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Clapboard exterior for strawbale?

 
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In the design phase right now, looking to save a bit of time and use the wood from my land.

Can a strawbale home built in timber frame be clad in clapboard? Will the bale walls still breathe properly?
 
pollinator
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Yes
 
pollinator
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They may breathe too well.  At least a scratch coat of plaster will help cut air and keep bugs out of the bales.
 
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You could use the wood for clapboard, but I’m not sure how that might save time. If you were to cover the bales with clapboard you would need to add nailers, but you will still need to have plastered the bales to control moisture and rodents. I don’t see that it could increase performance, unless you’re worried about rain or other weather. That said, maybe it’s just aesthetic preference, which is understandable.  
 
John C Daley
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From my experience. there is no need to plaster the wall for any sort of protection.
The new cladding will deal with rain and moisture humidity will be the same.
Plaster is used to give a neat tidy finish rather than anything else.

From; https://www.strawbale.com/debunking-the-rodent-myth/
I know, I know…For most of you this is old information, BUT if there are any folks out there that still have a worry in their minds (and it’s OK if you do!) about straw bale houses and rodent invasions, this latest “Straw Bale Minute” is a must watch. Trust me, rodents would much prefer to set up residence in your neighbor’s conventionally built house with lovely pink insulation than live in straw bale walls. '

You can hear why in this “Minute” below:
debunking-the-rodent-myth/

 
Michael Adams
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R Scott wrote:They may breathe too well.  At least a scratch coat of plaster will help cut air and keep bugs out of the bales.



This was the initial thought.
 
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Sorry to be late to this discussion--just back after a long absence.

I stopped counting the number of straw bale buildings I had a hand in working on--gotta be approaching one-hundred or so--and the only ones with rodent problems were a handful of older buildings (i.e. built well before our current level of best practice evolved) that had been poorly detailed at the foundation and at joints where exterior plaster met different materials like timber frames, top plates, etc.--all places where rodents could gain access.  On one project we were called in specifically to address rodent issues, mice had gained access by scurrying up the walls and into poorly detailed soffit vents--nothing to do with the bale walls themselves.  

I agree with John--rodent control shouldn't be a concern when the wall is properly clad--whether the finish is plaster or siding.  But a well applied plaster--whether just a scratch coat or the entire three-coat regime--goes beyond appearances.  

I depend on the exterior plaster as my primary air barrier. Doing this well doesn't just eliminate entry routes for critters of all kinds, it keeps air from streaming through the walls.  My secondary air barrier--probably as important as the first--is the interior plaster layer.  We're careful about how we flash windows and doors and seal any wall perforations, and about secondary stops behind joints where plaster meets framing so as these joints appear to open and close, they are still sealed.  This attention to detail doesn't take that much extra effort, and performs very well on the blower-door tests we need to pass.  We're generally  testing below 1 or 2 ACH (air changes per hour), which compares favorably to the average Oregon home built to minimum code, which is testing at 7 ACH.
So if you don't plaster the exterior, you need to do a really, really good job on the interior plasters.

An initial coat of plaster also makes the wall surface less likely to ignite should embers or flame from a wild fire happen along, or as likely, someone working near the wall with an angle grinder or welding torch.  And there is some evidence that even a single layer of exterior plaster--a scratch coat--offers some moisture management benefits in that it absorbs ambient moisture from high humidity and re-releases it to the space between the bale wall and the backside of the siding before it can pass into the straw.  Not that the straw couldn't handle the seasonal moisture fluctuation--it could, so long as the moisture level stays below 20%.

There's yet another option for some of us who live in temperate areas like S. Oregon and most of California.  And this method may have application in other regions--it's worth exploring.  That's to stack the bales against 1/2" plywood sheathing.  Has to be plywood as the glue in OSB makes it less vapor permeable. Even then, 1/2" plywood is at the very edge of not vapor permeable enough, so it may not work in all locations.  But where it can work, it solves a number of problems.  Although 1/2" of plywood isn't as good a fire barrier as 1" of lime or clay plaster, it is a rated fire barrier, which at least protects the bale wall from flash burning.  The plywood may play a structural role (shear), it can definitely be used to exclude critters, and if supported with thoughtfully placed framing, can be clad with plaster or siding, and either of these over a rain screen gap.

We didn't go into a lot of detail on this topic in CASBA's book Straw Bale Building Details: An Illustrated Guide for Design and Construction, but there are a couple of applicable details there.

Jim Reiland
Many Hands Builders
 
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