[GSBN] Inward Vapor Drive

[Jacob Deva Racusin]

Greetings,

I am working with a client in the mid-Atlantic region of the United 
States (southern Maryland), and a concern has come up regarding inward 
vapor drive of a plastered wall.  In this case, the insulation is 
dense-packed cellulose, but may as well be straw for all practical 
purposes of this conversation.  Exterior lime-stabilized base plaster 
with a finish lime plaster is designed to be directly coupled to the 
insulation, using traditional wood lath as a substrate.  The concern is 
that a moisture-charged reservoir of plaster directly adjacent to the 
insulation will result in a strong inward vapor drive during wetting 
events followed by sun (solar vapor drive), on top of what is already a 
inward-dominant vapor drive for that climate (despite the fact that it 
is a heating-dominant climate - this is what the local performance 
consultant tells me).  We discussed the application of limewash or 
silane/siloxane to reduce liquid absorption into the plaster, as well as 
the creation of a vapor-open assembly allowing the vapor to drive all 
the way to the interior and the use of clay plaster to store excess 
moisture.  This building will be relatively tight with adequate 
ventilation.  That said, the local performance consultant is very 
concerned about the assembly's ability to deal with inward vapor drive 
and the lack of drying potential in that climate.  For reference, I 
practice in the relatively colder Northeast.  To that end I'm looking 
for advice on a few things:

- Do any of you have case studies and/or hygrothermic models (i.e. WUFI) 
of plastered exterior systems in humid mid-Atlantic climates? By 'case 
studies', ideally some degree of moisture testing would be helpful for 
convincing the skeptical, but basic empirical "this is working under 
these conditions" would be great too.  The performance consultant is 
going to run a model through WUFI Passive, but I'd be interested in 
seeing if anyone has already looked at this, particularly for this climate.
- Has anyone seen failures in plastered exterior systems as a result of 
vapor migration?  Not from liquid, mind you - or rather, not bulk liquid 
penetration through the plaster into the straw (or cell, in this case) - 
but from condensation or significantly-elevated RH?
- One recommendation is to back-vent the plaster.  I fully understand 
the benefit of this in regards to interrupting vapor drive and allowing 
penetrating liquid to drain, and the air-seal would then be provided by 
an exterior air barrier/weather-resistant barrier on the inward side of 
the gap.  All other considerations of feasibility and cost aside, I'm 
concerned about saturation of the plaster and a greater propensity for 
damage from freeze/thaw conditions with atmospheric temps being able to 
reach both sides of the plaster.  Any reason I shouldn't be concerned 
about this?  Any recommendations on appropriate detailing for 
back-venting a clay plaster?  I'm inclined to use wood lath as a 
substrate, but wonder if this would only exacerbate damage from 
freeze/thaw...

Thanks in advance for your insights,

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