[GSBN] Can bale buildings be air tight?

[Dusty]

Nicely Done Chris and Jenn!
I'm following your updates, I've been wondering if you have a technique for sealing the backside of electrical boxes. I've done quite a few blower door tests on Straw homes, some are detailed better than others, but I always find that exterior wall outlets leak a bit. We've achieved better air tightness with spray foam and caulk at the wire penatraions and box openings, wondering if Siga would work.
I'm cheering for your 0.6 ach50, you can do it!

Dusty Szymanski
COSBA

Straw & Timber Craftsmen INC
www.strawandtimber.com
dusty at strawandtimber.com

Register now for the International Straw Builders Conference at www.strawbaleconference.com 

--- On Tue, 11/13/12, Chris Magwood <chris at endeavourcentre.org> wrote:


From: Chris Magwood <chris at endeavourcentre.org>
Subject: [GSBN] Can bale buildings be air tight?
To: "Global Straw Building Network" <GSBN at sustainablesources.com>
Date: Tuesday, November 13, 2012, 5:13 AM


Hi all!

Maybe the question of whether or not a bale building can be built air tight hasn't been keeping you all awake at night, but it's caused Jen and I some long evenings of thought and lots of detail drawings on our plans! Maybe that's just what keeps northerners warm at night...

We've been honing in on good results over the past couple of years, and this year we finally nailed it (but didn't put a nail through it... that would have been counter-productive). We just did a blower door test on our most recent project and ended up with a result of 0.99 ACH50. That's approaching the PassiveHaus goal of 0.6, and we realized after the test that we hadn't covered up the open sump pit in the basement, so we may actually get to PH levels with the final test.

The building has a mix of site-built, clay plastered walls and prefabricated, lime/cement plastered walls. We've long used a system that uses a flexible air barrier (house-wrap type membrane) at the edges of the walls where the plaster will meet ceiling, floor and intersecting walls. The membrane wraps down behind the plaster by 3-4 inches. What was interesting with this building was to find that in some areas that detail worked very well, and in others it didn't help much at all.

What this tells us is that unprotected plastered edges leak... a lot! Our first blower test helped us discover that some of these seams were leaky. One leaky wall accounted for a shocking 74 square inches of leakage! By eye, it just looked like  the usual plaster shrinkage around the edge of the wall, maybe 1/8 inch. But multiply that around an entire home and it's no wonder that many bale buildings under perform in blower door tests. You can see photos of these areas on our project blog at http://endeavourcentre.org/2012/11/blower-door-test-1/

What we don't know is why some areas stayed tight and others didn't. Our best guess is that it can come down to quality of work. How long was the barrier tail left? How well was the mesh installed over it? How vigorously was the plaster pushed into the mesh? The plaster pulling away at the edge as it shrinks also seems to cause some slight bending of the skin inward, taking it away from the barrier.

One good take-away from this project is about the beauty of clay plasters. The leaky edges of the clay plastered walls were relatively easy to address... moisten the edges a bit and squeeze in more clay mix. Everything bonds together and the seams went from very leaky to completely tight!

Now, if anybody happens to ask if them there bale houses can be air tight, it's possible to answer yes.

Cheers,

Chris

-- Chris Magwood
Director, Endeavour Centre
www.endeavourcentre.org

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