Cob/Cordwood Masonry Hybrid

[Renewables at aol.com]

Michael,

I'd say that Rob Roy's area in Upstate, NY is a wee bit colder than N.
Illinois. We're only about 7,500 degree years for heating here. A good thing
is that we get about 50% more sun exposure here.

I have and am considering Rob Roy's method of earth sheltering, but that
involves using cement blocks with surface bonding cement along with 2-4" of
closed cell insulation. Although I haven't ruled it out, I decided to seek
more natural alternatives first. The average earth temperature at the eight
foot level here is 55 degrees F and it would do well in helping the tempering
of the thermal mass.

One of the building sites I am toying with is relatively flat with a slight
gentle southern slope. No chance of digging into the grade much. There is an
old silted in pond about 1/4 mile away. The silt makes really great material
for an earth berm and the living earth roof. 

We'll see what happens. The clay soil and cordwood will have to be hauled on
site, but not very far (1/4 mile at most). Sand will be the largest scarce
item on the property. The nearest sand quarry is about 7 miles away at about
$55 a truck load. Straw can be purchased locally, but is all oat straw, the
least desirable kind I am told. There is no wheat straw locally to my
knowledge. Cement blocks for an earth sheltered arrangement can add quite a
bit to the cost unfortunately (even though the thermal tempering effect is
desirable). Subgrade cordwood has been a risky venture so far, even failing
for Rob Roy's experiments.

We may end up building a 16 foot diameter round  shed with earthed roof that
would be used as a construction shelter and/or temporary living quarters at
some point. That will probably be a cod/cordwood hybrid also. 

The big thing about a cob/cordwood hybrid is that the wall thickness more or
less has to maintain a uniform thickness for the full wall height and must be
strong enough to support an eight inch thick earth roof. Cordwood masonry
divided into three parts, 2/3 inside/outside mortar with 1/3 insulation in the
middle. The thickness of this cordwood wall would possibly need to be adjusted
to support the earth roof. I don't see this as much of a problem for a 16 foot
to 20 foot shed, but it certainly would be a more ambitious project for a 38
foot diameter (~1,000 SQ ft) earth roof cob/cordwood home. The roof rafter
centers would be supported by a circular stone masonry heater. There would
also be a girder ring at midpoint of the radius. Cob/cordwood wall thickness
would have to be carefully calculated to support this load. As of yet I
haven't had an expert calculate these loads. Of course the appropriate
overhang would protect the wall too from driving rains. The cordwood log ends
do breathe, so moisture absorbed during driving rains would quickly dry out
the cordwood.

I don't own the land yet, so practice building is a ways off. I can almost
taste it though! I have to do more reading and sample gathering to determine
the best clay soil from nearby. It is not far away to the different kinds of
soil though. There is another 9 acre field that was on a federal land set
aside program for a number of years. It has allot of popular trees that have
competed with the blackberry vines on the South edge of the field. Some of
these are more than five inches thick, making for excellent cordwood log ends.
Soon they will be cut down to reestablish the field size.

Dave

In a message dated 11/1/98 11:54:25 AM Central Standard Time, HandyM2 at aol.com
writes:

<< Hello Dave!  Love your website!  Kudos for putting such effort into play
for
 the rest of us.  Reference your below Cob query I must ask you to remember
 that Regional Housing is just that.  Regional.  Cob was most common in
 England where such weather extremes are FAR from common.  
 
 I'd suggest your interest in Cord Wood be a bit better in your region.  Hvae
 you given thought as to Underground Cord Wood Housing?  IIRC Rob Roy is
rather
 big on that and his weather area is very simular to yours.    
 
 << I have been a fan of Cob for a long time, but wondered how it would
perform
 in
  the really cold winters of N. Illinois and Wisconsin. Sustained temperatures
  of thirty degrees below zero on either temperature scale can wick the heat
  right out of most any dense thermal mass. Also, the sustained 90+ degree F
  temperatures with the high 90+ percentage humidity of late August would
  eventually raise the internal home temperatures to the uncomfortable stage.
  You would end up having a building with its weak points in March and
  September. The cold would be catching up with it in March and the warmth in
  late August (at least in N. Illinois).  >>
 
 Really good analysis of Cobs weakness in such extreme weathers. 
 
 Michael 
  >>