[GSBN] Cellulose in unvented roof

[David Arkin, AIA]

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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*  *  *  *  *
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