Cob in really cold climates

[M J Epko]

At 09:47 PM 8/29/96 -0500, Eric Hart wrote:

>Minnesota has a cold climate.  So my question is, what is the R-value of
>cob?  Does anybody know? When it gets down to -30 F every year for a week or
>so, you care about the R-value of a building system.   I have heard that
>rammed earth does not work in cold climates because earth is great for
>thermal mass but poor for insulation.  I don't see why cob is much
>different.  

        Seems to me that rammed earth is like man-made rock, often
stabilized with concrete which further reduces thermal resistance. The stuff
is pounded inside forms, these days commonly with air hammers. No air
pockets, no plant stuff. As I understand it cob is hand-packed. It probably
doesn't have the compressive strength of rammed earth, but I think it would
certainly have better thermal characteristics.

        My rammed earth book by David Easton is out on loan, or I'd get it out.

        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?

>Just want to get all the issues out on the table so we don't think that one
>building system is the solution for all climates.  It seems to me that each
>climate requires a different type of building system that utilizes local
>resources.  

        I agree with this.

*

        I can't answer the r-value thing. But I'm sitting here trying to
make sense of the formulae in Earth Sheltered Housing Design (1979,
University of Minnesota) to get some idea of what an answer *might* be. I'm
reading the section on thermal loss through earth roofs (which would not
necessarily be the same as thermal loss through cob walls.) I recognize that
this book is talking about loose earth, not packed, but they do consider
topgrowth which may simulate the characteristics of straw mixed in
throughout the soil.

        Anyway - fully mindful that this entire 318-page book was written
with consideration of weather data collected from 1940 to 1970 for the
Minneapolis / St Paul Minnesota area - they wrote: 

"Seasonal temperature fluctuations reach a depth of several meters into the
soil whereas the penetration of short term temperature fluctuations over
periods of hours or days is almost negligible... wide daily air temperature
swing is essentially eliminated below 0.2m [which is - like, what - 8
inches?]; this demonstrates the advantage of even 0.2m of earth cover."

"... the thermal conductivity of soil is approximately 25 times greater than
modern insulating materials..." (So 1" fiberglass = 25" [2'1"] loose earth -
however, fiberglass has no thermal storage capacity, which I imagine would
significantly temper the bad thermal resistance characteristics of earth.
For instance, the initial quote indicates that not much depth almost negates
the effect of daily temperature swings. I take this to mean that a
decently-thick cob wall would be heated to a temperature from the interior
during the winter, and that temperature would be more easily-maintained due
to the thermal mass of the earth as opposed to maintaining the same
temperature with fiberglass [or sb] as insulation, which has no mass.)

They compare two roofs: the first being 18" soil atop 4" rigid foam; the
second being 4.6" rigid foam. They selected average January high & low
temperatures of 5 & 15 degrees, then sent in a five-day cold front with
highs & lows of -5 & 5 degrees. The second roof, with no mass, required 196%
more additional heating energy during the cold front. Quoting: "Roof A ...
requires a full day before the ceiling begins to indicate that more severe
conditions now exist outside..."

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?)

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.

*

Shannon's message responding to the same post just came in; I seem to be on
the right track (oddly enough.)

Shannon wrote:

>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.        

*

Shannon, I understand the copyright cringe; I'll summarize it when I'm
awake. It'll be easier & more comfortable. I can also snail you a photocopy
of the article if you want. If not, it's Backwoods Home #39, May / June '96.

                Backwoods Home Magazine
                PO Box 40
                Montague CA   96064

                email   backwood at snowcrest.net
                URL     http://www.snowcrest.net/backwood

*

M J