[Cob] R-Value: effective versus actual

[Shannon Dealy]

I am concerned that some people may have gotten the wrong impression about 
cob r-values in the context of recent discussions, so I thought some 
clarification was in order.

Some simplified definitions:

   R-value: A measure of a material's resistance to transfering heat
            from one side of the material to the other side.  This is the
            material's "actual" R-value.

   Effective R-value: the R-value a material will "appear" to have when
            measured in a specific context.

Cob's actual r-value depending on the mix will have a rough order of 
magnitude in the 0.3 per inch range.  This means we could expect an 18"
thick wall might be around R-6

So where do all these high r-values you keep hearing about come from?  The 
key is "Effective R-value" and the phrase "specific context".  In the real 
world, temperatures fluctuate a great deal and conventional houses use 
heating and cooling systems to deal with these changes.  This is where 
thermal mass can be a big win.

To give a simplified example, suppose you live in a climate which never 
changes and all year round the temperature drops down to 60 deg. F at 
night and climbs to 90 deg. F each afternoon.  For each 24 hour period, 
the temperature average is 70 deg. F (due to differences between how long 
we are at the maximum and minimum temperatures).  In a conventional house 
with the thermostat set to 72 deg. F, you would be running airconditioning 
in the afternoon to keep the temperature down and at night running the 
heating system to keep the temperature up.  In a cob house (in our 
simplified example), the temperature of the cob would remain close to 
the 70 deg. F outside average temperature at all times, so you would only 
use a little bit of energy to raise the temperature in the house from 70 
to 72 deg. F.  The result is that even though cob is a very poor 
insulator, the cob house would use only as much energy for heating 
and cooling as a building with a lot of insulation, giving it a very high 
"effective r-value".

Now change our example so that the outside temperature is always 60 deg. 
F or always 90 deg. F year round.  Now our cob building will behave just 
like its actual R-value of around 0.3 per inch because we aren't gaining 
any energy savings from the thermal mass stabilizing temperature swings.

In reality, effective r-value is much more complicated, because we can use 
overhangs, trombe walls, mass floors and glass to increase winter thermal 
gain and damp summer heat.  In addition, the local climate will make an 
enormous difference.  A location which gets bitterly cold in winter, but 
has a couple hours of sun almost every days may allow for much better 
effective r-values than a location which never gets below freezing, but 
has heavy cloud cover which blocks the sun for most of the winter.

So effective r-value depends not just on the materials you are using, but 
the design of the building and the climate you are building in.

It is the "effective r-value" that is being used in the context of Janet's 
code approved cob building project.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
Phone: (800) 467-5820 |          - Natural Building Instruction -
    or: (541) 929-4089 |                  www.deatech.com