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What color house?

  • Thread starter Vladimir Vladimirovich Nabokov
  • Start date
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Vladimir Vladimirovich Nabokov

Jan 1, 1970
0
I assume a dark color will absorb the heat and awhite will reflect heat, so if
you live in the Midwest, what's a good compromise?

Pure white would seem perfect in the summer to keep out the solar heating, but
in the winter you would welcome the heat gain. Am I stuck with middle gray, or
is there an established scientific answer?
 
Morris Dovey said:
The color or the paint is nowhere near as important as the insulation
inside the wall and the caulking around openings like doors and windows.

There's a link below to photographs a white building (not a house) that
was kept comfortably warm through the winter by two vertical black solar
panels set into it's south wall.

What happened on cloudy days?
I doubt the building would have been much better heated if it'd been
painted black.

NREL says 930 Btu/ft^2 falls on a south wall in Des Moines on an average
24.4 F December day, so 1 ft^2 of white R20 south wall with a U2 airfilm
conductance would lose about 24h(65-24.4)1ft^2/(R20.5) = 47.5 Btu/day.

We could model a dark south wall like this, viewed in a fixed font:

930/24h = 38.75 Btu/h
--- 20
|--------|-->|------------www--- 65 F
--- |
|
1/2 |
24.4 -----www-------

which is equivalent to:

1/2 20
---------www-------------www--- 65 F
|
| 24.4+38.75/2 = 43.78 F
---
-
|
-

with a heat loss of 24h(65-43.78)1ft^2/20.5 = 24.8 Btu/day.

With an 8'x48' wall, the difference is 8x48(47.5-24.8) = 8698 Btu/day.
If a single-glazed air heater gains 0.9x930 = 837 Btu/ft^2 and loses
6h(80-24.4)1ft^2/R1 = 334 on an average December day, for a net gain
of 503, the dark wall is equivalent to 8698/503 = 17 ft^2 of air heater.

Nick
 
M

mike

Jan 1, 1970
0
Vladimir said:
I assume a dark color will absorb the heat and awhite will reflect heat, so if
you live in the Midwest, what's a good compromise?

Pure white would seem perfect in the summer to keep out the solar heating, but
in the winter you would welcome the heat gain. Am I stuck with middle gray, or
is there an established scientific answer?
Don't remember all the details, but some kid won the national science
fair prize for a roof that looked like a stairway.
Black on the vertical surfaces and white on the horizontal surfaces.
Average reflectivity went down as the sun moved south for the winter.
 
mike said:
Don't remember all the details, but some kid won the national science
fair prize for a roof that looked like a stairway.
Black on the vertical surfaces and white on the horizontal surfaces.
Average reflectivity went down as the sun moved south for the winter.

PE Norman Saunders patented "solar staircase" has mirrors for treads
and transparent risers...

But shading based on detailed summer sun angles isn't as effective as
80% vertical shadecloth over the outside of a window, since a lot of
summer sun is diffuse, eg 65% on a south wall in Austin in August.

Nick
 
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