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Understanding thermal mass  RSS feed

 
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Hi there, just wondering if anyone can recommend any good reads on how thermal mass works? I guess what I’m having trouble with is understanding how cob would stay cool in the heat, considering the fact that it seems it would be absorbing heat from the sun. Same with the cold, wouldn’t the walls just retain the cool temperatures from outside?
 
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I don't have a book to recommend.  My understanding of thermal mass is that it takes a while for heat to move through the mass and due to its mass it holds onto a lot of heat (or cold).  It doesn't fight heat transfer nearly as much as insulation would.  I believe it basically averages out the conditions around it.  So if you have mass with a 70 degree house on one side and a 100 degree exterior on the other, it should average that out to 85 and resist changing.  If you have 70 inside and a day to night variation of 60 to 100 (equaling an average of 80), the thermal mass should approach 75.  I believe in a hot climate you'd want your walls to be shaded with large overhangs so the sun isn't adding thermal energy to them.

This is why I believe cob and adobe isn't recommended for frigid climates.  If you average 0 outside and 70 inside, the walls will be averaging 35. 
 
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josh ober wrote:I guess what I’m having trouble with is understanding how cob would stay cool in the heat, considering the fact that it seems it would be absorbing heat from the sun.


It does indeed absorb heat, but different materials can absorb more energy before they actually start to feel warm.

I appreciate that could be a bit hard to wrap your head around. The fundamental reason for this property is highly technical and beyond my understanding, but is explained on Wikipedia:

https://en.wikipedia.org/wiki/Heat_capacity
https://en.wikipedia.org/wiki/Thermal_mass

An example given on Wikipeida (see this table) is that water requires about 4.5x more energy (measured in Joules) to raise its temperature by 1°C compared to an equivalent mass of concrete. So in layman's terms, concrete starts to feel warm four to five times faster than water does. This means water is the better material to use as thermal mass. Of course, water has a habit of leaking out of containers and destroying stuff. On the other hand, water can also be pumped through heat exchangers which allow you to push the heat around much faster. So there are a number of factors to consider.

Here's another useful table showing heat capacity. Sandy clay requires about 1.6x more energy than concrete to raise the temperature.

An interesting (and counter-intuitive) detail to note is that air has a higher heat capacity than concrete. But of course, 1 tonne of air occupies a much larger volume than 1 tonne of concrete.

Finally I just want to point out that I am not a scientist or engineer, so I would love somebody who is to confirm what I have written above.
 
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Cob has a insulation value of 0.3 to 1.0 per inch, Depending on the amount of fiber used.
So at best a 12 inch thick wall would only have a R-12 value, probably closer to R-6. http://www.greenhomebuilding.com/QandA/cob/insulation.htm
So you are correct in assuming that this will not work.

Except it does work. Under certain conditions aka the usual it depends

A) If your summer day time high is 100F and night time low is 65F with a daily or weekly avg temp of 78F.
The thermal mass will make your indoor temp 78F

B) If your winter day time high is 70F and night time low 40F with a daily/weekly avg of 50F.
Your indoor temp will be 50F. Fine with a blanket and a bit of fire

C) In the tropics with a high of 88F and low of 78F. A cob vs a metal housing (car), will provide quite a bit of moderation from the direct sunlight.


Where it doesn't work is where you a avg temp of 20F. High 35F, low of 5F. So subtropical/tropical/desert areas are where it works best year round.

SOLUTIONS:
1) Make the walls super thick, as in 36inches thick for a R-value up to R-36.
2) Place insulation on the outside wall to increase the R-value. Insulation could be strawbale with a thin plaster of mud.
 
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I am not sure your different systems of averaging are correct.
I will try and explain what thermal mass does.
Irrespective of any temperature ranges inside or outside, the thermal mass absorbs heat, say in the case of a wall with sun on it or even hot air, slowly.
If you think about the effect of that same wall if it was glass.
In the former there is resistance to the heat going through the wall, whereas the glass will transfer the heat almost imeadiately.

As the day cools, the thermal mass then starts to transfer the heat back to the coolest area, outside in this case.

At times when the temperature is hot for a very long time, the wall may heat up entirely.

That is rare, but the benefit of the thermal mass is to slow down the transfer of the heat and create a more stable environment.
 
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