Help find an old thread—Cooling the roof with sprinklers

I can state that I live in Texas where it's hot as Fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuck and I've never seen this.

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It doesn't work

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It kinda has to, laws of thermodynamics and all. The only issue is how efficient it is which can vary a lot depending on many variables.

Years ago there was a clothing manufacture in Eunice. Think it was named Jantzen. Big metal roof building and they had a water cooled roof. Not sure how much it helped. Would imagine that modern insulation techniques would be better and cheaper than running water all day.

I'd imagine it would work better on a metal roof than on asphalt shingles

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kinda has to, laws of thermodynamics and all. The only issue is how efficient it is which can vary a lot depending on many variables.

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I will rephrase, it does not work in a cost effective manner.

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I will rephrase, it does not work in a cost effective manner.

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Do you have data or just your gut? If it is just gut then I would point out it is a damn complex system with many variables that would directly impact the economics of the endeavor.

I remember it. I believe I gave my 2 cents on the subject.

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I still don’t understand how we have come up with something better for roofs.

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It would seem solar panels would be the best.

The sun hits the panels, not the roof, and the barrier between them could act as an insulator. The energy from them is langiappe.

From one of the replies on stack exchange

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Yes, water on the roof will help cool it. Cooling with liquid water running off from a sprinkler is not efficient, but evaporative cooling from a small amount of water (like a periodic sprinkle) is very efficient.

1 gallon of water consumes 8000 BTU as it evaporates. If you spread 1 gallon of water on the roof once an hour and it evaporates you've made the same cooling effect as adding an 8000 BTU air conditioning unit. (They are rated in BTU of heat moved per hour.) That's like a 0.66 ton AC unit dedicated to cooling your roof surface! (In HVAC, a 1 ton means moving enough heat to melt 1 ton of ice in 24 hours, which takes 12,000 BTU per hour. So 8000 BTU / 12000 BTU = 1 ton of cooling).

Add too much water (or too often) and it begins running off, at which point not all of your water is doing evaporative cooling. It's doing regular heat transfer via thermal mass, which has a much smaller impact. It takes 1 BTU to heat a pound of water by 1 degree. If you put 1 gallon (8.34 pounds) of 70-degree water sealed in plastic on a 160 degree roof, and the water heats up to 130 degrees, you've consumed 500 BTUs of heat from the roof (60 degrees x 8.34 pounds). If it's not sealed in plastic and evaporates, you consume an additional 8000 BTU as that gallon evaporates.

Once the evaporation cools the roof to, say, 100 degrees, an additional gallon of 70 degree water can only warm up to at most 100 degrees, a 30 degree increase, which would only consume 30 x 8.34 = 250 BTU.

Simply put, your cooling is very efficient if little or no water runs off. Add as much water as you can without any runoff to get maximum efficiency cooling. In industrial buildings, this kind of evaporative cooling doesn't just reduce the heat coming in from the roof - it can reverse the heat flow and make the roof a cooling element! Warm air inside a large plant will rise and will often be well above 100 degrees inside the building; an evaporation-cooled roof could bring roof surface temp down from 165 F to 90F degrees and will be carrying heat out of the building.

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Some engineering calculations for the industrial level: LINK /