Cob inside

[Eric D. Hart]

At 11:46 PM 8/29/96 -0700, M J Epko wrote:

>        Perhaps you (Eric) could help answer this question, actually. I know
>you were working on precasting building components of a straw / cement
>mixture, which would, I think, likely have worse thermal characteristics
>than cob as pertains to thermal transfer. How's that going? Have you had the
>opportunity to find the magic recipe & try the concept out? Did it behave
>responsibly for our climate?
        Haven't made any further progress on the straw/cement mixtures, we
have been building instead of experimenting.  I don't know if we will pursue
this further.  I would think that it has better thermal characteristics than
cob because its mostly straw (just enough concrete added to make it strong
and stick together).  

>So sure - it ain't strawbale (which has great insulation and crappy mass.) I
>find the idea of south-facing cob in frigid climates for the thermal mass
>and rounding out the rest of the building with sb intriguing. (How 'bout cob
>for the greenhouse proposed on the sb list?)
        Cob or rammed earth would be a good choice for that transition
between the greenhouse and the house.  The thermal mass would help to temper
the greenhouse temperatures (especially to provide heat at night) and also
provide radiant heat for the house.  If the wall radiated enough heat, you
could collect the heat that rises from it and distribute the hot air to the
rest of the house using duct work.  The greenhouse and house could go up and
then the cob wall could be built after the two structures were enclosed.
Then you could take the time needed to build the cob wall up.  

>We here have been considering using standard sb construction, using regular
>stucco on the exterior, then using cob on the interior for its functional
>mass and artistic sculptability. That would give us the outstanding
>insulation from the bitter cold *and* a lot of interior mass for the sweet
>warmth. Did you see the sculpted bath alcove at the Cob Web site? Gorgeous.
        This idea occured to me also.  Only problem is how do you deal with
the post and beam system in the strawbale part of the structure?  I guess if
it was a load bearing design, then you could finish everything but the
interior finish.  Seems like a lot of labor just to get an interior finish
which has thermal mass.  I could see building a load bearing cob design and
then putting strawbales on the outside to add the necesssary insulation.  
        Can cob posts be constructed?  I could see making a series of
massive posts which were part of a strawbale infill system and which also
provided thermal mass to the interior of the building.  When I am thinking
about these ideas, I'm assuming that they would be tried in a place with no
building codes.  I've got enough problems just trying to get strawbale in
fill designs approved, let alone trying to add cob to the equation.  A cob
column would work good in a greenhouse where you need thermal mass and some
sort of material that will resist moisture damage.  One could start really
early in the spring (after the ground thawed out) building your cob columns
and then once they were ready,  put the roof trusses on them and then do the
strawbale infill.  I can't see doing a whole wall that has such a poor
R-value but using it sparingly for load bearing posts would be a good use of
the material.  


>>Cob Cottage Company's approach is to make use of the thermal mass of the
>>walls and store heat during the day to keep the house warm at night, though
>>in really cold climates or during prolonged periods without direct sun
>>this probably won't work to well.
>
>        This could work equally well storing heat from the inside to
>moderate net loss at night. The house would work along the same lines as a
>masonry heater.        
       One problem with masonry houses is that once they heat up, they stay
that way for a while.  I went through an 1830's stone house on the
Mississippi River here and they said that the house became unbearable in the
summer, even in the relatively cool Minnesota summers.  The same goes for
when the houses cool down, they get cool and stay that way.  Ever gone by a
parking garage (all concrete) in the spring and early summer and felt how
cold it is in there?  The inside of parking garages during the winter is
colder than the outside air and stays that way longer (due to the poor
insulation and the conductivity of the concrete).  I just envision living in
a refrigerator during the winter and an oven during the summer, so am going
to progress cautiously when it comes to using mass in buildings.  
        Last question, have cob structures been built in places other than
Oregon and Great Britian?  It may be that cob works well for places that
have relatively mild climates and small temperature variations from season
to season.  That's the climate in the Pacific Northwest and Great Britian
but not the Midwest.  

Eric Hart
Minneapolis, MN