[Greenbuilding] How green is cellulose infill?

[Keith Winston]

I've built with double studs, 2x4 + 2x3's on 2x10 plates, and then blown 
it with cellulose (much like Corwyn's Larsen trusses, but structural). 
Vertical studs. I don't really love the strapping approach, since you're 
hanging the weight of the interior walls (plus shelves, or whatever) on 
strapping... But maybe that's ok. I've heard people doing it with 2x4 
strapping installed on edge, for the extra 2", and I just don't think 
I'd trust that. Compared to the Larsen trusses, the plates in my 
approach (not that I invented it) function as thermal bridges. That can 
be gotten around by actually building double walls, which is cheaper and 
more materially efficient, but there is a great convenience to using the 
wide plates to lay everything out at once, and tilt up and plumb just 
one wall, not two. There are a thousand possible variations.

Also, Jefro, the reason for the strapping is twofold. One, to make a 
deeper wall and therefor a higher R-value wall. And two, to minimize 
thermal bridging. If the deeper wall were constructed simply of deeper 
studs (2x8, or 2x10 for example) you might have an R-24 or even R-35 
wall (cellulose is between R-3 and R-3.5, in my book), but it would have 
all these "stripes" where the studs are that would be more like R-8 or 
R-10. Besides the energy cost of this, it can also occasionally result 
in funny things like wall discoloration and the like. Hence people 
either do strapped walls (with the strapping running perpendicular to 
the studs, so the only bridging occurs at the intersections), or 
double-studded walls (either with offset studs, or not: I've never 
figured how much it matters. It's easier to build with aligned studs, 
since you have to do that at all the doors and windows anyway).

Finally: one of the effects of plywood sheathing is considerable 
resistance to racking forces. If you doff the plywood skin, you 
typically have to include diagonal bracing or shear panels or something. 
Oh, I see you probably knew that upon reread. Anyway, strapping doesn't 
do anything for racking/shear forces. A strapped wall can still rack 
quite a bit, since it just requires rotation at all the small joints 
between straps and studs.

Also: I really like cellulose, it's all recycled and appears to be quite 
mellow. You can blow the stuff with a regular mask and not feel like you 
just exposed your body to some toxic soup laced with barbed wire, as per 
fiberglass (even with a good respirator, full-body-suit, eye protection: 
which are no fun on a hot and humid day...). It uses the paper waste 
stream, and I think it can use low-quality paper, which is often not 
recycled at all.

Hope that helps!

Keith


Mike O'Brien wrote:
> Hi, Jefro--
>
> Our general contractor, Dave Heslam of Coho Construction, has built  
> staggered-stud double walls on 2x8 plates, and wanted to try the  
> strapped wall. He thinks it might be easier. Also our structural  
> engineer wanted 2x6 @ 16" OC for seismic requirements (we are in Zone  
> III and the main floor is very open).
>
> Best,
>
> Mike
>
>
> On Jul 9, 2006, at 12:21 PM, Jefro wrote:
>
>   
>> Excellent information, thanks.  Can you tell me who your supplier is?
>>
>> Out of curiosity, why is the strapping necessary?  If we do go with  
>> cellulose we'll likely use a double wall and hang plywood on the  
>> outside, which would far exceed any shear requirements.  Are you  
>> building without plywood?  (well done if so!)
>>
>>
>> Mike O'Brien wrote:
>>     
>>> Hi, Jefro--
>>>
>>> Here in Portland, we are building a house with framed walls, 2x6  
>>> on 16" centers and interior horizontal strapping to create a  
>>> 7-1/2" wall depth. We plan to blow cellulose into these walls.
>>>
>>> As far as I can tell the fire retardant, which also acts as a bug  
>>> retardant, is boric acid and borax pentahydrate. The cellulose  
>>> fiber is from recycled newsprint. And from all the research I have  
>>> done, the borates are safe unless you eat them.
>>>
>>>       
>
>
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-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com