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

["Phil Scott" <philscott@xxxxxxxxxxxxx>]

"Robert Bonomi" <bonomi@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:11bs1lfok861n40@xxxxxxxxxxxxxxxxxxxxx
> In article <42BDB7E5.456E1146@xxxxxxx>, Some Guy
<Some@xxxxxxx> wrote:
> >It's a mostly sunny day here (SW Ontario) and the mid-day
outdoor
> >conditions are about 87 degrees and 50 to 55% relative
humidity.
> >
> >I have new shingles (100% asphalt, relatively light in
color) on a
> >roof (pyramidal) with a 4:12 pitch covering an area that's
about 36' x
> >40'.  Large vent fan and 3 passive vents near the peak.
Soffits are 2
> >foot wide on all 4 sides (but ventaliation is only good
along 1 side
> >for the moment). Small fan is jury rigged in the attic
space to
> >provide forced air circulation of the attic space and is
powered with
> >the main roof fan.
> >
> >Temperature sensor is positioned 1/2 inch from the
underside of wood
> >deck - so I can read the air temp immediately under the
decking but
> >not the actual deck or exterior shingle temperature.
> >
> >Around 1 pm today the attic temperature read 120 degrees.
I rigged up
> >some garden sprinklers on the roof and adjusted the flow to
achieve
> >maybe 1 gallon per minute total flow.  Naturally, the water
exiting
> >the roof through the downspouts was warm to medium hot to
the touch
> >(didn't measure the temp).
> >
> >Within about 15 to 25 minutes the attic air temp was about
100
> >degrees, and it's almost 4 pm as I type this and the temp
is 98.4
> >degrees (outside temp is about 87).  I'm sure I don't have
total
> >coverage of roof with water spray.
> >
> >Since I'm currently re-working the soffits (adding
pot-lights,
> >speakers, cables for CCTV, etc) I'm going to be running
stuff into the
> >attic from the basement.  I'm thinking that maybe it might
be "cool"
> >to run a copper water line as well and permenantly mount a
couple of
> >sprinklers that would give proper coverage to the roof and
I could
> >turn on manually or automatically.
> >
> >Is there anything written up about residential roof cooling
with water
> >spray?
> >
> >Are the benefits (reduced interior cooling load and
increased shingle
> >longevity) outweight by (maybe) shingle dammage by water
spray if the
> >water is used on the hottest, sunniest days in the middle
of the
> >summer?
>
> Back in the days before ubiquitous A/C, and when 'skimpy'
insulation was
> commonplace, "watering the roof" was a _common_ method of
making things
> inside a bit more livable.
>
> The idea being to just wet down the roof, and have the water
_evaporate_
> to carry off the heat.   evaporating water pulls off
hundreds of times as
> much heat energy as does just warming the water up does.
>
> 'Evaporative cooling' is one of the most energy efficient
cooling methods
> known.  However, there are a whole bunch of limits as to how
much cooling
> you can get that way.  The higher the ambient 'relative
humidity', the
> less cooling you can produce.  The lower the temperature,
the less cooling
> you can produce.  And, of course, you have to have a means
to 'discard'
> the 'used up' air.
>
> As a practical matter, you "don't care" how hot the attic
air is _near_the_
> _roof_, if the temperature just above the insulation to the
inhabited space
> stays rational.  Decent 'convection' air-flow, possibly with
power-assist
> does a good job of _that_.
>
> Also, if you have 'good' insulation between the inhabited
space, and the
> attic, a 'spike down' in the attic temperatures will -not-
make a noticable
> difference in the inhabited space thermal load -- takes too
long for the
> change to 'penetrate' all that insulation.
>
>
> I'd try putting some temprature sensors at, or even a little
ways _into_
> the top of the insulation at the attic *floor*, and see how
bad the
> temperatures get there.  And then try to optimize the
air-flow to get
> those readings close to 'in the shade' outdoor temperatures.

   Thats good advice

Phil Scott
>