Re: [GBlist] Qustn: Rigid Insul above exist'g roof

[John Salmen <terrain@shaw.ca>]

It is a moot point to some degree as most codes require ventilation. The
origin of the code requirement is not something documented in any 'building
science', rather it is something I would call anecdotal - a decree built
upon decree. There was some more extensive discussion of this recently on
the list.

If you look at Joe Listurburek writings he makes an extremely good case for
not ventilating in hot and hot humid climates. CMHC studies in Canada also
make a good case for not ventilating in cold arid and cold moist climates.

If however you have a leaky roof assembly a ventilated attic will mitigate
the problem to some degree and a 'hot roof' assembly will not. If there are
major air leakages in a hot roof assembly there can be problems. In other
words a vented attic will disguise a myriad of problems, whereas if there
are problems in an unvented assembly they will become a much more visible
problem and potentially harder to deal with.

I feel comfortable designing and having built unvented assemblies and
generally I think they are preferable but the detailing and craftpersonship
becomes important.


----- Original Message -----
From: "Scott Schreffler" <ScottS@bumgardnerseattle.com>
To: <greenbuilding@crest.org>
Sent: Friday, November 30, 2001 3:03 PM
Subject: RE: [GBlist] Qustn: Rigid Insul above exist'g roof


> I don't think you need to vent.  A rule of thumb is to ventilate when the
> insulation is on the inside.  No vent if insulation on the outside.
That's
> why you don't have to vent when you use a SIP.  Essentially it's on the
> outside.  If you do the "green" thing here, and stack ventilation on the
> existing roof (no waste to landfill) then it's adding insulation onto the
> outside.
>
> No ventilation needed.
>
> Scott Schreffler
> Seattle
>
> <<so the qwuestion is...to vent or not to vent>>
>
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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
______________________________________________________________________