Cob: Thermal properties of cob

[Shannon C. Dealy]

On Sun, 11 Jun 2000, W wrote:

> Poor Insulator?
> uh oh.  I was under the impression that cob was second only to straw
> bale in insulative properties.  (with cob walls being the same 16-20
> inches thickness of bales)?
[snip]

Nope, while there has been quite a bit of debate over how much
insulation cob provides, I know of no one who considers it to be a really
good insulator.  This does not mean however that it is necessarily a poor
choice for any climate.  Cob provides excellent thermal mass, and at least
some insulation value (depending on your mix and who you believe a 16-20
inch thick wall is probably somewhere between R-8 and R-18.  It is
important to remember that in many if not most climates, the insulation
value of cob will be far less important than the thermal mass properties
for most of the year.  Thermal mass acts as a stabilizer for temperature
swings, if the outside temperature reaches 100 degrees F for one hour each
day for a week or more, but the average temperature for each 24 hour
period is only 75 degrees, then your cob walls will tend to hold the
temperature of your house to something near that 75 degree average.  This
can of course be altered (for good or bad) by the use of large areas of
glass facing the sun, trees for shading the house, open/closed vents
or windows, etc..  These items will alter the average temperature in and
around your house, but the cob will still hold your interior temperature
somewhere near that average, rather than letting it go to the temperature
extreme.  The same holds true for cold climates - the cob will act to
average the temperature swings, and by careful use of glass facing
the sun, shade trees, barriers to block high winds, etc..  It is possible
to build a cob house which will require little or no additional
heating/cooling (depending on your personal temperature requirements) year
round in a fairly wide range of climates.  The area in which cob generally
will have a problem with temperature extremes (that cannot be dealt with
just by using passive solar designs) is places where it gets cold and
stays that way for many days or weeks at a time.  For climates like this,
I would recommend a layer of good insulation (possibly straw bale) wrapped
around the exterior of your cob building.  This will make for very thick
walls, but with the cob walls inside the insulation, your interior
temperatures will be very stable, and in the event of a failure of your 
heating system, it could take several days before the interior temperature
drops enough to be a significant problem, by which time you would
hopefully have it fixed.

Note: rereading the above, I may not have made it clear, while cob
evens out the temperature swings, it is over a period of several days, not
just the swing for one day, the more cob (or other thermal mass) in your
structure, the longer the averaging period.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
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