Home Automation - Cooling attic (and home) by spraying water on roof (shingles) -

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Cooling attic (and home) by spraying water on roof (shingles) -

Some Guy

2005-06-25, 6:25 pm

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?

Robert Bonomi

2005-06-25, 11:25 pm

In article <42BDB7E5.456E1146@Guy.com>, Some Guy <Some@Guy.com> wrote:

quote:

>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.

Some Guy

2005-06-26, 4:25 am

Dll wrote:

quote:

> "Some Guy"
>
>
> Funny thing is that a coworker tried this. He said it made his
> shingles look really old quickly and didn't really do a whole
> lot other than that.
>
> Perhaps you are good (or bad) at math. You might start by looking
> at the accuracy of your thermometer. Only a few I know will
> discern accurately the 1.6 degree drop in temp you say, and
> they are expensive.

The thermometer I'm using is similar to this one:

http://tinyurl.com/d2u7x

It's an older version of the one on that page, except mine is
temperature only (so it shows simultaneous temps of both the sensor in
the unit and a remote sensor on the other end if a wire attached to
the unit). Mine also shows the temp with 1 decimal place resolution.
When the remote sensor is brought close to the unit and left to
equilibrate for 10 minutes both readings agree with each other to
within about 0.5 degree.

Whether or not it is off by a few degrees compared with a calibrated
thermometer is not really the point. If it shows a daytime attic temp
of 120 degrees and then later 74 degrees at night, and if other
sensors I have are telling me that it really is 74 degrees outside,
then I think I can rely on it.

quote:

> Did your living space get cooler? Probably not.

Hard to judge, given the fact that my thermostat is on the main floor
and is regulating the AC based on the temp it is seeing on the main
floor.

It seems obvious that an attic with an air temperature of 100 degrees
would present a much better thermal load on the air space of the house
below it vs 120 or 130 degrees in the attic.

quote:

> Would a $50 attic fan do a 50x better job?
> They do seem popular.

This is with an attic fan going.

quote:

> I suspect there is a reason the world isn't equipped with
> roof sprinklers.

Houses aren't equiped with a lot of things - today or 10 years ago or
30 years ago. My house was build in 1976 and (as I have discovered)
has 1/2 inch drywall panels directly behind the aluminum siding on the
second floor (no foam-board or styrofoam insulation panels).

But getting back to roof-mounted sprinklers, something like that will
never become part of standard residential construction because if it's
not done right it will more of a liability than an asset (freezing or
bursting pipes or condensation in attic, need for a good control
system, etc).

There is no doubt that water-cooled roofs in industrial/commercial
situations have been proven to be useful and cost effective (according
to some google searches I've done). I can't find anywhere where
they've tried it in a residential situation.

Some Guy

2005-06-26, 4:25 am

Probably another reason why you don't see this in residential
settings:

Patent 4175703: Spray cooling system for gable roof

http://www.freepatentsonline.com/pa...703/4175703.pdf

In the USA, you can patent almost anything, regardless if it is
similar to what is already in practice, or what is generally "known to
those skilled in the art".

In this case, the above patent (granted in 1979) at the time the use
of evaporative roof cooling was well known for industrial / commercial
flat roofs. How this guy got a patent for use on a gable roof is
beyond me.

The patent office is full of examples of things we don't see as
consumers because some bone-head thought he could get rich by
patenting something trivial and obvious. All he ends up doing is
wasting his money on a patent.

I would think that the patent has now expired.

Some Guy

2005-06-26, 6:25 pm

Dll wrote:

quote:

> The readout doesn't mean anything. I'd bet you're getting 2 C
> max accuracy.

I'm satisfied that when I bring that particular unit (with it's
internal and external transducer) along with several other (different)
units together in the same place, and see them display a temperature
within 1 degree (F) of each other, that there's little chance that
they're all out of whack by the same amount.

quote:

> Pretty scientific test you had going there...not!

My hypothesis was that spraying water on the roof would cause a
decrease in attic air temperature.

My temp display before applying the spray was showing 120+ degrees
(f). Within 1/2 hour of applying the spray the temp went to 100
degrees and leveled off to about 98 degrees after an hour.

This was scientific in that

- I had reasonable confidence in my measurement device
- I varied only 1 parameter during the study
- there were no other variables that could have affected the
results (like a change in outside temp, cloud cover, etc)

During the entire time the roof-mounted exhaust fan was running.

quote:

>
> Vent fans are tried and true energy savers.

I don't know what the temp would have been without the spray and
without the fan running, but I agree that a vent fan (combined with
proper soffit ventilation) is absolutely necessary to prolong shingle
life and reduce the heat load to the house.

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