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I have slightly inclined (50 cm over 6 meters) metal roof over my studio. The roof has bad thermal insluation -- 15 cm of mineral wool between sheet metal.

The location is Kiev, Ukraine. Ambient temperatures extremely rarely exceed 32 °C, and most of the activities in the studio occur after 6 p.m., when it's never over 30 °C. I lived through a year without AC, with only fans for air intake and circulation.
With ventilation turned off the hottest time inside is 7 to 8 p.m.. With ventilation on, and 20 people inside the temperature inside is 1-2 °C more then ambient air.

I want to cool the inside for a few degrees by pouring a water on the roof. My questions are:

  1. Can I hope for outside roof surface to get as low as a wet-bulb temperature? The graph gives me 25 °C at warmest hour of ambient 32 °C 55 % humidity.
  2. Which method of water distribution is more effective:
    a) Drip tube. Would be easiest to install. The drawback being the risk of water joining into a few streams instead of uniformly wetting the surface.
    b) Misters.
    c) Sprinklers.
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  • $\begingroup$ I am aware that I'll spend immense amount of water on it, but with a cubic meter of water heat of evaporation in 620 kW h , the operational cost of water is orders of magnitude less than comparable electricity to run AC. $\endgroup$
    – Gleb
    Jul 5, 2016 at 10:10
  • $\begingroup$ You will likely not reach the wet bulb temperature but may get close depending on how well you distribute the water. Wet bulb is measured by wetting a cloth and swinging it in the air. The resulting temperature of the wet cloth is the wet bulb temperature. It relies on reaching full saturation in the area of the measurement. Given that, your best bet is to increase the amount of evaporation and heat transfer. Mist will work the best. Drip tube will barely work unless the flow rate is so great to remove heat by conducting it away from the roof. $\endgroup$
    – Byron Wall
    Jul 8, 2016 at 14:41

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What is the general range of relative humidity? You won't get anywhere unless the air is reasonably dry.
Next, you want to maximize the evaporative area or most of the water will just roll off the roof. Essentially, you are looking at turning your roof into a radiator aka heat exchanger. This will involve bonding a lot of finstock or evaporative towers to the existing structure. And even if you do that, the existing insulation will hamper the flow of heat inside the room to the cooled roof layer.
All in all, you're probably going to be much better off with a swamp cooler set up with a fan just outside a window.

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  • $\begingroup$ Relative humidity is mentioned in the question, it's up to 55% at 32 °C, 65% at 28 °C. The roof area is 160 square meters, I don't think I need any fins to increase that surface furthermore. Essentially I want to compensate for solar radiation with evaporation, going below ambient temperature is a welcomed bonus. $\endgroup$
    – Gleb
    Jul 5, 2016 at 12:05
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    $\begingroup$ No, you did not state that that was the existing RH , just that that was some expected value for something on a non-accessible graph. To your comment about solar insolation, consider that a wet roof may actually absorb more light than a dry one, due to index-matching behavior. $\endgroup$
    – Carl Witthoft
    Jul 5, 2016 at 15:20
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One way to accomplish evaporative roof cooling is to install a green roof (also called bio roof). Basically moss, grass or other plants growing on the roof. The plants and soil will retain the water until it evaporates, and also increase the surface area available for evaporation. Usually this is done on flat roofs, but it might work for such slightly slanted roof also.

The cooling effect of green roofs is widely recognized, though I couldn't find any actual measurements on how close to wet bulb temperature you would get: https://www.epa.gov/heat-islands/using-green-roofs-reduce-heat-islands

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