thermal mass in a ceiling SOLAR.KnowledgePublications.com

thermal mass in a ceiling
11 mar 2003
still wondering how to store overnight heat from warm sunspace air.

what was the name of that scottish practice (pooging?) of filling
the rafter cavities in a ceiling with soil or sand or ashes? what
was the purpose? frugal funerals? insulation? noise reduction?
mass to store heat from a woodstove? do they still do this?

we might lay flat polyethylene film duct with a few inches of water in
a rafter cavity to store overnight heat in a house, with some drywall or
welded-wire fencing beneath. this could cost less than a mass wall, and
water stores 2-3x more heat by volume than masonry. ceilings might work
better than walls for storing overnight heat, with stratified warm air
from a sunspace near the ceiling...

suppose we keep an r16 8' cube 70 f for 6 hours on an average winter day
by controlling the flow of 100 f sunspace air which enters the cube just
under the ceiling and floats around in a uniform layer and loses heat to
the ceiling and then descends to the lower part of the cube at t degrees f
and mixes with air which returns to the sunspace or air heater at 70 f.

with q cfm of airflow, the ceiling gains about q(100-t) btu/h, and the
average air temp near the ceiling is 100-t/2. a ceiling with area a and
temp tc would gain roughly p = 1.5a(100-t/2-tc) btu/h (or maybe 3 vs 1.5a
if the top of the duct is also exposed to warm air.) for the cube,
q(100-t) = 96(100-t/2-tc).

if it's 30 f outdoors, the non-south walls lose (70-30)3x8'x8'/r16 = 480
= q(t-70) btu/h, so (ignoring the ceiling heatloss), q = 480/(t-70) and
by substitution and cranking above, t^2 - (280+2tc)t + 15k-140tc = 0.

meanwhilst, a 64 ft^2 ceiling with water depth d" that gains p btu
in 1 hour would warm from tc to tc + 0.00293p/d f. 

how much water is needed to make the cube at least 60 f at dawn?

nick