re: Help find an old thread—Cooling the roof with sprinklers

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For it to work I would think you’d need water running on your roof all day. If you started in the afternoon your attic would already be too hot for the watering of the roof to make a noticeable difference.

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That would make it not work. It needs to be just enough to evaporate quickly, then when it evaporates, you do it again

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For it to work I would think you’d need water running on your roof all day.

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First, you really don't want "running water" for an efficient system. You want a system that keeps the entire roof surface covered with just enough water to be constantly evaporating to take advantage of the high latent heat of vaporization of water. In the perfect system no water would drip off the roof surface at all.

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If you started in the afternoon your attic would already be too hot for the watering of the roof to make a noticeable difference.

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This really makes little sense. To illustrate that mist down a section of extremely hot patio in the sun and check the temperature delta before and after, you will see a HUGE drop almost immediately.



Again, there are a huge number of variables and it will be more efficient for some homes than others. Just a single example is the better your attic insulation is the less effect it will have.

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I’ve seen mist systems that can help your AC run more efficiently. They operate around the unit.

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We just discussed this on the H&G board in the last couple of days.

The main issue with those are the dissolved minerals, particularly calcium, in the water and their impact on the condenser coils. They do absolutely reduce head pressure and in turn, reduce current draw.

ColdTurkey


Shouldn’t you be in the ER?

If putting water on your roof gives you a lot of relief then you need to add more insulation to your attic.

The heat in your attic shouldn't be able to reach the living space.

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Would imagine that modern insulation techniques would be better and cheaper than running water all day.

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Water is fairly cheap.

I would only do this in a heat wave though.

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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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shite like this is what makes me love the OT. How cool is that?

Those Taco Bell tile roofs seem to last

Why do you think that summer showers are called Nature's AC?

How do you think that cooling towers work?

When water evaporates it has a cooling effect.