Re: Cooling attic (and home) by spraying water on roof (shingles) - good/bad?

[Jim Baber <jim@xxxxxxxxx>]

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Jim Baber's comments:

wkearney99 wrote:

>>Is there anything written up about residential roof cooling with water
>>spray?
>>
>>
>
>Besides the glaring fact of WASTING WATER?
>
Bill has a point here, but if you were to use misters (like I do between
my solar panels and my roof), you might see the same kind of drop in
attic temp that I have had.  My roof that has my solar system is a 6:12
pitch facing due south.  I have noticed a 10 degree reduction in the
attic temperature just due to the shading by my solar panels, down from
160 to 150 deg. on a 100 degree day.

I installed those commercial cooling misters for patios and yards,
behind my solar panels to cool the panels themselves, figuring that I
did NOT want mineral deposits on the front.  I was not to worried about
roof deposits, because I intended to only run the misters on days over
90 when I had measured a drop in power generated by the panels because
of temperature degradation above 95 degrees.

This supposedly only uses about 2 gal. an hour per the Mfg. claims, and
has not deposited significant or noticeable minerals in 2 years of
operation.  The roof does get damp sometimes but not usually, since
there is considerable convection air flow upwards behind the panels that
evaporates most of the mist before it wets the roof.  I do use a
demineralizing cartridge in the supply line, and they are available
where I got the misters.

When I turned on the misters behind the solar panels the temperature
dropped an additional 30 degrees in the attic to 120 degrees.  This
isn't cool by anyone's idea, but I noticed that the A/C has reduced the
number of minutes the it runs each hour by about 15% on those 100 degree
days we have too many of here.

By the way the misters also accomplished what I wanted in the first
place, I got back the 0.83 kW of peak solar production I had lost
because of the solar cell heating. (that's about $1.62 for any day over
90)   All in all I found it to be a very worthwhile effort since we had
67 days over 90 last year for an additional estimated $108.00 worth of
power we produced.  I have not estimated how much the reduction in A/C
time was worth but I could from the cost of running it vs not running it.

>Do the math on your water bill ............
>
>
At 10.8 gal. a day for 67 days that's not much water, but I can't put a
value on it since we are not metered at all.  You could put a value on
it at your cost, but I can't even make a guess as to that.

>........ (or the electric to pump your own well water) and you'll probably find it's
>a helluva lot cheaper to just leave the roof alone.  If anything, put in an
>attic vent fan.  Water's not as harmless a material as one might imagine.
>The various minerals causing deposits, the likelihood of algae other
>mold/fungus growth and even it's weight are all factors to consider.  Enough
>that unless the roof was designed with in mind would make it not only a bad
>idea but a potentially dangerous one at that.
>
>-Bill Kearney
>
>
I can't see how it would be dangerous, in fact in So. Calif. some people
do this for protection from brush fires,  and I would do it here if I
still had a shake shingle roof.

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