[GSBN] Plaster Reservoir?

Tue Jun 2 00:05:37 UTC 2009

Have you all ever considered the possibility of using compost fleece in
strawbale construction?
This material allows a compost pile to breathe but blocks the entry of
rainwater.
See: http://mailman.cloudnet.com/pipermail/compost/2000-June/006945.html

Paul

-- 
Paul A. Olivier
27c Pham Hong Thai, Ward 10
Dalat City
Lam Dong Province
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 0906941573 (in Vietnam)
Mobile: 84-906941573 (outside Vietnam)
Skype address: Xpolivier

On Tue, Jun 2, 2009 at 6:43 AM, John Swearingen <
jswearingen at skillful-means.com> wrote:

> 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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