Cob: to insulate or not

[Darel Henman]

Drew,
  I wasn't disagreeing with you that a 40 deg F thermal mass is 40 deg
F.  I was pointing out that that if the soil was heated for better
winter performance and even to provide heat inside it would not be a
heat sink, but a heat source.

Drew Neve wrote:
> I'm going to disagree with most of the comments questioning what I said,
> based on the source I quoted in the original message and my own
> experiences. 

Your source would no doubt agree with what I said.  


> If I go into my 40 degree uninsulated basement and put on a
> space heater while I'm working on something, the temperature warms up right
> nearby the heater only.

That would indeed happen.  But, what I think confused you, is that I was
describing a different case in which the soil was heated from the
original 40 F to 80 F over, say a six month summer period using solar
energy.  If a lot of thermal mass is involved it might take two years to
stabilize, but each year would be better than the original 40 F. 


> If I take that same heater to the same sized
> insulated room immediately above, even at night, the heat warms the entire
> room, even when it is below zero outside. 
That's because you are only heating the air.  If you turn off the heater
it will cool down quicker than if you had a heated thermal mass nearby.

> As soon as I set the heater down
> and walk back downstairs, the temperature still says 40 degrees.

If it was 40 F then it would definitely still be around 40.  The mass
your talking about would take a long time to heat.   As I mentioned, six
months of free solar heating of the soil would work wonders.

> Apparently that heat is being absorbed by the earth outside my basement
> walls.  A thermal mass will hold heat, but the earth is more of a thermal
> mass than I am interested in heating.  I also don't want to live in a 40
> degree house, so I'm going to insulate against the earth.

Perhaps you should read John Hait's book entitled, "Passive Annual Heat
Storage".  This book should still be available at libraries or the 
Rocky Mountain Research Center.

Here's an interesting comment about the process or a site in Wisconsin,
which has an annual temp in the low 40's.   Note, the deep ground would
be this temperature to.

Ernest Martinson mentions that:
"John Hait in
Passive Annual Heat Storage suggests the 20 foot perimeter insulation
because heat requires 6 months to
travel 20 feet and at a depth of 20 feet the temperature approaches a
constant that is a reflection of the
average annual air temperature for the site. Suppose the average annual
temperature to be in the forties in
northern Wisconsin. Then, in a few years, the temperature of the earth
surrounding a passive-solar
earth-sheltered home can be raised to the deep earth forties and then
towards a comfortable seventy due to
the insulation/watershed umbrella and an earth buffer over the umbrella.
The insulation/watershed umbrella
can be cheaply constructed of alternate layers of earth and plastic
extending outward with a gradual slope
for 20 feet."

> Also, looking at the figure in "The Hand-Scupted House" that compares cob
> to straw bales, they point out that cob (earth) is not a good insulator.
Again, you are not considering the thermal mass function.  

Here's more information for you from:
http://www.thenaturalhome.com/frostwalls.htm

"The Underground Space Center at the University of Minnesota did some
excellent research in
the late 70's and early 80's pointing to the fact that horizontal "wing"
insulation was preferable to
vertical foundation wall insulation.  Wing insulation was shown to keep
the earth near foundation
walls dry, greatly increasing the insulation's efficiency.  Expanding
greatly upon this concept, John
Hait published Passive Annual Heat Storage (PAHS) in 1983.  This work
was dedicated to a very
basic concept that is ignored by nearly all architects: keep the ground
under and around the home
dry and you can use it for an amazingly effective heat sink and seasonal
temperature moderator. 
After all these years, I am still shocked and amazed at the ignorance
shown by designers who
insist on leaving the foundation walls in contact with wet earth. "

> I don't know Darel, and I don't want to be confrontational, but I find his
> points puzzling, perhaps there is a misunderstanding as to the context.
It is confusing to many people.   But, I recommend considering the use
of thermal mass heated by solar energy for annual passive heating. 

Here's the Rocky Mountain Research site:  Apparently they changed their
name, but see 
"Example 2: Storing summer's heat for winter" on the following site:
http://www.rmrc.org/coolscience/coolscience1.htm#ex2

The above page's home site also has some interesting information for
people interested in energy and cooling.

> All of this really is not peculiar to cob. 
Cob walls are made out of soil and are thick.  They will hold a lot of
thermal energy, but depending on the area may not provide enough thermal
lag to float the home through the winter, so back up heating would be
needed.   If the cob house can be made to function without or with as
little purchased fuel, I consider that a good thing.

> As an aside I would not
> recommend using cob below grade, pour concrete or use stone or earth-filled
> tires against the hill and use cob above grade.

I agree with you.  I would be very wary of a cob below grade wall.
 
> Drew Neve

Hope I managed to be a little clearer this time.

Darel