[GSBN] [nbne] Earth plaster and Cellulose

[Derek Stearns Roff]

If it is of interest, the Green Building Advisor has a few articles on this question.  Perhaps the most specific is "When Sunshine Drives Moisture Into Walls" http://www.greenbuildingadvisor.com/blogs/dept/musings/when-sunshine-drives-moisture-walls.

It contains the quote below, and insists that the third point, the interior vapor barrier, is a critical part of the problem.  Without the vapor barrier, the author says, the problem is very unlikely.  Air conditioning is also described as an essential part of the problem.  I'm guessing that in the location (mid-Atlantic) of this project, air conditioning might be part of the plan, but if not, this further decreases concern with solar vapor drive.  ("Solar vapor drive" sounds like what I wish my car would run on.)

Derelict

Inward solar vapor drive problems require four elements
The phenomenon that destroyed Zaring’s walls came to be known as inward solar vapor drive. The classic disaster requires four elements:

  *   A “reservoir” cladding — that is, siding that can hold significant amounts of water;
  *   Permeable wall sheathing like Celotex or Homosote;
  *   A polyethylene vapor barrier on the interior of the wall; and
  *   An air-conditioned interior.



On Apr 25, 2013, at 8:46 PM, Jacob Deva Racusin wrote:

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<http://www.newframeworks.com/>
jacob at newframeworks.com<mailto:jacob at newframeworks.com>

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>


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