[GSBN] Drainage within a plaster system

Tue Jul 9 14:23:21 UTC 2013

In any climate with a real winter, the temperature of the plaster on a strawbale wall will drop below freezing not long after the adjacent air does.  This will be delayed slightly by the thermal mass of the plaster, but very little by heat working its way out through the walls.  We picked strawbale for its insulation, right.  So, temperature-wise, the exterior plaster is on its own.  Left out in the cold, so to speak.

In comparing the rate of freezing between the thick monolithic plaster layer that many SB houses use,  and the thinner outer plaster layer of the ventilated rain screen design being discussed here, I'm sure you could measure how much faster the rainscreen's plaster dropped below freezing.  If you had a stopwatch.  An egg timer might be accurate enough.  During Spring and Fall, there might be some nights where the thicker plaster wouldn't quite freeze, while the thinner plaster layer of the ventilated rain screen would.  But plaster layers of both types will drop below freezing so often during a New England winter, that frost avoidance based on plaster thickness is irrelevant.  If either type of plaster configuration can't handle freezing and thawing at the level of moisture involved, then it will fail during the first winter.

Keeping the plaster dry (drier) is the best defense against freeze-thaw problems.  I think the thin outer layer of the ventilated rain screen plaster has three advantages.  Firstly, it can dry to both inside and outside, and can even be releasing some water to the air gap inside while it is still receiving rain on the outside surface.  After identical rain events, the ventilated rain screen plaster should always be somewhat drier (sometimes, a lot drier) than monolithic plaster, which gives it an advantage in avoiding freezing damage.  Secondly, a thin layer is less likely to crack than a thick layer in any expansion-shrinkage scenario, because the forces build up to lower maxima.  Thirdly, when cracks in the plaster do occur, the rest of the wall is protected, by the air gap and the drainage plane material, to a greater extent than a monolithic plastered wall.

A non-plaster ventilated rain screen design, with the rain screen placed in front of the plaster, increases the advantages mentioned above, and would give even more protection to the plaster in an area where driving rain and freeze-thaw are big concerns.

Derek

On Jul 9, 2013, at 6:25 AM, Jacob Deva Racusin wrote:

I was looking at integrating a drainage layer into a plaster system, and the concern came up about freezing in cold climates.  If you have a 'ventilated' (not just 'vented') rainscreen and the drainage gap between the scratch and brown/finish coats allows sufficient air flow as to drop the temperature to atmospheric temps, is there an increased chance for freezing of the exterior plaster layers?  If those exterior layers get saturated (thinking esp here of clay- and lime-based plasters, though I suppose the same issue would occur for cement stucco) and then it freezes hard that night, would there be a greater likelihood for freeze damage on those outer layers with cold air venting behind them than if they were in direct contact with the rest of the wall and 'protected' from behind by heat loss from the building?

Cheers,
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

On 7/9/2013 8:10 AM, John Straube wrote:
I is an engineer, but I would not trust engineers much without lots of input from the reality of the building site.
The inner stucco is likely the best shear resisting layer and I would rely on this.  Also a good fire barrier and wind barrier.
The exterior stucco needs to be attached well enough to not fall off in an earthquake or windstorm.  14 ga wires will do it if they are often enough.  In other uses, I have seen screws through metal lath used to support exterior stucco, but this requires about one screw per 1-2 square feet.

On 2013-07-08, at 9:09 PM, Frank Tettemer <frank at livingsol.com>
 wrote:

I really like what you're doing with this, John!

The stitching of 14g wires answers all my concerns of the outer layer of render flaking off, or pulling away from the structure.
And I imagine that this stitching would also reinforce the lateral resistance of the plaster, when it's ask to act as a wind bracing,
and when it's contributing to seismic resilience.

Would any Engineers on the list want to confirm this? John and I are just builders, after all, and speaking for myself, I'm only qualified for driving nails, not engineering.
(other than "intuitive engineering".
I _may_ be qualified for that.).

Frank
........................................................................................................................
John Rain Skeemer Swearington wrote:

We/ve used a similar product, with good success, by Benjamin Opdyke: http://www.benjaminobdyke.com/visitor/subcategory/subc/rainscreenProducts.

We applied it more or less as Frank says:  we used a thick scratch (base) coat of plaster over the bales, and then tied the rainscreen (Homeslicker plus Typar) and light stucco lath onto the building with 14g wires through the bales. The Typar stops the next plaster coat from filling the gap.  This sounds more laborious than it was; it went pretty quickly. Then we applied  a regular three-coat plaster, though a thick two-coat would also work.  We used a custom flashing at the bottom of the bottom of the wall to carry away the water.I see

This was in a highly exposed round tower sticking up through a tile roof, a fairly challenging location that receives high winds and driving rain.  We've gone through the first winter, including some hurricane force winds, with no sign of leakage :)

John "Rain Skeemer" Swearingen

On Mon, Jul 8, 2013 at 2:04 PM, Frank Tettemer <frank at livingsol.com <mailto:frank at livingsol.com>> wrote:
Hi Sven,

While this flexible drainage plain may well provide the needed path for intrusive water, and allow drainage from within the wall plaster,
would it not also prevent bonding of the render to the straw bales in the wall assembly?

I'm imagining a 'brown' coat of plaster, worked into the exterior of the bales, and the application of the Duraflow material to this first plaster coat, while it is still wet and workable. Then the second coat of plaster covers the Duraflow material, yes?

Would this compromise the structural needs of the outer skin, in it's function for structural engineering and seismic resiliency?

Or, to say it all a different way:
Wouldn't the use of the Duraflow weaken the strength of the wall assembly?

--
Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884

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John F Straube
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Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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