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

"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. In other words, the temp could well have gone up with the measurement error added in. Another thing to consider is the heat capacity of the water, and where that BTU capacity is best used - perhaps not cooling your roof. Did your living space get cooler? Probably not. Would a $50 attic fan do a 50x better job? They do seem popular. I suspect there is a reason the world isn't equipped with roof sprinklers.

- Nate

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Its done very rarely but is a good idea in hot climates... especially as we are going to time of use billing on residential shortly... with roof cooling you could keep from running the AC so much under peak conditions and save a nasty demand charge billing.

Using city water though leaves mineral deposits that embed into the roofing and are probably not real good for most roofs over time. thats why its not more common.

If enough water can be used so that the roof stays wet and runs off then mineral deposits are less but its still an issue...some areas a big issue... others not.

Poultry farms use 3000 psig water atomizer units (pumps as in pressure washers) to force water through tiny nozzels so it evaporates completely in the air...cooling the air and any mineral deposits come out as solid particles and land on the ground.

You could cool your attic that way and get good results.. those are fairly expensive systems though.

Or you could use an evaporative cooler to cool the attic. 1/6 hp fan motor... would save about 3/4 hp of cooling load. No mineral depost issues... but then you have a swamp cooler to maintain.

Phil Scott

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Thats good advice

Phil Scott

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

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.

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.

This is with an attic fan going.

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.

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

Patent 4175703: Spray cooling system for gable roof

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.

What is your reason for spraying water on your roof?

Besides the glaring fact of WASTING WATER? Do the math on your water bill (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

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.

BTW:

In searching for the roof spray patent I found this:

That may be prior art that precedes one of the latest patents from NASA

--- its for a golf ball that has aerodynamically more efficient ellipsoidal dimples. The wonder of socialism at work In America !!!

.........

The water spray idea could be a nice product business where there is plenty of water. There may be room for some new inventions there.

BTW, If you are working on an idea, write me, and I'll get you in touch with the right people. Do talk to your Patent Agent first or we'll help you to find one, and do not talk to others regarding your idea. Least of all on Usenet. No, you don't pay anyone to market your ideas - that's a scam. We help you to get your idea developed, drawn, patented, prototyped, and presentable. Sometimes we can help you to find capital for you for your development purposes, or more importantly, we are a self-help forum for technical ideas of all sorts.

Ralph Hertle Vice President and Trustee National Society of Inventors

"Some Guy"

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

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

Vent fans are tried and true energy savers.

- Nate

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.

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.

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.

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.

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.

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.

That may be true, but it's probably safe to assume that the temperature decrement which the gentleman read was not off by 2ºC. Even assuming the "before" and "after" temperature readings were off by a degree or two, the delta reading was probably accurate to within a small fraction of a degree.

That said, I'm not sure this is an optimal use of natural resources. Where I live (southern Florida and northern Brazil at various times during the year) water is more expensive than electricity. A good fan can bring the attic temperature to within 5ºC of the outside air temperature. Operational costs are minimal.

In Sarasota private well water cannot be used for this purpose for two reasons. First it's usually full of sulfur and iron deposits which would stain the roof horribly and stink like rotten eggs. Second, it's illegal. We have strict environmental laws here to keep anyone other than strip miners from wasting natural resources and destroying the ecology.

There's one method of using water to cool the roof/attic that can help. I use solar panels to heat my pool. The pool holds ~30,000 gallons. There are eight 4'x10' solar panels on the roof. Sunlight hitting them warms the pool -- not the attic. I've noticed a significant improvement in cool-down cycle (time it takes to get the house cool again after we've been away) with the pool heater running. Since the pool water is being circulated by the filtration pump during the day anyway, there's almost no *extra* cost to use the solar panels.

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Misting is a vastly under used technology... sounds like you have a great application there. Ive been trying to sell it to super markets with air cooled refrigeration for years... a tough sell..but it would save them a bundle and cut repair costs. There are commcl systems on the market for that application but they dont sell well.

They sell for chicken ranches and misting public areas the desert mostly.

Phil Scott

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The solar heating system should be a required part of any pool installation. It should be written into the code and rebates should be offered by the utility companies.

But that would make sense...

R

wind will

The latent heat of evaporation for water is somehing like

970 btu's per lb... one gallon of water is 8 lbs...so its about 7800 btu's of heat required or the equivalent amount of *sensible heat removed from the air to evaporate a gallon of water. Thats equivalent to about 3/4 of a ton of airconditioning (average home takes about 3 tons).

So you would have to evaporate about 4 gallons of water an hour to cool the average home in a very dry climate... say 10 hours a day.. thats 40 gallons a day,..or 1200 gallons a month. The average home uses about 500 gallons of water a day the bill is say 25 dollars for that 15,000 gallons a month. So the water used in 100% evap cooling sells for around 3 dollars a month. or lets say 10 dollars max.

Cooling the house with refrigerated HVAC will cost 200 to 300 dollars a month or more.

The cost of a gallon of city water is less than a cent...so its a very good deal cost wise.. and ecologically much better than running a one HP motor (approx HP required to deliver 3/4 ton of cooling if you count the fans). That power is generated by burning foscil fuels in most cases.

Evaporative cooling would be limitlessly popular if it did not add an equal amount of humidity in the form of 'steam' to the air it was cooling to *sensibly lower termperatures. There is sensible heat, measured with a thermometer, and Latent heat thats water vapor in the air..it takes about 100 times as much heat to create steam as it does to raise the same amount of water 1 degree F.

So latent heat is a big deal in humid climates... in dry climates you can use evap cooling to take advantage of the situation.

The human body feels both this humidity and the sensible temperature as one...so evaporative cooling only works in very dry climates where an acceptable rise in humidity fits well with human comfort requirements.

It will become a lot more popular in commercial buildings in the south west shortly..as fuel prices go up. Right now its straight refrigerated air.

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That would be what is called a 'ground water sourced heat pump system' those work very very well. And are popular. For some reason you dont see them piped to pool water very often. But its workable....to a limit. Depending on the size of your pool, in the summer the system would warm the pool...but you wouldnt want it to go over 85 degrees or so...then it would have to switch back to air cooled or a ground loop.

In the winter you would be cooling the pool while heating the house... the pool temp would then drop to 40F or so and start absorbing heat from the surrounding ground in most climates... and if the pools was large enough and had a pool cover .. it would make a good heat source for winter heating...but of course then too cold to swim in..

so for that reason pipes burried in the ground are common. The engineering and install can screwed up easily and often is..then its a mess. If done right its a very good deal.

On the attic temps though the poster who mentioned temp at the top of the insulation had the best response.. myself in your case I would not wet the roof. I would ventilate the attic with a fan.

Phil Scott Mech Engr HVAC contractor since 1829.

Doesn't a mister waste water by atomizing it and then the wind will blow it away?

I would think the ideal way to cool something with water would be to completely wet the surface of something you want to cool with a low volume of flowing water. That would avoid staining from minerals in the water. The heat you're taking away is the difference between the inflow and outflow temp multiplied by the volume flow rate. Some evaporation will also happen.

I know that water can cool best if (all of) it evaporates, but that will leave mineral stains, and also it probably won't get you down to the low temps you can reach by continuous flowing water.

My tap water comes from lake Huron and is probably around 50 degrees (f). I pay 3.86 cents (CDN) per cubic foot (3.165 USD). I think that's about 0.516 cents (CDN) per US gallon (or 0.423 cents USD).

I pay 5 cents (CDN) per kWh for the first 750 kWh (per month) and 5.8 cents per kWh after that. That's about 4.1 and 4.8 cents USD respectively.

That's 5.6 cents (CDN) per day, or $3.74 (CDN) for 67 days ($3.06 USD)

Deposits, yes - but perhaps negligable if the water is not allowed to evaporate. Mold/fungus - I'm thinking no because this would only be used on mostly sunny days for maybe 1/2 hour to maybe 2 hours of the day. When the water is turned off, the roof will dry up and I wouldn't think that would give any mold or fungus any ability to grow given that the shingle temp would probably climb back to well over 100 degrees. A thin layer of flowing water on the roof is equivalent to a gentle spring or fall rain shower and nowhere near the weight of the snow loads we see during the winter.

PS: Given gasoline costs at $2 to $3 per gallon (or $1 CDN per liter) what are the costs to generate electricity (on a kWh basis) using a gasoline powered generator?

PPS: Are there gasoline or propane-powered AC units, and are they more economical to run vs electric?

PPS: What are the pro's and con's of immersing your outside AC condensor coil in your swimming pool (and therefor doing away with the cooling fan) ??? Heat your pool and remove heat from the coils much more efficiently?

I still don't see an effective way to arrange a residential cooling system based on the evaporation of water.

I don't think you can concentrate and deliver a substantial amount of heat to a surface you can cool via evaporation. Evaporative cooling would be effective if the heat inside a house could be brought to an out-door device who's surface temperature is well above boiling and the application of water would result in evaporative cooling.

So what you're saying is to mistify the air circulating in a house in order to cool it. I don't think that would work once you've saturated the air (ie humidity > 75%) not to mention the effects of saturated air on items in the house, the wood, the machinery, etc.

I think only the roof makes a workable item to cool with a water cooling. It's designed to get wet, to collect runoff, and gets very hot in the summer, and cooling it can lead to reduction in cooling requirements of the house.

The only thing that makes more sense is a temporary tarp or canopy to cover the roof during the summer (to be taken down during nasty weather, storms, etc).