[GSBN] Earth plaster and Cellulose

[Jacob Deva Racusin]

Hello,

My apologies for the cross-post, but I'm trying to do a very quick and 
broad survey.

I was just contacted by a former student I am advising, who is project 
managing a building currently in design phase, in the mid-Atlantic 
region in the eastern US.  The wall assembly was initially to be 
straw-clay, with plaster finish on both sides. Budget and logistics have 
directed an insulation switch to cellulose.  He is currently advocating 
for wood lath with lime-stabilized earthen base coat, with finish lime 
coat (our standard approach for finish exterior plaster).  The architect 
is balking, suggesting that the moisture storage and release dynamics 
for which we rely upon the plaster in straw-based designs will not play 
out the same way with cellulose, that the cellulose will be overwhelmed 
with moisture and push the dew point deep into the wall.  This is all 
second-hand reporting of the conversation - I know no more than what I 
just reported - but I am due to speak with the architect and client 
tomorrow on my student's behalf, and I'm curious as to whether or not 
any of you have direct experience with earth base plaster over wood lath 
outboard of a dense-packed cellulose-insulate wall, or any reason to 
believe the cellulose would not behave adequately in concert with 
adjoining plaster (as opposed to straw).

The architect is spec'ing (outward from the framing) plywood, housewrap, 
drainage gap, stucco lathe, plaster.  My impression is that this 
suggestion would certainly work, but is hardly the natural wall system 
initially intended.  It seems to be playing off of the concerns of 
sun-powered vapor drive pushing vapor deep into the wall cavity as a 
result of a reservoir cladding adjoining the insulation.  As long as 
there is no interior vapor barrier or Class II vapor retarder, I feel 
this dynamic is identical to that we've created with straw-insulated 
walls, and if anything the cellulose would stand up better to incidental 
vapor or liquid moisture intrusion.  Also note generous overhangs and 
24" grade separation are spec'd in the design, as is multiple coats of 
limewash (and potentially a silicate top coat for durability), which 
will further reduce bulk water absorption and minimize incidental 
solar-powered vapor drive.  The wall is designed to dry to both sides of 
the assembly, in keeping with the variable moisture drive of that region.

Perhaps there is another concern here I'm not understanding - I'll find 
out more tomorrow - but in the meantime if anyone has any perspective on 
why this situation would work for straw, and not cell, or any direct 
experience in a comparable climate executing such a system, I'd be 
grateful for your input.

Thanks so much,
Jacob

-- 
Jacob Deva Racusin
New Frameworks Natural Building, LLC
P.O. Box 15, Montgomery, VT 05470
(802) 782-7783 (c)
(802) 326-2209 (h)
www.newframeworks.com
jacob at newframeworks.com