SB: Re: [GBlist] Re: SB: Venting cathedral ceiling/roof insulated withcellulose?

[John Salmen <terrain@shaw.ca>]

I agree that an air barrier has to be tight meaning specifically that you
can't allow large flows of moist interior air to some potential point of
condensation - but this is true of any roof assembly and in a cathedral
assembly w/ venting the moisture laden air will most often find some point
to condense before the ventilation gap and in the case of dense materials
there will be little air movement affecting any drying into the insulation.
In a typical loose batt insulated attic the airflow through the insulation
will affect some drying but the reduction in insulation value is major.

It may be nice to think that ventilation is going to accomplish something
but I just don't see it despite the fact that ##$% happens blowing air on it
is not going to make it go away. If anything the ventilation gap creates a
cooler roof which in the case of duroid may extend the life slightly but in
the case of metal is just more potential for condensation and perhaps a
bigger less controllable problem - your roof is then only as good as the
roofing felt below. In the case of humid environments you are just creating
holes for humid air to move through.

The best insurance against ##$%^ happening is to make sure the materials
chosen can deal with a little and then to go to the source of the problem
which is interior moisture levels.






----- Original Message -----
From: "Robert W. Tom" <ArchiLogic@yahoo.ca>
To: <strawbale@crest.org>; <greenbuilding@crest.org>; "Whitney Burgess &
Tristan Kelley" <raven@tetontel.com>
Sent: Wednesday, May 15, 2002 9:07 AM
Subject: [GBlist] Re: SB: Venting cathedral ceiling/roof insulated with
cellulose?


> 5/14/02 12:22:11 PM, Whitney Burgess & Tristan Kelley <raven@tetontel.com>
> wrote:
> [snipped & pasted]
>
> >...whether or not to vent the cathedral ceiling/roof ...that will
> >be insulated with dense-packed cellulose.
>
> Whitney & Tristan;
>
> Theoretically, you should not need to vent the ceiling/roof if the
interior air
> barrier at the ceiling is perfectly air-tight.
>
> In reality, $#!+ happens and theory (as well as the air barrier) is, more
often
> than not, imperfect.
>
> In addition to the likelihood of an imperfect air barrier, the reality of
"$#!+
> happens" also means that moisture will likely find other ways to get where
it's
> not intended to go.
>
> Without a ventilation airspace to permit moisture to cook off in the event
that
> it does find its way into the ceiling insulation, damage is almost certain
to
> occur.
>
> In providing a ventilation airspace to accommodate drying, you lose a bit
of
> space that otherwise could have gone towards insulation. That lost R-value
is
> pretty easy to regain elsewhere.
>
> --- * ---
> Rob Tom
> Kanata, Ontario, Canada
> (remove the "chaff" from my edress if you hit "reply")
>
> Please visit http://www.theHungerSite.com daily
>
>
>
> ______________________________________________________________________
> This greenbuilding dialogue is sponsored by REPP/CREST, creator of
> Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
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> ______________________________________________________________________
>


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