[Cob] Re: hybrid straw bale/cob

[Barbara Roemer & Glenn Miller]

Damon wrote:      " SInce the north side of the building never gets sun,
would it be a good idea to construct the northern wall with strawbale
because of it's superior insulation value over cob? I was planning to
install a wood burning stove next to the northern wall too. Will strawbale
be the best option, or should I just go cob all the way? Another question;
I've heard cob has a heat transfer rate of about 1 inch per hour. What
about the heat transfer rate of a strawbale wall?"



> To which Pat replied in part:

  "Outside the walls if you can insulate during winter only, say, by
stacking straw bales and keeping them dry, that's the best of the mass
and R value world's together."

Several pertinent points have been discussed on the SB REP and Yahoo SB R
Us lists.  If you are in a predominantly heating climate, then the
tightness of your building is probably just as important as the amount of
insulation you have.  Plastered straw bale walls are about 70% more
insulative than unplastered bale walls: the plaster is very efficient in
excluding air movement through the bales.  The effectiveness of bales
stacked against the summer shaded wall (north, above the equator) would
probably only lie in deflecting wind around the building, but would
contribute far less insulative value than a plastered bale wall
incorporated into the structure with the cob.
Isn't Ianto Evans now suggesting plastered bale walls on the outside as
the insulative layer and very heavy, cob-like plaster on the inside, the
thermal mass.  Seems like I'd seen something here on treating particularly
the "north" side like this, esp. when one has a solar orientation that
allows sunlight to penetrate the building and heat the inside of that
"north" wall.  Such a "northside" wall will function well in hot summer,
too,  since the summer sun is so far north, and would otherwise heat the
exposed house from early in the morning.

If you're interested in determining how much heating and cooling you'll
likely need, go here for heating and cooling degree days and average max
and min temp. data for your latitude and longitude, and good explanations
of same:

http://www.worldclimate.com/

Of installing a woodstove on a plastered bale wall, again, see the
archives on the bale lists.  An Israeli woman had a fire in her bale wall,
having relied on the plaster to provide "clearance to combustibles," and
was advised by list members to relocate the woodstove to a non-bale wall,
if I remember correctly, or to replace the bales in that section with a
non-combustible material.  Bales are cellulose, and can char over time,
just like wood beneath a plastered wall that gets hot enough, enough
times, and can then combust.  I remember the fire in the cob
oven/fireplace at Black Range Lodge in Kingston, New Mexico some years
ago.  It was a Sun Ray Kelly desig
n, and built by volunteers.   Too much
straw in the cob mix too close to the flame path, and a year or so after
it was built, caught fire and had to be pulled apart.  The message is to
treat woodstoves or any other appliance which relies on combustion just as
the UBC requires with a specified distance from combustibles.

Lastly, for an excellent discussion of the amazing properties of lime
plaster, see a post from today on the REPP bale list.

Barbara in the Sierra Nevada Foothills