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[Cob] Coblist Digest, Vol 6, Issue 178

Barbara Roemer roemiller4 at
Mon Dec 22 15:58:32 CST 2008

Shody, I like the solar attic, too.  We may use the AGS system for our home
when we build, having convinced a friend in our climate to provide for it
even if she doesn't need it.

Ed, your point about earth bermed walls being constructed (usually) of block
or concrete is well-taken.  concrete has to be waterproofed/isolated or at
least damp-proofed with excellent curtain drainage, too, or it will wick
water up many stories and into the interior.  Berming an earthen or cob wall
assures that moisture will migrate, but in contrast to concrete which is not
damaged by moisture but does damage by carrying moisture to other materials
which are harmed by water, cob will lose its strength as straw in it rots
with moisture.  The cob must be isolated from the earth berm to keep it
dry.  Building a berm right near your cob wall but not flat up against it
will allow air to circulate and keep moisture from migrating into the cob
wall, but it probably doesn't affect the temperature of the room inside the
cob wall too much as an open air channel provides neither thermal mass  nor
significant insulation.  Such a berm might keep you more comfortable and
reduce drafts as it reduces air pressure from wind against the side of the
house, but if that's the order of convection you're working on, you'd do
better to caulk the leaks in the wall and save yourself the work of a berm.

I'm certainly no expert, but wonder about earth bags and moisture wicking.
I've seen earth bags used for foundations with an isolating barrier between
them and the cob or bale. The bags around here are woven, so if moisture
stood against them, the clay inside would still be able to absorb it.  Maybe
on a rubble trench with no concrete cap, water would just drain away.  We
used a concrete cap on the RTF for our little bale shed, and then builder's
paper (tar paper) atop that before the bales.  I live in a climate with 60"
of precip in about five months, so I think about moisture - a LOT.  If I
lived in an area with 11", I'd be far less concerned about it.

Also, of foundation insulation, this source

states that except for places like California and Nevada, in the US, R10 is
usually specified.  We will insulate the perimeter well, and run insulation
beneath the outer foot or so of the slab, and of course a vapor barrier
under it all.  Especially in cold climates, where you can create good
drainage, the perimeter insulation can be more significant than increasing
under floor insulation, and is recommended by RTF builders as well as
typical slab/perimeter builders.