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re: calculating "ideal" thermal mass for residence
14 aug 2003
michael dewolf wrote:
>> >i am trying to determine which construction method makes the most sense:
>> >insulated concrete panels (eco-tierra) exterior walls or sips.
>> sips. they are easier to move, and they allow larger heat setbacks at
>> night and larger ac setbacks during the day, so you can save more heating
>> and cooling energy when you aren't home.
>i'm not sure what you mean by a "setback".
a thermostat setback. an r24 sip house with 1/2" drywall (0.5 btu/f-ft^2)
might have a 12 hour time constant. you might go to bed when the house
is 70 f on a 30 f night with the thermostat set for 40 f and wake up to
a 30+(70-30)e^-(12h/12h) = 45 f house 12 hours later, saving lots of heating
energy. with 8" concrete (17 btu/f-ft^2) walls, rc becomes 400 hours, so
you might wake up to a 30+(70-30)e^-(12/400) = 68.8 f house with almost
no overnight energy savings.
>i will say that sips don't seem to have a lot of thermal mass.
exactly. i suggested sip walls, with enough thermal mass in the ceiling
(eg 2 layers of drywall with some low-emissivity paint or a poly film duct
with a few inches of water over a dropped ceiling) to store overnight heat,
heated to 90 f or so with warm air from a low-thermal-mass sunspace. a
thermostat and a slow ceiling fan might control the room air temp and
allow large setbacks when the fan isn't running.
a shelfbox inside the sunspace might provide hot water and heat for five
cloudy days in a row.
nick
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