Re: [GBlist] vapor barrier

["David Porter" <dporter@porterarchitects.com>]

>I need a vapor barrier above a t&g decking ceiling under a
> thick layer of EPS roof insulation and metal roof.  I would like to use
> taped 30# felt rather than taped 6 mil poly because it will be much safer
> to walk on while the insulation and hold downs are being attached.  Will
> this provide an effective vapor barrier?

It sounds like you will have a waterproof and vapor proof barrier above the
insulation (e.g. the roofing underlayment as well as the roofing) and now
want to also install one underneath the roof insulation; sort of like
sandwiching the EPS insulation between a top and bottom layer of vapor
barrier.

If so, I don't know if that is such a good idea.  When one installs a vapor
barrier on a typical house ceiling, there is usually an attic space above
the ceiling that allows air circulation above the insulation to take out any
moisture that builds up in that cavity.  Same holds true for cathedral
ceilings where the construction has insulation in the rafter cavity but
there is a couple of inches of air space above the insulation and air flows
through each rafter bay to a ridge vent, thus also evacuating any moisture
and heat that builds up in that cavity.  It is a no-no to build a cathedral
ceiling with rafters where the entire rafter cavity is stuffed solid with
insulation that is trapped between the roof sheathing and the ceiling board.
It is always required to allow for a minimum 1 1/2" air space above the
insulation in that situation.

I think if you sandwich the insulation between layers of vapor barrier
material, you might be forming a cloud chamber in the insulation board.
There will be no where for moisture or vapor to escape out of the insulation
if any should get into it.  Maybe I am wrong and others will chime in to
correct me but I would not install a vapor barrier underneath the rigid
insulation if it is directly beneath the vapor barrier for your roofing.

David Porter AIA
Palm Beach Gardens, FL USA
dporter@porterarchitects.com
www.porterarchitects.com




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