[Greenbuilding] energy retrofit, asphalt shingle roof, integrated solar thermal air collector
nick pine
nick at early.com
Sun Nov 20 13:25:35 CST 2011
Haudy Kazemi <kaze0010 at umn.edu> writes:
>I'm planning an energy retrofit on an old house in Minnesota and was
>wondering if anyone has used an asphalt roof as a solar thermal air
>collector...
Walls work better than roofs for wintertime heating, unless they are
transparent and steeply-pitched. See
http://www.ece.vill.edu/~nick/Soldier...On.pdf
>The idea is to use a 1.5" air gap (created by 2x4 furring strips between
>the shingles+OSB and 6" of polyiso) as a solar air collector to heat air
>for a porch...
Sun shines on unglazed shingles over OSB and you collect warm air from the
other side? :-) NREL says 430 Btu/ft^2 falls on the ground and 820 falls on
a south wall on an average 17.9 F December day with a 10.2 mph windspeed in
Minneapolis. If 1 ft^2 of black 4:12 roof with an 18 degree slope receives
430cos18+820sin18 = 667 Btu/ft^2 of sun in 6 hours at 111 Btu/h with a 2
Btu/h-ft^2 slow-moving airfilm conductance and warms to 17.9+111/2 = 73.5 F
with no heatflow into the attic , it can only provide about
6h(73.5-70)1ft^2/(R1+R0.5) = 14 Btu/ft^2 per day to 70 F attic air on the
other side of the R1 shingles :-)
>The front porch is on the south side of the house...
Why not just glaze the south wall of the porch? R2 glass or polycarbonate
glazing with 80% solar transmission can provide about
0.8x820-6h(70-19.7)/R2) = 500 Btu/ft^2 per day of warm air for the house.
Nick
My message to architects and engineers is: Look at the whole picture. In the
trade press recently, there was an article hailing a custom,
9,000-square-foot, architect-designed house as the latest in environmentally
responsible design. Its principal claim to fame seemed to be the use of
natural, nontoxic finishes on the woodwork. In the rush to commercialize
"Green Architecture," no one noticed that this house consumes more energy
than a small New England town.
If your goal is trying to build an environmentally responsible building,
you're missing the whole point if you get all lathered up over a nonvolatile
natural finish on the handrails, while you're connected to a plutonium
generator down the road. It's the same "out of site, out of mind" again,
with a new face. "I'm doing all I can for the environment, my architect
specified beeswax on my new woodwork--someone else will just have to figure
out what to do with
all this radioactive waste"... and acid rain and oil spills and global
warming and ozone depletion and unhealthy air quality and...
You hear a lot about sustainability these days. I've been at this since
1973, long enough to be certain that, without addressing the energy issues,
you're in the weeds. All the fuss over "my milk-based paints transported in
from Europe" is just a myopic distraction from the issues that really matter
on a global scale. True, natural-based finishes are desirable, but they fall
far short of the answer. Establishing an energy infrastructure based on
renewable resources
is a necessary and fundamental precondition to establishing a sustainable
society or to achieving sustainability at any scale. If you are not
addressing the energy issues, don't even pretend that your building is
environmentally responsible.
Architect Steven Strong in The New Independent Home by Michael Potts,
Chelsea Green, 1999
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