[Cob] R-Value: effective versus actual

[Howard Switzer]

Wonderful information, thank you Shannon.

I would add, and you probably all know this, that my experience with radiant
heat, which is what we are talking about, is that the comfort level might be
below the 72 mark.  My home has radiant floor heating in a slab with bale
walls and it is often too hot at 72 and I prefer it down around 65 because
of the warm surfaces all around in the house, the floor and 1.5" thick
plaster. Not having such cold surfaces around means my body heat isn't going
anywhere fast, except maybe toward the windows. I did a high mass passive
solar home in 1980 for Albert Bates mother, Dorothy Bates, that was that way
also.  She was an elderly woman, a retired successful realtor, who wrote
books on  vegetarian cooking and was impressed with how well it worked and
how comfortable her rooms were at 65.  Thermal mass along with passive solar
design should have become a convention in the building industry years ago.

h


On Sun, May 15, 2011 at 11:53 PM, Shannon Dealy <dealy at deatech.com> wrote:

>
> 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
>
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-- 
Howard Switzer
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Linden, TN 37096
931 589 6513
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