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

[bonomi@xxxxxxxxxxxxxxxxxxxx (Robert Bonomi)]

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.