[Cob] Thermal mass

[Lance Collins]

Hi Paul,

The basic physical properties you are chasing are 'specific heat' and 
'thermal conductivity'.

Specific heat is the amount of energy required to raise a unit of 
mass through one degree of temperature.  Generally the denser the 
material the higher the specific heat.
And for conductivity the better the molecules in a mass are connected 
the faster heat travels through it.

Generally the molecules/particles in a lump of earth are poorly 
connected so cob, adobe, rammed earth are better insulators than 
concrete.  A rule of thumb is that heat travels through cob at an 
inch an hour so that the sun energy that shines on your foot thick 
cob wall at 9 am will be warming you inside at 9 pm.

So a cob wall at two feet thick will have twice the thermal mass but 
will not be as comfortable as one foot thick.

I can't remember the rule for concrete off-hand but 4 hours might be 
a good guess.  If so then you would want the concrete wall to be 
three feet thick!!

Trying to work out the best physical properties to use is an 
interesting intellectual exercise but no more than that.   A house is 
a living machine and smarter more practical people than you or I have 
experimented for a long time with ways to take available materials 
and construct a comfortable dwelling.

Have a look at Annualized Geo-solar Storage (AGS).  There's a lot of 
practical physics discussed at Don Stevens site which may help. 
(www.greenershelter.com)


>Can anybody post a table that compares the thermal mass potential of 
>cob versus concrete and other materials?  I've had terrible luck 
>finding such a comparison.  I did find one interesting table that 
>basically showed that concrete has an unusual ability to store heat, 
>and that water stores even more than that (like twice as 
>much).  Water turns out to be the best thermal mass but so far I 
>can't figure out how to build a house out of water.  I'm working on it though.
>
>I like the idea of passive solar but it's kind of useless to believe 
>that it can heat your home in temperate zones unless you have a huge 
>thermal mass and that angled glass (probably pretty expensive) that 
>takes the rays of the sun and puts them directly into the mass. I 
>believe that can do wonders, but aside from concrete I don't know of 
>another building material with that kind of thermal capacity.
>
>Any links or even memories of, say, how massive a cob or adobe wall 
>would have to be to equal so many inches of concrete?

Lance
(in Aus) 


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