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re: dome home
13 jun 2000
phxbrd@home.com wrote:
>> ...tests on a 200' diameter monolithic coal storage warehouse
>> in pittsburgh showed a 5 f temp difference from top to bottom.
>> warm air rises up to the top, and the well-insulated concrete
>> conducts the heat back to the bottom.
>that statement makes no sense to me.
pity.
>convection takes place when hot air expands...
bouyancy happens when denser cool air displaces expanded hot air.
>as hot air rises, it tends to cool,
hmmm. new physics. why yes, rising things cool. the cia has only
recently revealed stories of fly balls returning to earth with
mysterious coatings of frost. (you don't see this on tv. they wipe
them off first--watch closely next time.)
>causing it to rotate back down toward the floor again in an endless cycle.
yes, i can see this. a convective doughnut with warmer air rising
up in the middle and air near the sides cooling and falling, with
a faint shimmering image of dorothy and toto under the peak.
>when a building is really well-insulated, less cooling takes place near
>the ceiling & hot air tends to just stay there. forced air circulation
>becomes necessary for maximum efficiency, but "well insulated concrete"
>will return heat to the bottom of a structure by itself....
believe the tests, or try some arithmetic. a slowly-moving air film has
a surface conductance of about 1.5 btu/h-f-ft^2, concrete has a us r-value
of about 0.2 per inch, with a bit more for 10% rebar. urethane foam has
an initial r-value of about 7 per inch; 2" of concrete under 3" of foam.
on the one hand, warm air tends to stratify. on the other, the concrete
conducts heat downwards, with little heat loss to the outdoors...
nick
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