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An Oklahoma City house

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Posted by Nick Pine on January 7, 2004, 7:01 pm

Mr. Okie writes:

I know the feeling. SIP (structural insulated panel) houses are excellent
insulators, and they can go up fast. Is there a manufacturer near you?
You might look at the national SIP association web site. Premier
www.pbspanel.com has an office in Mead, NE (800) 228-4412. Draw your house
on the back of an envelope, send it to them, get back a CAD drawing, look
it over, send it back, get another back, and after a few iterations, it
arrives in the form of big (up to 8'x24') flat panels on a flatbed truck
with precut holes for doors and windows, and they screw them together
in a week or so.

A solar closet can keep a house warm for a few cloudy days... A "shelfbox"
improvement can also make hot water for showers.

...a fine climate for solar house heating. NREL says 870 Btu/ft^2 of sun
falls on the ground and 1340 falls on a south wall on an average 35.9 F
January day with a daily max of 46.7. East, west and north walls get 570,
590, and 230 Btu/ft^2.

If sunny days are like coin flips, a house that stores overnight heat can
be at most 1-1/2 = 50% solar-heated. Storing heat for two cloudy days can
be 1-1/4 = 75% solar-heated, 3 makes 1-1/8 = 88%, and so on.

That can work. A solar closet is a glazed box that normally lives inside
a sunspace with its own glazing. The sunspace provides heat for the house
on an average day, and the solar closet provides heat on cloudy days.

A 48'x48' house with 8 8'x24' 8" R32 SIPs for walls and a flattish roof
with 12 moree and 192 ft^2 of U0.32 windows with 50% solar transmission,
96 ft^2 on the south, 48 on the east, 24 on the north and 24 on the west
and 30 cfm of air leakage would have a thermal conductance of 192ft^2xU0.32
= 61 Btu/h-F for windows + 1344ft^2/R32 = 42 for walls + 2304/32 = 72 for
roof + 30 for air leaks, a total of 205. It needs 24h(65-35.9)205 = 143.2K
Btu of heat on an average January day. If 600 kWh/mo of indoor electrical
use provides 68.2K Btu/day of that, it needs 143.2K - 68.2K = 75K Btu/day
of "other heat." The windows would provide 0.5x24(4x1340 +2x570+590+230)
= 87.8K Btu, more than enough to keep the house warm, with some thermal
mass (how much, if the house is 70 F at dusk and 60 at dawn?) near the
windows to store overnight heat, so we don't need a sunspace for average-
day heat.

This house would need 5x75K = 375K Btu of heat for 5 cloudy days in a row,
which might come from 375K/(120-80) = 9375 pounds or 1172 gallons or 146 ft^3
of water cooling from 120 to 80 F over 5 days. A 4'x8'x8' tall shelfbox
with a 4'x8'x4' tall 128 ft^3 tank under 12 4'x8' shelves with 2" of water
in a poly film duct on top of each could provide cloudy-day heat and preheat
water for showers using a 42 gallon pressurized galvanized tank in the lower
tank and a simple greywater heat exchanger.


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