[Cob] Cob R Value

[Shannon C. Dealy]

On Mon, 20 Jun 2005, Anna Young wrote:

> Hi Cobbers,
>
> We have an home energy consultant reviewing our cob house plans. She
> wants to know the R value of cob. I heard around 0.7/inch, but only
> anecdotally. Does anyone know of any tests or work done to come up with
> an accurate range for R? To contrast the consultant had heard the value
> was closer to R1 for a 24" wall.

This question comes up on the list every year or two, and I have seen all
sorts of numbers run by, many of them are simply not even remotely
possible (probably due to failure to adequately control for the behavior
of the thermal mass).  Having said that, the R-value is going to vary
depending on your particular mix, but some reasonable (though very
imprecise) bounds can be estimated based on the type of materials it is
made of and comparing them to other similar materials with known values:

   concrete          - 0.08/in
   plaster           - 0.12/in
   face brick        - 0.11/in
   low density brick - 0.20/in
   stucco            - 0.20/in
   hardwoods         - 0.91/in

Low density brick and stucco are probably the most similar in composition
and density to cob, though the straw content of cob might make it's
density even lower and should further reduce it's ability to conduct heat
(increasing it's R-value).  I included the R-value for hardwoods because
some people have claimed R-values on the order of 1/in (or higher) which
is clearly not remotely possible as any type of wood is definitely a much
better insulator.

Using the above, I would consider 0.2/in to be a reasonable conservative
value, and would speculate that the upper end of the possible R-value
range (for a high straw mix) might possibly run as high as 0.4/in.
The conservative value of 0.2/in would give R 4.8 for a 24" wall.
If you are in a moderate climate and/or have good sun exposure in winter,
the termal mass of a properly designed building could be considerably more
important than the actual R-value.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com