Re: [GBlist] vapor barrier

["John Salmen" <terrain3@home.com>]

In a humid climate venting an attic will bring in moisture laden air from
the outside - it will not remove it. In colder climates the source of moist
air is from the inside of the building not the outside.  So providing
ventilation makes no sense for either scenario and this has been
documented - CMHC has conducted studies in western canada comparing vented
to unvented roofs and BSC covers this topic quite well. Though most codes
still call for min. 1-1/2 ventilation this will change. ASHRAE has already
done so.

The key is air sealing to the interior - in Sacies case the thickness of the
EPS insulation more than provides sufficient impermeability but she is
looking to protect the wooden roofing in the course of construction. Any
additional waterproofing member in a roof  assembly is an ok thing outside
of the waste of material. I'm not fond of using any bitumen type product
though within the interior envelope and am not sure how it would react with
eps over the long term.

John Salmen
Terrain EDS

----- Original Message -----
From: "David Porter" <dporter@porterarchitects.com>
To: <greenbuilding@crest.org>
Sent: Tuesday, July 31, 2001 3:59 AM
Subject: Re: [GBlist] vapor barrier


> >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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______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________