[Cob] RE: Foundations for stoves and interior cob walls

[Barbara Roemer]

Anna wrote:

A question about stove and interior cob wall foundations. We are doing the
formwork for our cob/strawbale house concrete bond beam at present (thanks
to living in an earthquake zone). The plans specify the same 2-3' wide, 8'
deep footing for an interior cob wall and masonry/woodstove stove as for the
main outside walls. This is a lot more cement and feels like overkill to us.
However, if we do a stone or urbanite foundation starting higher up, above
the waterproofing barrier, it will be on a lot of infill earth. Well-tamped,
but it might settle over the years.

Where are you, Anna?  What is your seismic zone?  We are in zone 3 backed up
to the western slope of the Sierra Nevada, west of Lake Tahoe.  We have no
frost line (that is, frost doesn't penetrate even 1"), but we and the
Building Dept take our seismic zone rating very seriously.  Since it's
generally counter productive to put steel in cob, where you have thick
interior walls, they'll need a broad foundation, and on disturbed soil, even
with well compacted soil, you'd probably be well-advised to go deeper than
normal.  How about considering a light straw clay partition wall so you
don't have the weight?  You can vary the straw clay mix some so that it's
more dense and less insulative, but of course no where near as dense as cob,
and only 9-12" thick.  If you're using your cob wall as distributed thermal
mass, you might do your energy calcs just based on your floor mass and the
masonry heater, increase your solar collection, and come out with plenty of
solar heat.  

The masonry heater itself will require significant foundations.  Here, on
undisturbed soil, it's at least 18" thick plus a foot in either dimension
larger than the heater footprint, and including plenty of steel (set out by
code).  We will use a masonry, refractory core and throat of the chimney for
our heater, but the rest of it will be rammed earth block for less embodied
energy.  If you are anywhere nearby, email me for details about an August
workshop in same.  If you are anywhere near Virginia, you might enquire
about the April Masonry heater Assn annual meeting at which rammed earth
block heater building will be taught.

Barbara