Cob: Re: Thermal Mass and R-values

[Mike Carter and Carol Cannon]

From: Michael Saunby <mike at Chook.Demon.Co.UK>
Date: Monday, July 26, 1999 6:15 AM


>On 25 July 1999 17:06, Mike Carter and Carol Cannon
>[SMTP:cobcrew at sprynet.com] wrote:
>[SNIP]
>>   Consider also two samples that have the same density but in one, the
insulating
>> material is in a solid layer in the middle instead of being dispersed
evenly
>> throughout.
>
>OK, how do you suggest folks do that?

Let me try and restore context to the statement at the top.  People are
trying to figure out how to add more insulation to their cob walls,
especially those of us who live in "extreme" climates.  There has been a lot
of discussion about how to add such insulation, including use of materials
like straw bales, perlite, vermiculite, pumice, cellulose, fiberglass, etc.
Our young German carpenter friend suggests that it is all hopeless based on
a well-known equation for the thermal resistance of straw-clay mixes.  I
wanted to show that (1) two samples with the same density can have different
"R" values depending on the distribution of the insulating material (2) the
equation is for clay and straw; you extrapolate to other materials at risk
(3) the equation handles mixes where the means of thermal transfer is
conductance; as you get towards a mix where the insulating materials
dominate, that assumption may no longer hold and increasing density (of the
insulating material) can give better thermal resistance.

Can someone tell me why, if "light clay" is such a poor insulator, that my
cob oven with 3" of "light clay" can be 400 degrees F inside and after two
hours it is barely warm to the touch on the outside?  Try that with a
conventional metal oven.

I have long advocated an outer cladding of straw "mini-bales" impaled into
the cob with bamboo stakes and then plastered over.  If we can find a cheap
souce of small bales we will do this; possibly even retrofitting one of our
3 cob buildings.  The best we were able to find were breadloaf-sized bales
for $3 each.  WIsh I could find the machine that made them.

>What mathematics should be applied
>to these types of problem?
>From what I've read, there are so many variables that computer modeling is
about the only way to answer these questions non-empirically.

>Consider the possible combinations of 10m x 2m x 0.1m of polystyrene or
>some other insulator and 10m x 2m x 0.1m of brick or some other structural
>material, e.g. cob. To build a wall of 10m x 2m x0.2m how should these
>materials be combined?  Which will be the most comfortable? Which will use
>the least energy to heat?  Which will use the least energy to cool? Is the
>answer the same for all?
Put the insulation on the outside of the thermal mass.

>How DO builders calculate this today? Are they
>getting the right answers?
That's what we are all struggling with here.  I haven't seen a "magic
bullet" for predicting how a cob house will perform.

>Should the costs of the materials be considered?
Of course.  And cob is costly (not so much to make, but to move and place,
especially on multi-storey buildings).

>If so what would be the trade-off between structural material, i.e.
>something that will hold the roof up, vs. insulation which will just
>collapse under the weight?
I think this is the wrong question.  The trade-off is between the cost of
the insulation materials and whatever heating/cooling would otherwise be
used over the lifetime of those materials.  If we can't come up with a good
way to add insulation to cob walls, I will start building thicker walls
(currently averaging 22" at base).

Thanks for the insightful questions,
Mike