[GSBN] Plaster Reservoir?

Mon Jun 1 23:43:00 UTC 2009

A moisture buffer seems important when (a) there is a relatively impermeable
surface behind the cladding that is incapable of storage and/or (b) there is
a great deal of moisture migrating through the wall, like wind-driven rain.
Straw will act as a moisture buffer (storage) in the case of (a), absorbing
moisture and holding it until it can migrate to the outside.  In California,
we've used a rain-screen design as insurance in cases with extreme (coastal)
rain/wind combinations.

Interestingly, only place I've encountered problems (in conventional
buildings) with moisture pumping, has been in San Francisco, where the fog
rolls in in the afternoon, dampens the wall, and then is burned off in the
morning, with the sun striking south facing walls and, potentially, driving
moisture inward.  We see the results, not so much in damage from moisture,
as in popped nails in wood siding.  The frequent expansion/contraction of
the wood works nails loose--so we use screws.

John

On Mon, Jun 1, 2009 at 3:31 PM, Mark Piepkorn <mark at buildinggreen.com>wrote:

> Another question from the Northeast. The liaison is normally ej now, but
> since I stuck my foot in this one I thought I'd go ahead and forward it for
> any discussion and replies.
>
> - - - - -
>
> From: Jacob Deva Racusin <buildnatural at googlemail.com>
>
> I just read a great article by Joe Lstiburek on rain prevention in
> brick-clad structures here:
> http://www.greenbuildingadvisor.com/blogs/dept/building-science/bs-podcast-rain-control-energy-efficient-buildings-part-1
>
> Quickly summarized, brick cladding today is considered to be reservoir
> cladding (stores moisture), and that a drainage plane is required behind the
> brick to control liquid moisture.  Moreover, a ventilation plane should be
> coupled with that drainage plane, to deter the potential of vapor migrating
> into the wall cavity behind, specifically if it is vapor-permeable (using
> plywood sheathing, tyvek, among other common exterior wall treatments).  One
> of the main drivers of this migration is the vapor pressure born of sun
> heating the brick, driving vapor deeper into the wall cavity (I know, sounds
> unintuitive at first, but heat moves from warm to cold, and vapor from
> highest concentration to least, so heat-charged vapor will migrate into a
> cooler, drier interior cavity).
>
> So my question is - and I've been thinking about this for awhile, but not
> yet gotten it out of my head until just now - since we are primarily
> building vapor-permeable wall systems with reservoir renders (holding
> moisture), are we doing a greater disservice to our wall systems by opening
> up the potential for inward-driven vapor pressure from the exterior when the
> sun comes out after a rain storm, or are we doing a greater service by
> providing a moisture control medium for potential condensation issues along
> that exterior bale wall plane by having a 'moisture battery' in direct
> contact with the straw, able to mitigate liquid condensate?
>
> In simpler terms, is the moisture pressure greater from vapor pressure
> through the exterior plaster into the bales, or from condensation through
> the bales to the exterior plaster plane?
>
> I appreciate any insights, I'd like to be able to answer this question with
> confidence...
>
> Warmly,
> Deva
>
> - - - - -
>
> From: Robert Riversong <turningtide at ponds-edge.net>
>
> Deva - that's an excellent question!
>
> Where a moisture buffer (reservoir) is needed is on the inside, contiguous
> with the interior environment, just as heat buffers (thermal mass) are
> relatively ineffective on the outside of a building. An interior moisture
> buffer will keep indoor relative humidity far more constant and contain
> excess moisture without forcing it into the thermal envelope.
>
> Exterior reservoir claddings, such as brick, concrete, some stone, and
> thick earthen materials can be problematic if they are allowed to absorb
> significant quantities of water and exposed to direct solar gain. That is
> why brick cladding has traditionally been separated from the
> structural/thermal elements by a drainage space (with weep holes).
> Lstiburek's recommendation to also vent that space would make it a true rain
> screen and more effective at mitigating moisture migration from outside to
> in.
>
> In winter, moisture drives are happening simultaneously in two directions,
> with indoor temperature and vapor pressure pushing moisture outward (by air
> transport, temperature gradient and vapor diffusion) and outdoor high
> relative humidity pushing liquid water inward (through concentration
> gradient, liquid diffusion and capillary action).
>
> In the summer, all moisture drives are from outside to in (with the
> exception of indoor positive air pressure - a bad idea in an air-conditioned
> home), and the strongest of them can occur at any time of the year when
> liquid water is stored in reservoir claddings and the intense sun (low and
> more perpendicular to walls in winter) is pushing Btu's into the cladding.
>
> What is needed for a well-performing wall system in a heating-dominated
> climate is a low-permeability (but not impermeable) and air-tight interior
> wall surface, some interior and/or mid-wall moisture buffering capacity, and
> a highly-vapor-permeable but water resistant outer cladding. The more
> reservoir capacity the outer cladding has, the more important that it be
> shielded from rain and splash and/or resistant to liquid absorption.
>
> Straube's research indicates that lime wash can reduce absorptance of earth
> plasters by 90% and siloxanes can reduce it 99%, in either case without
> diminishing the vapor permeability.
>
> - Robert
>
> - - - - -
>
> From: Mark Piepkorn <duckchow at potkettleblack.com>
> In-Reply-To: <4A2431E3.5010107 at gmail.com>
> References: <4A2431E3.5010107 at gmail.com>
> Mime-Version: 1.0
>
> Also see
>
> http://www.buildingscienceconsulting.com/resources/walls/brick_veneers_rain_sun.pdf
> and
> http://www.ornl.gov/sci/buildings/2010/Session%20PDFs/164_New.pdf(especially the "Drying Phase" subhead on p5, and the "Constant Versus
> Cyclic Outdoor Environmental Conditions" head on p6 - even though the
> testing protocol doesn't replicate our materials or weather conditions).
>
> This is a question that's been considered in SB circles from time to time -
> usually California SB circles - and no final answer has ever cropped up as
> far as I'm concerned. The discussions usually revolve around the safe
> moisture storage capacity of bales, and providing a highly permeable escape
> plan.
>
> The trick, as always, is having a drying regime that exceeds the wetting
> regime. An unvented brick veneer offers less drying potential than a highly
> permeable plaster. As long as the safe moisture storage capacity of the wall
> system and materials isn't exceeded, the drying regime can be seasonal.
>
> Assuming best-practice design and detailing have been followed, I think
> interior-sourced moisture is a greater danger in our climate.
>
> Robert's response is good stuff, as usual. There are those in the SB
> movement, however, who argue against the rule of thumb that, in cold
> climates, the interior finish should be x-times (different sources have
> different numbers) less permeable than the exterior finish. The reasoning
> behind that position is that there isn't going to be enough interstitial
> vapor migration through virtually any high-permeable finish to cause
> moisture trouble. From the inside, the problem is always discontinuities,
> not permeability, and having the extra drying capacity to the interior for
> when it's needed outweighs any wetting capacity through the unbroken finish
> plane. The ORNL study above can be read to support that notion. (I'm not
> going to pretend to have enough knowledge to go to the mat in support of the
> argument, but it makes sense to me.)
>
> Mark
>
> - - - - -
>
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-- 
John Swearingen

Skillful Means
www.skillful-means.com
blog: https://skillfulmeansdesign.wordpress.com
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