[GSBN] Can bale buildings be air tight?

[Chris Magwood]

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