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Re: [Phys-l] paint your roof?

Yes, it is easy for the do-it-yourselfer. But what about as part of the
price of a new house, or a redo on an existing house. How economical is it
compared to other options?

In New Mexico they do sell solar upgrades to houses, and the price is fairly
competitive because of the huge amount of sunshine. I know someone who
bought it in Los Alamos. But in the NE the number of cloudy days and the
very low temperatures probably make it less economical. In The Pacific NW
the temperatures are not as low, but the number of cloud free days is very
small during the winter.

If there is to be a decrease in the use of fossil fuels and an increase in
solar, it needs to be commercially available at a competitive price. Of
course when the next oil/gas price spike happens, the game changes. And it
is certain that in the long run the price has nowhere to go but up.

Part of the problem is that the builders find it more economical to buy
standard systems to install in homes. If each home has to have something
different the builders probably won't offer it.

But the white roof certainly makes sense in a warm climate, mostly S of the
Mason-Dixon line, as long as the roof also is not reflectant in the IR. The
question is whether it makes sense in more northerly places. It might be
helpful in big cities to reduce the heat load. After all the large cities
are hot spots, and have a big effect on the local weather. Forests are not
hot spots because they store the solar energy biologically during
photosynthesis, and they release moisture. So if we could have a
photosynthetic roof, or something similar the heat problem would go away and
it would produce fuel. Yes, I know that expensive solar cells do that.
There is also now a coating which allows the cells to collect the light
efficiently without having to sunflower them.

Light reflecting roads could also be helpful, but I doubt drivers would be
willing to put on heavy duty sun glasses just to drive to the store 2 blocks
away. But getting rid of black roads might be OK. Is there such a thing as
whitetop?

I think the current proposal for light reflecting roofs is not as draconian
as a totally white roof. As I read it they are proposing more reflective
roofing materials, not totally reflecting materials. This might be possible
with negligible change in price. I doubt that a change from the black color
currently popular to a much lighter color would make a significant change in
the heating bill for a northerly well insulated house. Actually the
perceived color is not a very good indication of the reflectance of light or
the ability to radiate IR. Remember that color is a matter of perception
and not just what you measure with a photometer. So just making the black
roof somewhat lighter may not be noticed by many homeowners. One could even
possibly make it look dark at an angle, but much lighter straight on.

So having a heat load/ungoing cost index may be a good idea so homeowners
can rationally decide what type of roof is appropriate.

John M. Clement
Houston, TX


The problem with any roof treatment is that it gathers thermal energy
where
you don't need it, when you need it the least. Then heat infiltration
is
reduced by attic insulation. A method of storing the energy and
releasing
it later could be of enormous benefit, but that is currently fairly
expensive.
John M. Clement
Houston, TX
Acquiring the thermal input to the roof is ridiculously easy.
On the south (or both) roof faces, stapling film to the underside of
the rafters
to duct hot air to the apex, and siting a perforated flex aluminum pipe
(like the duct for clothers driers, but bigger) at the peak, provides a
means for moving hot air.
That's the easy, cheap bit. Say $40?

But storing that low grade heat - that's a whole 'nother matter.
Here's one suggestion. An outhouse or basement or a subfloor area where
water bottles or cans or barrels could be arranged to receive the warm
air flow.
Where later, a duct with thermostatic fan could circulate warmed air
from this store into the house. Some people have used rock beds,
pebbles, sand, swim pools for storage......