Cob: Real information

[Wagzie at aol.com]

I've only been on this list for a short while, but I have yet to see anything 
discussed about the construction of cob buildings.  What I have read so far 
has been people  discussing issues that are related to sustainability, which 
I would think most people in this group are very concerned with, but I 
haven't seen any one talking about cob construction except for insulation.

As an owner of a self built earth bermed passive solar house with the 
insulation on the exterior of the 8" concrete walls, insulation is MUST 
especially in northern climates.  Although my house does not go below 55*F in 
the winter, it is nearly impossible to warm up the mass with solar energy or 
the wood stove.  It is quite easy to heat up the air, but an entirely 
different matter to heat up the thermal concrete.  
A cob building in the north must somehow be insulated or the exterior 
envelope will loose all the heat that is generated by whatever form.  From my 
experience with an overly constructed thermally mass house, I think that the 
thermal mass must be isolated from the interior space conditioning unless you 
use a radiant floor and radiant wall heat, then insulate the mass on the 
exterior.

How would heating the thermal mass walls with radiant heat affect the 
longevity of the cob walls?  I sure do not have an answer for that.  I won't 
even get into the discussion of what to use to supply the radiant heating 
system.  From all my experience in energy efficiency, the only way to heat a 
mass is through radiant heat and unless you live in the Southwest, I don't 
think the sun will suffice.

However, thermal mass works wonders in the summer time, especially if you 
insulate on the exterior side of the insulation and I'm sure this is true of 
Cob construction.  Here in SW Ohio, summer moisture is the problem that is 
most difficult to deal with and thermal mass does little or nothing to help 
alleviate moisture problem.

The problem is still:  how to insulate a cob structure?  A difficult question 
with no easy answers if you want to use the cob as your interior finish.  
Insulating a cob structure is complicated and will probably increase the cost 
of construction significantly.  

One suggestion that I have not seen is to sandwich a 2" piece of polystyrene 
between  the cob wall with a method of tying the two walls together, which is 
a thermal break, but for structural integrity, it would seem that it would be 
required.  I guess the other way would be to use styrofoam on the interior 
and exterior with cob in between similar to the insulated forms that use 
concrete instead of cob.  However, then you are limited to straight lines and 
corners and can't utilize curved walls or roof lines as easily.

Spray on cellulose is another option to insulate the interior of the walls.  
This is not a DIY job because of the expense of the equipment, but it would 
work well on curved and unusual design elements.

One question I have about Cob construction is how to stop the moisture in the 
ground wicking up into the floor and walls?  Moisture migration is a serious 
problem in the midwest in the summers.

If there are any who have built with cob here in sw Ohio or SE Indiana, 
please contact me.  I would love to see a cob building.