[GSBN] Cellulose in unvented roof

[John Swearingen]

We've been encountering this issue also, due to the new fire-safe
regulations which discourage eve vents.  My understanding is that 1/4" mesh
may be acceptable.  This is, however, Sonoma County, where they tend to
re-invent the wheel as an octagon.  There are some details which have been
proposed using baffle systems to break the force of the wind-driven embers
and let them fall harmlessly to a metal pan.  A commercial system that might
be accepted is: http://brandguardvents.com/index.html

We've had dense, wet-blown cellulose permitted, as you have, in a variety of
jurisdictions. Isn't there good data on dense cellulose to make a case?  We
presented that in Los Altos several years ago with success. As a more
expensive alternative, soy foam insulation would probably be permitted in
Sonoma.

All the best,
John



On Tue, Jul 15, 2008 at 11:08 AM, David Arkin, AIA <david at arkintilt.com>
wrote:

> Hi GSBNoids:
>
> Kelly's note reminds me to comment on this also.  Tim and I talked
> about this issue recently.  Sonoma County has an (in my opinion,
> unwarranted) policy against unvented full-cavity, dense-packed sprayed
> cellulose insulation.  The building official observed some failures of
> unvented fiberglass batt cathedral ceilings while in Santa Cruz
> County, and he puts cellulose in this same category.  We've used
> unvented cellulose throughout Northern California without any problems.
>
> Venting is now a tricky issue thanks to the new building code,
> especially here in California with extreme new restrictions on
> exterior finishes and venting in fire zones.  No venting of the
> soffit, eaves or fascia are allowed, to prevent embers from being
> drawn into the attic.  Sonoma does approve of ventilated roofs, so
> long as the intake is on the roof, at least 6" in from the edge, and
> screened.  We've modified our 'Ice-House' roof detail to have the air
> intake above and back a couple feet from the edge of the roof.  The
> ice-house roof is essentially a second roof shading a lower one, and
> providing a space for air to flow between them.  Good for keeping cool
> on a warming planet.
>
> Tim, feel free to give me a call if you want to discuss more.
>
> Good day to all,
>
> David 'Have an ice day' Arkin
>
> On Jul 10, 2008, at 5:05 PM, timok wrote:
>
> > Hi All,
> >
> > I know this question isn't exactly straw bale but, it is a topic
> > about which
> > reasonably good builders disagree. and we've come accross a county
> > that
> > wants more legitimate reasoning than we're discovering on our end
> > soooo...
> >
> > Do any of you have an U.S. building code accepted report or testing
> > on the
> > use of cellulose in an unvented roof cavity. we have a roof design
> > that's
> > almost impossible to vent and it's pre-insulated with cellulose so
> > we can't
> > back up now. the best we've come up with is an supplement to the
> > 2007 IRC
> > that allows Air permeable insulation to be used with the addition of
> > R-15
> > rigid insulation on top as a condensation control. though this
> > sounds like a
> > good idea thermally and long term, there is already r-42 in the roof
> > and the
> > facia is bought, so I'd love to find a few reports that back the
> > concept
> > thoroughly enough to make our case stick.
> >
> > of course I'm open to being schooled by the great wizards of building
> > science should my thinking be flawed about the benefits of an
> > unvented roof
> > cavity. but thermally, and to protect against fire flow up and into
> > the
> > eve's, I believe we are better vent free and I think the
> > condensation is
> > better handled in drain planes below the roofing and in not drawing
> > extraw
> > moisture into the roof through the eve's. and from my humble
> > interpretation
> > of John Straube last CASBA presentation there would have to be
> > enough air
> > pressure to suck small children off of the side walk in order to
> > move air
> > through the small cavities significantly enough to matter; for
> > moisture or
> > heat exhaust, was my interpretation.
> >
> > just trying to build the best buildings we can,
> >
> > from the firey hills of Mendocino county, California...where we
> > would traid
> > a lot for some condensation, i extend my thanks for your help with
> > this
> > detail
> >
> > tim
> >
> >
> >
> > Tim Owen-Kennedy
> > Vital Systems
> > Box 751, Ukiah, CA 95482
> > www.vitalsystems.net
> > _______________________________________________
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> > GSBN at greenbuilder.com
> > http://greenbuilder.com/mailman/listinfo/GSBN
> >
>
> *  *  *  *  *
> Arkin Tilt Architects
> Ecological Planning & Design
>
> David Arkin, AIA, Architect
> LEED Accredited Professional
> CA #C22459/NV #5030
>
> 1101 8th St. #180, Berkeley, CA  94710
> 510/528-9830
> www.arkintilt.com
>
> "There is no way to peace. Peace is the way."
> — A. J. Muste
>
>
>
>
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>



-- 
John Swearingen
Skillful Means, Inc.
Design and Construction
www.skillful-means.com
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