Two observations:

Drip irrigation is highly effective in the middle east, but that’s because the water can be absorbed into the ground (in a focused way) and used. Virtually no water evaporates.

Your theory assumes almost 100% evaporation.

Would the roof type play a role? Tar/composite shingles, especially dark colored ones, absorb heat and would speed up evaporation. Would the cooling effect of the evaporation offset the heat effect of the shingles? Same with an aluminum/metal roof? It seems you would be reflecting away any possible cooling effect.

The only type of shingle where this might make sense would be wood/shake shingles or clay/tile roofs where the dripping water has a fighting chance to cool something before turning into humid air.

Are there any lesses to be learned from evaporative coolers?

I have a black, tar composite roof. On a sunny, hot day i can spray some water on the roof near the peak and it all evaporates before hitting the rain gutter. You can easily see the steam (evaporation) rising. If I broadly spray for about 10 seconds (about 2 gallons) none of it reaches the rain gutter. It is all gone.
That is what prompted me to do some calculations.
(The sun facing side of my roof is about 150 sq meters, about 1500 sq feet.)

Some thoughts:

Some areas have water shortages, and so this idea might not go over well. Would you be using reclaimed water?

I think every house should catch some of the water coming down off the roof during a rain, and reuse it for gardens and lawns, maybe you could use that water for the roof? Some sort of recycle system. I wonder how expensive it would be to have a holding tank, and a pump that gave enough water pressure to water the roof?

Parts of the country with warmer climates can easily build flatter rooftops, and growing gardens on the rooftop might be more effective, and more productive. I don’t know if flat roofs are more expensive?

Lots to consider. I would wonder about the space under the roof. Is it attic space or your living quarters? Arid locations where evaporation would work best are coincidentally where water is likely to be at a premium; though 2.7 gallons hourly during daylight hours seems more than cheap. For that matter, I wonder if the economy of shading the roof through the installation of solar panels might be the better deal over time. The solar power might be employed to drive an air conditioning compressor, though I have no idea if those panels have attained efficiencies as of yet where the square footage of your roof would sufficiently power an ac system. the

I like these ideas!
Certainly if the attic incorporated living space this would be really helpful. My house (ranch) has 12” of R-31 fiberglass batting on top of 6” fiberglass with vapor barrier on the floor to insulate the living area ceiling from the cold, or heat. The attic, which is used for storage, does get very hot in the summer when the sun shines. I have ridge and soffit vents but it is still hot in there.

@stanleybmanly Ideally solar panels could be used for shading and powering the A/C unit. unfortunately that would be an expensive proposition and would require installation and maintenance costs.
A dribble hose is cheap and easy to do. My garden hose has enough pressure to reach my rooftop so this would be easy to dry. It is a small mount of water indeed – about the same as one or 2 toilet flushes. But, it saves so much energy!

@JLeslie I like the idea of using reclaimed water or roof runoff. I can easily dedicate one of my rain gutters for water storage and use a small fish pump to lift the water to the ridge vent. That is more complicated.that simply putting a weep hose with a properly sized flow restrictions.

I’d really like it if someone could independently do the math. I can tell how I did it. but I don’t want to influence the answer.

I’ve never known anyone who has used the system of which you’re speaking. It sounds a lot like the swamp cooler used in the more desert areas where it is really dry. I looked up Swamp Coolers here & while speaking of using a swamp cooler unit in the home they commented…Because of the different ways they work, you can’t run a swamp cooler and a standard air conditioner in the same house. They would cancel each other out, just like running a dehumidifier and a humidifier in the same room. At the same time, you won’t be running 2 different types of units so it might not make a difference to you.

Logically speaking, it kind of makes sense to me. I’m the type that would try it to see IF it actually would do what I was expecting it to do. Then IF it didn’t, I’d take it back down & feel I was out of nothing but my time & I learned a new lesson. I provided the link because you have a better idea of what you’re thinking than I do & you might get some ideas from the info provided. It’s only 5 short pages.

When I was growing up, Gramps didn’t spray the roof the his house, but he also didn’t have an a/c either. We had fans strategically placed around the rooms where we congregated.

@LadyMarissa I read that article and the idea of a swamp cooler is similar to what I’m
suggesting – except I’m using the roof as the evaporation pad and natural wind as the fan.

I’m an engineer (and lazy) so I need to make sure I’m not violating any rule of physics before I get up on the roof and run some hoses. So far it seems the physics is right.
I calculated how much temperature rise I could expect to see in the delivery hose and it is insignificant when compared with the evaporative loss once it is sprayed on the roof.
I think this will work.

I’m wondering if this would not work in humid areas like the southeastern US due to saturation of moisture in the air? I’m certainly not an engineer or a physicist, but my gut tells me that the efficiency of this system would be inversely proportional to the humidity in the air. Also the fact that it’s not being implemented everywhere leads me to believe there is likely something you’ve overlooked that makes this impractical or impossible.

I would think heavy insulation in the floor of the attic and a couple heavy duty attic fans would work just as well. It would be pretty easy to string up a soaker hose and check with an infrared thermometer though. I’m lazy too so I usually try it rather than calculate it and then try it. You could try it on the ground just by laying a few shingles on a trash can or something and use a meat thermometer.
If you have solar panels on your roof I know for a fact that cooling them like this works and makes a dramatic difference in their output.

@gorillapaws I figure a dry climate is better but even on a humid day the incredibly high temperature from the sun will still cause the water to evaporate. On humid days there might be a limit to how much water I can dribble before the slow evaoration rate allows the water to drip into the rain gutter.

@ARE_you_kidding_me I will try it some time. There aren’t too many hot days left before winter sets in here. I have plenty of instrumentation to check it. Thermal Imager, T-couples, instant read thermo-pens, Non contact IR…
I’ll measure the temperature in my attic then spray a few gallons on the roof. Then I’ll check it again. It is just so itchy and hot up there. ;-)

It wouldn’t work very well because you would be raising the humidity which would make you feel warmer, and you’d have to turn up the AC so as to dry the air more for it to feel cooler.

@kritiper Remember, I am doing this outside on the roof. That steamy water should stay outdoors.

The insulation in the attic would be the barrier for heat transfer. The swamp cooler use the air being blown across the canvas to carry the coolness.

@LuckyGuy One of the basic principals of air conditioning is that the air is being dried so that the sweat on your skin will evaporate, thus cooling you. If you can keep the steamy water and moisture outside it just might work. No harm in trying…

I wonder if the temperature difference between the roof at noon and a well or reservoir say 20 feet below the house would drive a stirling engine with enough force to do practical work.

The thing I wasn’t factoring in was if you run the pump then it’s costing you electricity to do that, unless you have solar power to run the pump, then you have the initial cost of the solar panel. Sunny days would be the hottest time, so a solar panel pump might make sense.

Let’s just do a back of the envelope calc.
A garden hose with a timer takes virtually no energy to run. A small, high pressure pump in that flow range would take about 5–10 watts to run. A 2 ton A/C unit takes 4000 watts.

If the dribble system reduces the temperature of my attic from 160F down to 120F that would reduce the temperature difference between my ceiling and the attic from 90F to 50F – almost half.
My A/C unit load would be reduced. Even if the system reduced the A/C load by only 10% that would still be an energy saving.

I need data.

Interesting. Do you even need to worry about water conservation there? For 2 months straight my city was running the reclaimed water irrigation all day long all over the city to rid the retention ponds of water, we had so much rain for months we had to get rid of the water. Now, our rainfall has gone down significantly and the ponds we at more reasonable level that if we got a hurricane or a week of rain we shouldn’t flood. If it stays dry we will have to worry about conserving, but that’s not likely, but still possible.

On a side note, I wonder how much it affects our weather to be running the irrigation all day long like that? I’m sure it’s minuscule, but it does put more moisture in the air probably.

No, we don’t have to worry about water conservation. We have our own well and we have town water. The price for Town water is scandalously inexpensive: $2.54 for 1000 gallons!
Also ground water is close to the surface. That is why my unlined pond holds water.
Most of the homes in this area have, and need, sump pumps. My sump pump crock empties about 50–100 gallons per day in the Spring!

The system would dribble only a few gallons per day – during the peak solar hours. Figure about 10 gallons per day. But, it would save a lot of electricity and the CO2 associated with its production.

Get one of those misting hose things (or a sprinkler or soaker hose) and set it up on the roof. Pretty inexpensive and low water usage. Conduct your own experiment. Just watch the energy use on your power meter, or let the misting hose run for a month, whatever, to compute the advantage,

Due to dissolved minerals in your tap water, you might end up with a thick layer of minerals on your roof at the end of summer. And you think cooling your roof will cool your house that much? The attic and ceiling insulation might attenuate the cooling effect of the water on your roof.

@LuckyGuy You’re the engineer…I’m not…so I can’t see things in the same way that you see them. In looking further, I found a company that put up roof top sprinkler systems. Then I had a power failure & lost the link. Maybe do a search of “roof top sprinkler systems” & you might find something that enlightens your thought process. They had a lot of drawings of how the sprinkler system worked. Those told me nothing but might make sense to you or at least give you some design ideas that will reinforce what you are thinking!!!.

@RocketGuy Good point about the minerals. Rain might wash it away. It’s worth a try.
I don’t expect to only cool my home with this. I figure it would just bring down the attic temperature from 160 F to something like 120 F and that would reduce the delta between my attic and living space ceiling from 90 F to 50F. That might translate to reduced .A/C usage.

I looked at the heat capacity and heat of vaporization of water and calculated that, theoretically, a flow of only 2.7 gallons per hour is equivalent to a 24000 BTU /hr of Air conditioning.

(Other climates that Actually where we live A/C is not really necessary. We cool our entire house with one 6000 BTU/hr window unit and a circulation fan. I’m thinking of this for other climates with higher temperatures and more sunlight.)

@LadyMarissa I will check out Roof top sprinkler systems. Thanks.

@LuckyGuy I have two 1200 CFM roof fans for an attic area of less than 1500 square feet. I’m in North Carolina, We dropped the in-house ceiling temp by 25* F.
Two fans so that when one side of the house and roof get to 105* F that fan will kick in.

@Tropical_Willie That’s basically what I have too. Before that candles would melt in my attic.

My electrician thought it was overkill having two but installed them and was happy I didn’t try to get both to come on at the same time. They both run and reduce temps in the attic, some days most of the time on a hot day in summer.

@Tropical_Willie @Tropical_Willie That is a good data point. So cooling your attic does reduce your ceiling temp – and presumably your A/C costs.
Do you know the wattage of your fans? (I can find a similar one if you don’t know.)
I found it 0.4 hp , 3.4 Amp, so figure 350 Watts each.
Hopefully you save at least that much by having to run your A/C less often.

I had an attic fan in TN.

I had a radiant barrier in FL. It really worked, but it was pricey to install. Best installed while building. I don’t think they use them in northern states.

@LuckyGuy I think it was less than 3 Amps.

We live in N Cal, where summer temps cool down at night. We have a whole house fan that we turn on when it cools down. It sucks warm air out of the house and reduces AC use. It pushes hot attic air out when we do that.