[GSBN] Inward Vapor Drive

[Graeme North]

hi Jacob

The approach I have favoured where we can get days of wind driven rain and very high humidities is to keep by far the majority of the water off the walls either with exterior rain-screening or very good eave protection.  If the exterior pastors can not get saturated then the other issues that concern are not likely to follow.

This approach works a treat for me and I get to sleep at night.

cheers

Graeme

Graeme North FNZIA
Graeme North Architects
49 Matthew Road
RD1
Warkworth 0981

www.ecodesign.co.nz



On 29/05/2013, at 3:01 AM, Jacob Deva Racusin <buildnatural at googlemail.com> wrote:






> 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
> 
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