[GSBN] URGENT! Drying moisture out of SB walls

David Arkin, AIA david at arkintilt.com
Wed Apr 5 18:42:36 CDT 2017

Hello Global Balers:

 A CASBA member poses the questions outlined in the situation below.  I’ve attached my responses below the query and photo, and invite any of you to weigh in with further recommendations, follow-up questions or anecdotes that may be useful. 


David Arkin, AIA, Director
California Straw Building Association

ps: Joins us for CASBA’s 2017 Spring Conference, May 5-7 in the San Francisco Bay Area, featuring architect Craig White of the U.K.:  "Towards a Photosynthetic Architecture - Renewable Buildings for the Circular Carbon Economy”.   Registration is open:  http://www.strawbuilding.org/event-2497515

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I’m  hoping you can address some of my questions or direct me to anyone with experience dealing with this problem, or anyone who has any insights into causes and solutions.  
I was contacted this morning by a client just south of Portland who has measured high levels of moisture in their straw bale walls, and is asking for advice on how to deal with the problem.   
The core questions I have are these:
1.       Assuming there isn’t a bulk water leak from the roof, downspout, or window, can wind-driven rain account for high levels of moisture in a straw bale wall assembly?  In other words, what does it take for a properly installed lime plaster to be overwhelmed by wind-driven rain?
2.       What are the options for drying the wall out?   Waiting for dry weather (summer!) may not  be an option as wet straw bales may not survive that long.
3.       Once the wall is dried out, assuming there isn’t significant permanent damage to the bales, what surface treatments are available that would prevent liquid moisture from soaking into the walls, yet keep them vapor permeable.   I can imagine several landscaping and rain screen (siding) solutions, but am not familiar with surface treatments.
Background Information.
Details about the wall assembly.   The SB walls are on a raised floor.   The space between the sill plates was filled with rock wool insulation and capped with ½” plywood to handle the bale weight.  The wall assembly has 2-string rice straw bales laid flat, and is part of the building’s shear wall system, using 17 gauge lath and lime plaster (exterior and interior).   Instead of applying a finish coat of lime plaster the client chose to apply a lime based paint from BioShield.    I didn’t plaster the structure or apply the lime paint, but believe it was done by capable professionals in accordance with best practices.  The bales were stack in April-May of 2016.   Bale wall moisture readings just prior to plastering averaged 14.1%.   The wall was prepped to receive a lime plaster—2-layer building paper stapled to all wood framing, shingled to shed water, etc.  The windows have sills, the 4’ roof overhangs are guttered and the downspout installed properly.   The walls were plastered during late spring and early summer.   Three coats of exterior lime paint were applied in late summer-early fall.    When I visited in November, I saw vertical cracks at the corners only (where I always see them, regardless of how much corner-aid or exp. metal lath is underneath!).    
Building site.  The house is located in an open field and has no barriers to wind driven rain.    The general contractor, who happens to live next-door, told me the field is like a wind tunnel.  He reported that since it began raining in the fall of 2016 he hasn’t seen the walls look dry more than a dozen times.   
The problem first came to my attention about a month ago when the client told me they smelled something awful in one of their rooms—the one with the most weather exposure (S. W. corner of building, labeled “office” on the plans).   I haven’t visited the site, but advised them to first investigate and rule out all the other likely possible causes for an odor (e.g. decomposing straw piled near the house, something else rotting in the crawl space, etc.), and if the odor persisted, to gather quantifiable information, including using a moisture meter probed into the wall near outlets, which they have now done, (see below).   


The office is in the S. W. corner of the structure.  I’m not familiar with the probe they used, but it’s likely that the shaft is about 18” long, and if used as described to me, “poked in a 45 degree angle from the interior of the wall near the outlets”, probably penetrated about 5” into the wall when it reads 8”, and about 12” into the wall where the chart says “full in.”  From all the points they gathered data, moisture readings were higher towards the exterior of the wall.
My understanding is that lime plasters will absorb and then release liquid moisture from wind driven rain, and are quite able to handle regular, frequent wettings without compromising the straw beneath.   If bulk water isn’t entering the wall through a breach in the flashing or another leak of some kind, is it possible that an unusually wet winter (I believe the Willamette Valley is experiencing a well-above average rainfall year like much of the west coast) could create the moisture levels seen below?   Is it possible that the  water is soaking in, and just keeps soaking in, unable to dry out because of the constant rains? 

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[Arkin comments in reply]

The moisture readings aren’t as high as I would’ve guessed based on your description … that’s perhaps good news.  We had a wall at the Real Goods Solar Living Center that was an exterior site wall with very little overhang, and it would get pounded by the rain.  We had a moisture reading over 50%!  However, in Hopland’s sunny hot climate it dried out between rains and now with a new broad overhang it is doing fine, 20+ years later.  Similarly a small outbuilding on that same site was flooded to the middle of the second level of bales.  It was earth plastered and we advised to simply let it be and see what happens.  The building has no windows or doors (it’s a ‘welcome pavilion’) and once again it dried out promptly and has been fine.  

At the same time, I’m recalling an olive oil facility that was on top of a hill in San Luis Obispo County, that had wind-driven rain penetrate cracks in the Gunite finish on their bale walls, to the point of black goo oozing out the base.  That’s when you know you have real trouble.  They drilled holes and drove air into the bottom of the walls, and also put a layer of breathable waterproofing on the exterior of the walls.  Similar to your case here, it was the windward side that had the worst problems, but rain swirling around the building caused some issues on the leeward side too. 

Here are my opinions on your questions, but let me be the first to admit there are others who could answer these better than me:

1.  The photo sure makes it look like wind driven rain, and at quite an angle!  Another 10’ of overhang (aka a porch) along that facade seems in order. Exactly how much moisture it takes to overwhelm a lime plaster wall is difficult to say.  I’m recalling studies done by the University of Bath that placed plastered wall samples in very exposed marine climates to determine this.  You might search for this, perhaps starting with EBNet’s BuildWell Library.  Bruce King may be able to connect you with Pete Walker, or you could try to reach him directly.

2.  Again, the numbers aren’t so high that invasive measures need to be taken.  I’d suggest putting some more powerful heaters on the interior, and aim to drive the moisture out toward the exterior.  At the same time they should deploy tarps or some other means of keeping wind driven rain off the walls going forward, but let the sun and warmth at them otherwise.  

3.  Again, my first suggestion is a longer porch roof along that whole facade, perhaps with some landscaping or something to break up the laminar wind.  I suppose a deployable system of a rain screen of some sort could also be used.  Allowing the walls to see sun this spring and summer will be good though.  Xypex is a product that folks have applied to walls, but I’m more familiar with its application on cement stucco than lime, so research that a bit first.  David Easton suggests Glaze ’n’ Seal on his earth walls.  I believe both have that waterproofing effect while still remaining breathable.  

As you know, both the plaster and the straw have a significant capacity to store and release moisture, and it seems they are doing exactly that.  I can’t say for certain, but this being their first season they may not be damaged to the point of needing to be replaced, but the smell detected is concerning.  Getting them to dry and then keeping them dry going forward is key, and if necessary some replacement may be needed, but I’d advise trying to avoid that first.  

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Arkin Tilt Architects
Ecological Planning & Design

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Thank you!

David Arkin, AIA, Architect
LEED Accredited Professional
CA #C22459/NV #5030

1101 8th St. #180, Berkeley, CA  94710
510/528-9830 ext. 2#

"There is no way to peace. Peace is the way."
— A. J. Muste 

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