[GSBN] Inward Vapor Drive

Tue May 28 17:06:17 UTC 2013

I'm wondering how a significant amount of moisture could be driven to the
interior in a single event--rain followed by sun.  The amount of vapor
would be limited; a good deal of the moisture would dry to the outside, and
the amount driven in a single event would be not be more than that
contained in a plaster layer less what dries to the exterior.  It appears
you're assuming that the majority of moisture will be pumped to the inside,
and I'm not sure how this assumption is arrived at.

The only conditions that we've encountered with inward vapor drive are in
San Francisco, during the summer, when there is frequent (daily) morning
fog, followed by afternoon sun. We've seen south faces of building were the
nails of siding have worked loose because of the daily expansion and
contraction of the siding around the nail, but haven't seen moisture
effects--this is a mild climate.  This only occurs because it's a
*daily*phenomenon.

John "Fog In, Fog Out" Swearingen

On Tue, May 28, 2013 at 8:01 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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-- 
John Swearingen
Skillful Means Design & Construction
2550 9th Street   Suite 209A
Berkeley, CA   94710
510.849.1800 phone
510.849.1900 fax

Web Site:  http://www.skillful-means.com
Blog:         https://skillfulmeansdesign.wordpress.com
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