hybrid strawbale/cob

[Eric D. Hart]

At 09:50 AM 11/22/96 -0500, Mark A Hoberecht wrote:

>I built a hybrid cob/straw-bale structure this past summer in Ohio and
>discovered some interesting characteristics.  I started with a sandstone
>foundation that had a cob "cap" on it several inches thick to level out  the
>wall for the straw bales.  I then built an interior cob wall about 6 inches
>thick and an exterior cob wall a couple inches thick on the outside.  This
>cob mix needs to be pretty dry or the straw in the bales can absorb a lot of
>moisture.  Even cob that looks and feels dry to the touch can give up
>moisture to the bales.  This will dry out as the cob dries.  Once the cob
>becomes drier than the bales, the process reverses itself and the cob sucks
>moisture back out of the bales, just as cob does with buried wooden framing
>members (a kind of preservative).
        Was the cob or strawbales load bearing?  Was the whole wall assembly
load bearing?  You mention wooden framing members so I am guessing that the
cob and strawbales aren't load bearing.  I don't see why the cob couldn't be
load bearing.  How thick would one of the cob walls have to be to be load
bearing?  Six inches seems to be too thin to me.  Maybe a foot thick??
Personally I would make the strawbales or the cob load bearing.  No use
going to all that trouble with the cob and strawbales and then holding the
roof up with wooden framing.  Cob would seem to be a better load bearing
material since it won't settle like strawbales do.  The straw might settle
so much that the roof would end up resting on the cob walls anyway.  

Eric D. Hart			
Community Eco-design Network	  
Minneapolis, MN  USA		
(612) 305-2899 
erichart at mtn.org			
http://www.tc.umn.edu/nlhome/m037/kurtdand/cen