I hypothesize that keeping curtains wet continuously will help in cooling a room through rapid evaporation with an indoor ceiling fan set at full speed. So how to accomplish this, and will it really keep the room cool for the following parameters:

  • The room is closed to prevent hot air from outside coming inside the room during day time. The door of the room is open facing another bigger room which has only one small window open for the same reasons. The bigger room is generally cooler (we don't know why) and has one small window open which supplies fresh outside air passing through green plants. And the bigger room also has a full speed ceiling fan.

  • The area is a humid tropical region - coastal India

  • The ceiling fan is at full speed

  • The size of the room is 15ft * 15ft (4.6m * 4.6m), the height is 16ft (4.9m)

  • The dark curtain to make wet is on one wall having a 10ft * 5ft (3m * 1.5m) size

Based on the above I need to know:

  1. Will a wet curtain really help in reducing the temperature of the room or keep us cool during daytime?

  2. If yes (I assume yes after some research on this) then how to maintain wet curtains without manually keeping the curtain wet? I envision keeping it wet using an irrigation drip system, but are there any simpler mechanisms?

  • Coolers you can buy commercially in India, use a similar fundamental of Evaporating Water through Mesh (made of fibrous material; grass or non-natural) and use a Fan to direct Air flow from in-Vents to the Out blower.
    – Alex S
    Jun 2, 2016 at 4:17

5 Answers 5


I think it could work with a minor tweak. If you stick the curtain to the outside of the window and keep it wet (including the surface of the window), it will cool the glass from the outside, which in turn will cool the air on the inside.

You will probably end up with condense on the inside (which would prove that it works) but compared to the original idea, that would be a minor problem. In fact it will lower the humidity inside making it more comfortable. In some countries it is quite common to have a small drainage under the window to dispose condense.

Here's a small example section drawing:

enter image description here

If you use a white cloth, it could keep the window more or less transparent (especially while wet) providing light while also partially reflecting some heating sunlight.

A dark curtain would not be a good idea. It may accelerate the rate of evaporation, but only because it becomes hotter, so that defeats the cause.

I guess some experimenting is worthwhile to find the ideal thickness and material for the cloth. If this proves to work out well, it may be interesting to develop an adaptive irrigation system specifically for this cause.

Update: Experiment results

I finally did some experiments last summer (2nd of August, 2018). The humidity of the air on that day was 65%

My setup was with a table, having two shallow drawers under two glass plates, I had a digital thermometer for each. (this was taken in the morning while the table was still standing in the shade of a neighbors tree).

Setup with thermometers

The thermometers would differ between 0.1 and 0.2 degrees Celsius when having the sensors in the same place.

Comparing themometers

I moved the sensors to approximately the middle of each drawer under the glass. The top thermometer display is in the drawer on the right.

Censor placement

Then I added the sheet to the left (I chose the left because it would be the first to get sunlight when the tree-shadow moved far enough. I also had given the left drawer the thermometer that gave the highest temperature. Not wanting to Bias the results in favor of what I would like to see.

Sheet stuck with Duct-tape

The start was a little unfair off course:

Sun starting to hit

I kept the sheet wet by spraying it every now and again.

Wet sheet

And the results were quite startling!

Hot temperatures!

63.8 degrees Celsius! I never would expect it to get that hot! Any way, compared to 48.4, the difference is about 24%. Now I guess my experiment is not like a window, it's almost flat, lightly slanted in the direction of the sun, so in real life with a vertical window I guess the difference would be a little less.

Later I did the same experiment with a dry sheet. The results were:

With a plain dry white sheet

The difference, 49.4 vs 61.1 is about 19%, so the water is only contributing about 5% compared to the dry cloth. Note that the air humidity was 65%, so this 5% may be more if you live in a dryer climate, but less if the humidity is higher.

Sorry for taking so long to do the experiment and then the delay to report the results... better late than never I guess.

  • 1
    As compared to cotton cloth, jute will be better material which holds water for a longer time and not allow algae
    – user2232
    May 9, 2015 at 6:57
  • good, is this already been used somewhere? from where you got this illustration? May 28, 2015 at 7:30
  • I got the idea when reading the title of your question. As far as I know it has not been tried. I drew the illustration myself. I might try something like it out myself this summer on a sloping roof (not a window). I have seen cheap drip-irrigation systems, and I guess some jute bags will keep the water in place long enough to evaporate before dripping into the gutter. I don't think the cloth will hold long though due to mold or decomposition. It may bring relief in the hottest months. May 28, 2015 at 19:06
  • @LouisSomers Do let us know how it went!
    – Earthliŋ
    Jun 2, 2015 at 17:57
  • 1
    @Earthliŋ I finally did. But I used a white sheet instead of jute, and kept it wet by spraying it by hand every 20 minutes or so. Feb 10, 2019 at 1:56

With the given parameters you originally gave, the idea won't work.

To provide continued cooling, the water blown onto the curtains needs to be able to evaporate water. Without a substantial ongoing supply of fresh air that is below its maximum humidity (for its temperature), the humidity will quickly reach an equilibrium point and then will no longer cool at all.

If you were to use plenty of fresh air (that isn't already at its maximum humidity) blown over a wet curtain, then you would obtain a supply of cooled air using evaporation. A heat exchange or heat pump system could still make use of that, but unfortunately both those systems tend to be either inefficient or expensive.

Fresh air feels cooler to our skin when it is able to evaporate some sweat, so simply having a flow of fresh air that goes past the inhabitants may be the simplest solution.

Now that you've updated your question, I think it's worth pointing out the following:

  • Hot air rises, so if the larger room has a higher ceiling, then that could explain the difference. Otherwise, is there a difference in the solar load through windows, or even on the roof (a roof angled to face the sun will be hotter than one that isn't).
  • If you use a fabric curtain you are likely to have mould/algae issues. A longer lasting solution is probably to use something that is allowed to grow algae but is able to be cleaned, such as stainless steel or plastic. Water doesn't flow as well over those surfaces, but I suspect your ability to evaporate water is going to be heavily limited anyway by your supply of fresh air rather than the surface area of the water.
  • Ceiling fans may bring hotter air down and make the temperature feel higher than if you used ground level pedestal fans instead.
  • The volume of air leaving the building is equal to the volume entering the building. This means you could vent the hotter air from the ceiling, and allow fresh air (where ever it is drawn in) to flow past a wet object, cooling it on the way into the building. This technique, combined with fans directed onto the inhabitants, would be your optimum solution in my opinion.
  • but there is ventilation which means humidity can't reach that equlibrium? Mar 25, 2015 at 10:36
  • If the air outside is hot, and you keep it out like you describe, then you no longer have ventilation. This might just apply for part of the day. Mar 25, 2015 at 10:37
  • never thought of that!, but the room is facing another bigger room which has some supply of outside air and we assume that the bigger room cools the air which supplies it to this smaller room Mar 25, 2015 at 10:40
  • I have made my question more specific , after realizing that there is actually not much ventilation. Mar 25, 2015 at 10:47
  • 2
    @sustainablecoolingsolutions I've updated my answer. You could also keep in mind the possibility of burying a pipe and drawing air through it to use the ground temperature to cool air on the way in. The deeper the better; it would need to be fan-forced though, probably. Leaving the fan running overnight would help cool the ground for use during the day. Mar 26, 2015 at 5:18

Evaporative coolers are common in dry hot climates. It's different from what you are proposing. It's a box on the roof that has coarse pads (commonly coir) with a small pump that keeps them wet. Air is sucked thorugh the pads, and exhausted into the house.

At the cost of more energy, you can use the now cooler air to cool the input air of a second unit. In principle with stages you can get the incoming air into the building down to the dew point.

In humid climates this doesn't work well. Moving the air through a wet pad doesn't cool it. Indeed, if the air is truly saturated it will warm up from mechanical losses.

If you climate gets cooler at night, then you may get some relief by using the cooler night air to cool a mass of water, then use that mass of water for cooling during the day.


Due to your humid climate, an evaporative cooler is not likely going to be very effective. A 100% efficient evaporative cooler will only cool down to the "wet bulb" temperature:


For direct evaporative cooling, the direct saturation efficiency, measures in what extent the temperature of the air leaving the direct evaporative cooler is close to the wet-bulb temperature of the entering air.


The same equation indicates why evaporative coolers are of limited use in highly humid environments: for example, a hot August day in Tokyo may be 30 °C (86 °F), 85% relative humidity, 1,005 hPa pressure. This gives dew point 27.2 °C (81.0 °F) and wet-bulb temperature 27.88 °C (82.18 °F). According to the formula above, at 85% efficiency air may be cooled only down to 28.2 °C (82.8 °F) which makes it quite impractical.

Using Surat, India as a representative coastal city, the current temperature is 26c/79f and the web bulb temperature is 24.5c/76f, so even if your evaporative cooler were 100% efficient, it would only drop the temperature by a degree or two, but it would also increase the humidity inside your home, making it less comfortable.

This heat index chart shows the relationship between relative humidity and perceived temperature:

enter image description here

  • so what is the solution? Apr 25, 2015 at 18:14
  • ALso assume Mumbai, surat is not much of coastal Apr 25, 2015 at 18:15
  • One obvious solution is a traditional air conditioner, not an evaporative cooler. You didn't include your location in the question, you said just "coastal city", so I picked a random city near the coast -- you can look up the weather in your own city to see how feasible it is for an evaporative cooler, but since you said it's a humid climate, it's unlikely to be very effective.
    – Johnny
    Apr 25, 2015 at 19:11

Sorry for the short answer, I can't comment yet due to reputation rules:

You're also going to have to deal with the heat generated by the ceiling fan. If it's electric, then it has a motor and generates heat. :/ It may actually be more heat than you can store in water through evaporation.

Have you tried putting the water on your roof? If the air is not as saturated as it can be, you may get some cooling from the evaporation there; at the very least, it will offset the heat from the sun.


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