[Greenbuilding] energy retrofit, asphalt shingle roof, integrated solar thermal air collector

nick pine nick at early.com
Sun Nov 20 11:25:35 PST 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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