Cob: Insulation

[Shannon C. Dealy]

On Mon, 19 Jul 1999, Sojourner wrote:

> "Shannon C. Dealy" wrote:
> > 
> > On Sun, 18 Jul 1999 Keftydia at aol.com wrote:
> > 
> > It might help also to sink the floor below grade to get
> > a better cooling effect (with proper drainage).
> 
> Could you elaborate on "proper drainage" for a below-grade floor?  Also
> how to enclose such a beastie and seperate it from the regular ole
> exterior dirt?

If your water table is near the surface of the ground at any time of year,
don't even bother to try this.

The system I know is basically a foundation/drain known as "rubble trench".
Basically, it consists of creating a drain around the outside of the
perimeter of the floor, which consists of a trench going down two to
three feet below the level of the finished floor.  The bottom of the
trench should slope slightly toward the downhill side of the house (this
won't work if you don't have at least a little slope on your land).  You
will then need one or two additional trenches leading from the one around
your floor area away from the house (again on the down hill side) at a
slight downward slope.  Eventually (hopefully without making to long a
trench), the bottom of your trenches running away from the building will
be at the surface of the ground (due to the slope of the land coming
down to meet the trench).  This creates a channel for any water that gets
near your floor to run out away from your floor, to the surface of the
ground below your building.  Next, in the bottom of the trench you can
either place some drainage pipe (the stuff with holes all over the sides),
or you can use rocks to create a clear channel by putting rocks on both
sides of the center and then another rock across the top, all the way
along all the trenches, leaving about a four to six inch wide unobstructed
channel in the center of the bottom of the trench that water can use to
flow away from your building.  In either case, you then cover it over with
rocks, building debris, coarse gravel, etc., filling and tamping the
trenches so that they are tightly packed and just a few inches below the
level of the soil base that you are going to build the floor on.

Once this is done, you can build the stem wall for your structure
on top of this, and if you are going below grade with the floor, your
stem wall may need to be fairly tall, since it will need to be well above
grade before you can start putting cob on top of it (assuming that you
are doing cob walls to go with your cob floor).  The design and
construction of your stem wall depends on your structural needs,
particularly if it is acting as a kind of retaining wall to support the
earth around the building.  If you are going below grade, I would probably
make the trench wider, so that the stem wall only covers part of it, then
I would put at least a one to two foot wide band of gravel around the
outside of the stem wall in order to keep surface moisture away from
the building, and allow it to drain more quickly.  If you are using this
technique as a foundation, your trench may need to be deeper in cold
climates, since the bottom should be at least several inches below your
maximum frost depth.

As far as the floor itself (once your perimeter drainage is done), you
start with well tamped earth about 8" below the desired finished level
for the floor (which is still something like 1-1/2 to 2 feet above the
bottom of your trenches), and use a layer of gravel to provide a moisture
break, followed by your cob layers for the floor.

The basic idea here is that by surrounding the floor with a trench that
goes well below the level of the floor, the moisture will be prevented
from reaching the floor area, since the water upon reaching the level of
the trench will take the path of least resistance and flow out through
your drain.  Having the gravel under your floor provides additional
protection, and may give your drains some time to work if the water levels
get to high.  If you are in an area where you have large amounts of water
to deal with, this may not be enough, so it might be a good idea to
contour the surrounding ground so that water is channeled away from the
building (probably a good idea in any case), or even install a secondary
trench on the side of the building that is closest to the source of most
of the water runoff.

If you want to avoid having your stem walls acting as retaining walls for
the earth around the building, you could just shovel the earth out at the
end of the building where the ground is higher, until you have a level
base for the floor, plus few feet outside the perimeter of the building.
Next, contour the ground to get a slope that will hold its shape reasonably
well, and put in some grass, bushes, etc. to help hold it in place.

[snip]
> The only earthen floor info I have found on the web, the guy laid up on
> a concrete slab.  I'm planning on going directly atop the gravel.
[snip]

The Canelo Project has an inexpensive booklet on earthen floors, I haven't
looked at it, but I imagine some others on this list have:

    http://www.deatech.com/canelo/

I think I did some postings on the subject as well on the coblist,
probably in the 1996 archives:

    http://www.deatech.com/natural/coblist/


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