[GSBN] Cellulose in unvented roof

[timok]

Hi All, 

Thanks for the input on my detailing documentation dilema. as you can see 
from the pasted text below of the notes between JFS and I we have decided to 
err on the side of caution and live to fight the beauracracy another day. if 
any one is interested in the supporting documents we are using just post to 
me off list and ill be happy to send you a copy of them as well as our final 
roof detail. 

stay true, 

tim 

p.s. for those who have been watching our latest battle with fire the flames 
came up to my property three times and were beat back twice by the Fire 
service and once be me and some volunteers. And though we're not totally out 
of the woods (pun intended) the smoke is starting to clear. though I will 
miss the red moon rises and gloriously violet-red-orange-gold -green 
sunsets, it's amazing how valuable a really deep breath of fresh air can be. 

 

Thanks for the note John, 

We have concluded that adding R-5 rigid over the roof sheathing is cheap 
insurance: the client probably would have spent more on my services and 
other consultants to prove otherwise. I still believe that our roof as 
designed would be fine, but since I see the added insulation and 
condensation buffer as significant improvements, I can stomach the changes 
in aesthetics and cost. Our roof is currently planned for copper shingles, 
though with the price of copper to day we may trade them in for composition 
and somspend the balance on fancy plaster detailing. I think the moisture 
would condense well above the double #15 felt and not bellow the felt. But, 
since we don't 100% understand the reason some fail and others don't I like 
the redundancy that the rigid offers, especially when it comes to moisture. 
wow I'm even being more redundant in this paragraph than I am on this roof. 
well what was it Mark Twain said? if I'd had more time I'd have written a 
shorter letter. 

Well, I trust you are well and as always I appreciate the morsels of moist 
wisdom you drop on us peasants. Hopefully we can share some good wine soon. 
speaking of which, rumor has it that the SIP's failed over the barrel room 
at Ridge. I guess they should have taken your and my advice on that one. at 
least the Bales are still fine. I knew when I saw them putting electrical 
tape over all the staple holes in the visqueen that it was only a matter of 
time. 

enjoy being, 

Tim 

jfstraube at gmail.com writes:
> Hi Tim!
> The answer depends on your climate. I don't think there any code reports that will help you alas. It usually will work in warm dry climates when a dark roof is used. We have seen failures in places as warm as Tuscon when white roof membranes were used. The code clause was written to help define how much insulation was need on top of deck. It assumes you don't have any airtightness. Of course cellulose is fairly good just not good enough. We have seen failures in California and US Greenfiber no longer suppports the idea. The risk is low if you airtighten the ceiling plane but it is not negligible. A dry interior during cold weather helps too. Hope this helps a bit. 
>
> Be well. John. 
>
> Sent from my BlackBerry® wireless device 
>
> -----Original Message-----
> From: "timok" <timok at vitalsystems.net> 
>
> Date: Thu, 10 Jul 2008 20:05:15 To: (private, with public archives) Global Straw Building Network<GSBN at greenbuilder.com>
> Subject: [GSBN] Cellulose in unvented roof 
>
> 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
> _______________________________________________
> GSBN mailing list
> GSBN at greenbuilder.com
> http://greenbuilder.com/mailman/listinfo/GSBN timok writes:

> 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
> _______________________________________________
> GSBN mailing list
> GSBN at greenbuilder.com
> http://greenbuilder.com/mailman/listinfo/GSBN
 


Tim Owen-Kennedy
Vital Systems
Box 751, Ukiah, CA 95482
www.vitalsystems.net