[GSBN] Drainage within a plaster system

Tue Jul 9 12:07:38 UTC 2013

Language is important here, as lots of confusion can be caused.
The Typar and rain slicker product are vapour permeable.  They allow easy passage in both directions of water vapour.
Typar is water resistant -- it will not allow liquid water to pass through it, although leaks through holes will easily occur if sufficient water is present, and it will leak a lot under water pressure.
A Rainscreen "screens" rain, that is, it stops most of the rain on the surface but not all. Two-coat or three-coat stucco does not stop all the water that hits it and is not thick enough to store all the water that hits exposed locations.  Protected walls see so little rain deposition that the stucco can act like a sponge.  Eg normal SB construction.
Drainage behind a rain screen (in the "drainage gap") is critical to collect the small quantities of leakage through it, and allow this water to flow downward under gravity forces.  Then you need flashing and holes at the bottom to direct this water out. To stop water moving further inward you need a "drainage plane", something like Typar in normal walls, but can be painted wood, stucco (in SB rain screen) etc.
If the drainage gap is large enough, sufficient amounts of air can move through it to help dry out both sides of the drainage gap (the rain screen side and the drainage plane side).  This requires holes at the top and bottom, and a gap usually over 1/2" or so.  Then it is called a "ventilated" rain screen.  Just a gap with vents at the bottom is a "vented" rain screen.  

In short, wise and experienced builders like John S and Frank T have found that for high exposure walls, drained stuccos are worth doing.  Smart architects like Dan Smith and Kim Thompson have also used drained stucco in high exposure designs quite successfully) at least 5 years ago.

For more of my ranting and raving on this topic, go to building science.com.   BSD-112 is about straw bale.  But BSD-103 and BSD-030 are general background, and there are numerous reports on testing in the lab and measurements in the field of all kinds of drained walls. Use the search bar and "rain screen"

On 2013-07-09, at 1:16 AM, John Swearingen <jswearingen at skillful-means.com>
 wrote:

> A rainscreen is an air gap,  vented at the bottom and sometimes the top.  They increase the assembly's ability to dry rather than inhibit it.
> 
> John "Mind the Gap" Swearingen
> 
> On Jul 8, 2013 10:03 PM, "Misha Rauchwerger" <misha.rauchwerger at gmail.com> wrote:
> I'd be concerned about the use of a non-permeable sheet on the outside of natural wall systems that need to breathe.  Especially on residential construction where so much moisture is generated on the inside of buildings.  What's wrong with lime plasters that are quite hydrophobic, yet still permeable?  Can the moisture migrate out through these rain screens?
> 
> Misha Rauchwerger
> www.builtinbliss.com
> 
> 
> On Mon, Jul 8, 2013 at 9:15 PM, John Swearingen <jswearingen at skillful-means.com> wrote:
> Call me anything you like!
> 
> The structural element is at the scratch coat...the bales are wrapped with 2x2x14g ww mesh, and the scratch coat is thick.  So outside of that, the rainscreen/lath/plaster assembly is weatherproofing.
> 
> John
> 
> 
> On Mon, Jul 8, 2013 at 6:53 PM, Frank Tettemer <frank at livingsol.com> wrote:
> This is so embarrassing!
> I did NOT mean to write that John is "just" a builder.
> I meant to write that I am just a builder. I am also feeling "just humiliated".
> (Always proof-read what you've typed, Frank, before pressing Send.)
> 
> You are way over-qualified for that designation, and my appologees to you,
> John "Much More than Just a Builder" Swearington.
> 
> Regretfully Embarrassed Frank
> 
> ........................................................................................................
> 
> An hour ago, Frank Tettemer 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
> 
> 
> -- 
> Frank Tettemer
> Living Sol ~ Building and Design
> www.livingsol.com
> 613 756 3884
> 
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> 
> -- 
> John Swearingen
> Skillful Means Design & Construction
> 2550 9th Street   Suite 209A
> Berkeley, CA   94710
> 510.849.1800 phone
> 510.849.1900 fax
> 
> Web Site:  http://www.skillful-means.com
> Blog:         https://skillfulmeansdesign.wordpress.com
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John F Straube
jfstraube at uwaterloo.ca
www.JohnStraube.com