose of a vapor barrier is to prevent
the transfer of moisture cointaining air to an area of lower
temperature at which the water can condense on a colder surface
(eg, your insulation or roof boards).  It's this condensed moisture
(water) that does most of the damage.  This kind of temperature
drop probably won't happen in the 1/2 distance of the sheetrock in
an insulated ceiling (an uninsulated one is another matter).

Eric Kasten   Tigger@ibm.cl.msu.edu   all standard disclaimers apply

PS - With the recent high interest in insulating homes, there have
     been a lot of books written on the subject - might want to
     check out the library.

=====================================================================
The subject mostly says it. I am planning on putting wall board in my
"Bonus room" over the garage. The room is insulated with faced batts.
Should I put another layer of 4-6 mil plastic over the backing or there
is no point? What about the ceiling I've heard that one should not put
a plasic vapor barrier on the ceilings (the rational was that moisture
would be trapped between the wall board and the barrier and the little
infitration between the stud and the insulation backing would allow the
moisture to escape.) what do you think?

Also on a side note, months back someone posted some hints on how to
sheetrock (Obviously I didn't save it) can someone mail me their copy?

Thanks
--
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Ali Dogrusoz | SAS Institute Inc. | Phone (919) 677-8000 |sasa3d@unx.sas.com
             | SAS Campus Dr      |                      |
             | Cary, NC 27513     |                      |
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