I want to estimate the outcome before going into hassle of installing the sprinklers/mist emitters.

I have a place on the last floor, you can call it an attic, directly under a flat roof which is metal, insulated underneath with 15 cm of glass wool. 160 square meters of roof that is.

Most of the year temperatures in Kiev are below 28 °C, and I have decent ventilation, so the temperature inside is not more than 1 degree hotter than outside with full occupancy of 50 people.
But those few hours in few days when the temperature rises above 30 °C is a problem for me.

Online calculators tell me that the heat is coming in mostly from sun radiation through windows and roof, and I need 19kW of cooling power to compensate for it.

I want to avoid installing air conditioning both because of up-front cost of more than 1000$, and operating cost of electricity to tun it.

The numbers below are in hryvnas, exchange rate is roughly 20 hryvnas per 1 US dollar.
Ballpark calculation is very promising: with a cost of water supply at around 10 ₴/cubic meters, [evaporation heat cost of water would be 0.02 ₴/kWh](http://www.wolframalpha.com/input/?i=%2810%20%E2%82%B4%20per%201000%20kilogram%29%2F%280.6194%20kilowatt%20hours%20per%20kilogram%20%29), which is 2 orders of magnitude less than cost of electricity which is around 2 ₴/kWh.
Estimated water usage would be 100 liters/hour, which is reasonable.

The question is -- how much of that cooling power would work for me cooling my space through 15 cm of glass wool between my ceiling and roof surface. That is versus cooling ambient air just above the roof.
 And how much water can I expect to evaporate on 160 sq.meters of roof at 30 °C, humidity rarely rises above 40% at such temperatures in Kiev.

Good answers to related questions:  

[Is a roof sprinker a good idea?](https://diy.stackexchange.com/a/28394/38229)  

[Would putting a sprinkler on my roof help cool my home?](https://diy.stackexchange.com/a/6745/38229)