[GSBN] Plaster Reservoir?

[Mark Piepkorn]

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.

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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

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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

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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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