[GSBN] Inward Vapor Drive

Wed May 29 01:55:26 UTC 2013

Thanks John and Graeme for the replies.  They validate my thinking on 
this, I'm glad to hear I'm not over-trivializing the issue.  That said, 
I'm hoping to drudge up something more specific to have a well-informed 
(i.e. case studies, test data, modeling) response to the performance 
consultant's concerns.  I think his concerns are valid, but overblown, 
fueled primarily by a lack of familiarity with how these systems work 
(possibly mixed with a fear of the dreaded solar vapor drive that has 
sunk well-publicized brick-clad, vinyl wall-papered development 
houses).  That said, in the language of data, and with my climate 
outsider status, I'll need a bit more to convince him than theory - 
despite how convincing it is.

This is exactly why I'm developing the in-wall temp/RH sensor systems 
(not sure if I mentioned these yet on this list?  I've spoken with at 
least a few of you about this).  Having access to long-term data from 
real buildings in a variety of climates will really help inform these 
types of conversations.  I'mabout to deploy the alpha-beta version, will 
be asking for interested beta testers in another month or so, more to 
come...be in touch if you are interested in putting sensors in your bale 
walls...

Thanks, glad to be a part of this group!

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

On 5/28/2013 6:22 PM, Graeme North wrote:
> 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 <http://www.ecodesign.co.nz>
>
>
>
> On 29/05/2013, at 3:01 AM, Jacob Deva Racusin 
> <buildnatural at googlemail.com <mailto: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 <http://www.newframeworks.com>
>> jacob at newframeworks.com
>>
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