Cob more straw in cob=more insulation?; sun tubes

[Bob Bolles]

Hi Will et All,

I enjoy the list, but seldom participate since I/we have had little
opportunity to experiment with Cob.
We are. however, working with clay/straw mixes that we use for plasters. 
Because we are working in a number of locations, and have access to a
cross-section of soils that range from nearly 100% clay, to others where
the clay content is virtually nonexistent, I am beginning to learn a bit
about how this stuff works.  Because we are out on the "cutting" (blunt
bludgeoning) edge, we do a lot of experimentation.  I don't encourage a
practice that pursues a path that others have found that would likely
result in failure, (that said) however, we do pursue a lot of directions
that may (and have) resulted in less-than-desirable results - Lots of "what
ifs"!

> I have no direct experience but it seems to make sense that additional
> straw would increase it's insulation value. I have heard that the R
> value for cob is approx. R1 per inch, whereas straw is over double that.
> The other factor is the thermal mass of cob. If the cob R value is
> increased the thermal mass will also be decreased however slight this
> effect may be. 

It seems to me that we are dealing with several different things here:
Thermal Mass and Insulation. Certain materials lend themselves well to
certain climates - Adobe block, for example. Adobe, as I understand it, is
a clay/sand/fiber brick or block, which was made in this form so that the
brick/block could be made in one location (where the materials were
available), and transported (in this form) to the building site (kinda like
straw bales ;-)
The material/system worked, not because of the (virtually non-existent)
insulation characteristics, but rather because of the way that the Thermal
Mass worked relative to the warm days and COOL nights.  In the morning, the
walls were cool, and thus the interior of the building was cool - during
the day the wall mass absorbed heat, an so that by the time the sun went
down and the air became cool again - the interior walls were radiating
heat. During the night, the walls became cool again, and the cycle began
anew.  Consider how that would work in a very hot or cold climate, where it
would not cool down in the summer or warm up in the winter.    We don't
have to look very far - Every year people die in the brick and stone
buildings in Chicago, Kansas City, New York from the heat and cold.  I
spent one miserable winter in Missouri in a brick home where the walls were
2 feet thick - there wasn't enough heat in the world to heat up that place.
 We finally moved into the living room where there was a monster fireplace
(no question why it was there), and I spent the winter cutting firewood,
just to stay alive.
Insulation is an entirely different thing.  Insulation reduces thermal
transfer from one side of a surface to the other.  In climates where it is
hot during the day, and does not cool down appreciably during the night -
or visa versa in the winter, than exterior walls with high insulation
values would be appropriate. 
> 
> The concern is if too much straw is used the cob will not be strong.
> Obviously the thicker the wall the greater its insulation value and
> thermal mass will be. The insulation value I expect is mostly due to the
> air space in the cob. While building the cob wall you could also poke
> lots of holes to increase the air space within the wall. You could
> increase the insulation value of the cob on the outside of the house by
> using straw rich cob on the outside side of the center of the wall. Then
> using good strong cob to hold the straw rich cob in-place.

That could well be true.  One thing that we have learned is there is no
"right" mix - it really depends on how the soil mixes up.  In some
clay-rich soils, we have been able to really load-up the straw content - in
others, the straw will reach a point where it really will not bind together
with the clay mix,  I would be rather cautious about loading up the cob
with a too-rich mix of straw.  Consider making up a test wall, and see how
it reacts to stress-tests when it dries (you know, beat on it - push on it,
weight it down - look for failure!)

 One could
> also increase it's mass on the inside of the house by imbedding heavy
> rocks on the inside side of the wall interior. 

I'm not really convinced about that.  Cob (or the plaster mix) has a
tendency to draw away from dissimilar materials - you may end up with voids
with rocks in them. By all means, avoid smooth round rocks - if your going
to use large aggregate, use stuff with rough and irregular edges.

> 
> If the wall is thick on the North Walls you could also corbel pockets
> near the outside of the wall that could be stuffed with straw then
> enclosed as you are building the wall.

Kinda like concrete blocks with insulated cores, eh?

> 
> Another option is increase the passive solar incorporated in the design
> to make up for the slight loss in insulation value.

Absolutely! - I would suggest rethinking "slight".
> 
> The solar tubes do look good but are really expensive. If you do find a
> good alternative please post the details. Two earthships I have seen in
> Kamloops & Vernon both had an extra set of windows in the roof to get
> the sunlight to hit the back walls for additional light and for
> capturing more solar heat.

Polished Aluminum sheeting - pop-rivet & caulk - insulate the tubing
through the attic space.

Regards, Bob