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I've been researching options for cooling a south facing exterior wall on my house. One thing I've been thinking about is what if I just spray the wall with water and let it evaporate?

The wall is wood siding, 1.5 stories. I say 1.5 stories because the top story is a bonus room over a garage. On the bottom story is a 3 car garage (two garage doors).

The idea is not to use misters, because I'm not trying to cool the air, but to use something like drip micro sprinklers to wet the whole surface of my south facing wall (maybe mounted under the eaves pointing at an angle downwards). The sprayers would turn on for x seconds, then turn off to allow time for the water to evaporate and cool the house. This would repeat on some interval (maybe based on the temperature of the wall).

Apart from the cost of the water, do you see any issues with this plan? I've considered the mineral deposit problem, I'm hoping I can just put an extra calcium filter on there?

Update
After doing more research, apparently this is known as "Spray Cooling", and not "Evaporative Cooling", because what we are cooling is what we are spraying down, instead of cooling air through evaporation. I've found several published papers on the application of this in manufacturing, electronics, etc... Papers like this one, Spray Cooling Using Multiple Nozzles: Visualization and Wall Heat Transfer Measurements, have some interesting charts showing the cooling curves based on temperature.

Then I came across someone with a slightly similar idea to me, 1977 Patent US4175703A, "Spray cooling system for gable roof".

From the patent:

Periodically, the valve is operated whereupon water is forced through the pierced holes in the pipe and sprayed as a sheet of water in an overlapping pattern directly from the pipe onto the building roof. Thereupon solar heat evaporates the water upon the roof absorbing some 8500 BTUs of heat in the process for each gallon of water employed.

....

they have not met with any marketed degree of acceptance in the residential building market. Probably the foremost reason for lack of success here is attributed to the absence of system aesthetics.

:)

Update 2
I finally found a few DIY examples online. These are for roof cooling, which is what I'm starting to lean towards. I think that continually spraying down wood siding might not be the best option... https://www.builditsolar.com/Experimental/RoofCooling.htm.

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Evaporative cooling is less effective in places with high humidity. There are mathematic formulas for how much water will be needed to cool a specified surface area by some number of degrees and they are not simple. The wiki article on this has some good information. It would be good to get some idea how much water will be needed before heading down that road.

Before I went that direction I'd look into something that is a one time cost vs. a recurring cost:

  • Planting trees to create shade.
  • Awnings - there are a variety of types. Some have the ability to retract and deploy based on the time of day.
  • Reflective coatings on windows
  • Vertical trellis (linked as an example, not an endorsement)
  • Painting the wall white, or a very light color.
  • Build a porch with a roof

Or some combination of the above.

  • Thanks for the ideas. Some additional info: the entire area under the wall in a concrete driveway. So some of these won't work. Also, I live in a low humidity area, South Western Idaho – Peter May 9 '19 at 20:58
  • I updated the question, I found that the correct term for what I'm looking for is, "Spray Cooling". – Peter May 10 '19 at 14:11
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This is not a well-known practice around here, so there's not going to be a lot of battle-test advice to be given. Therefore, you should figure it out yourself. Here's a possible way to do that.

Get an infrared heat thermometer, non-contact, then measure the temp of the outside sunny wall and the inside wall. If there's little in/out difference, it's a good candidate for your method, or at least further testing. If so, proceed to hose down the wall with a spray hose, nothing fancy, until it's good and soaked and rinsed for a few mins with cold water; as cold as your water can reasonably get the wall. Read the outside wall temp again.

Now some math. Take the difference between the two outside readings and divide it by the difference between the first-round outside and inside measurements, (out1-out2) / (out1-in1). Basically; how much improvement versus how much heat leaks into the room. If that number is less than 2, it's probably not worth it. The higher that number is, the better your result will be.

I would image that if that number is something like 4 or 5, then you should see the inner wall temp drop shortly after even just the test spray.

  • Thanks @dandavis. I may need to wait for the summer heat to kick in before doing this test, right now the temperatures aren't really high enough. But come 3PM in Summer with 100+ degree temperatures, I see this is a great test to do. – Peter May 10 '19 at 19:39
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Evaporative cooling on a wood wall will lessen the useful life of the wood siding. You will also need to think about foundation damage when all that water drips down in a way the house wasn’t designed to accommodate.

If cooling the bonus room is your goal, evaporative cooling the wall won’t help much with that, because the air you’ve cooled is outside the room (whether you’re misting or dripping the water, evaporative cooling still works by cooling the air, not the surface.) Instead, focus on reducing solar gain through the windows and giving the hot air at the ceiling someplace to exit.

South-facing windows are prime candidates for well-designed awnings, and here is a calculator for designing one: https://susdesign.com/tools.php

  • Fig, I will openly admit I'm not an expert in this area. But your saying that if I spray down a hot wall with water, the evaporation of that water from the wall won't provide a cooling effect? The useful life of the wood siding is a real concern; in this location it's slab foundation with a the driveway providing a downward slant away from the house towards the street, also I planned to experiment and find the right amount of time to spray to minimize water waste onto the ground. – Peter May 10 '19 at 13:30
  • @Fig so, if we assume you are correct, that evaporative cooling does not cool the surface, then why do we sweat ? – Solar Mike May 10 '19 at 13:55
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Paint the wall white

My preferred paint supplier offers a paint with 91% albedo (reflectivity). It results in the roof (and below it) being significantly cooler.

You might think "My wall is already white-ish because basically all architectural coatings are pastels"... but no. Near-white colors don't have nearly the albedo of white. Even their other "white" colors like "cloud white" have an albedo in the mid 80's. It falls off fast.

So have the "reflectivity" conversation with your paint supplier. (they won't know the word "albedo"). You want their whitest paint, and if you pick their highest-reflectivity 3 white paints, there will be some difference between them. If your paint supplier can't talk about reflectivity, get a better paint supplier. This is not a new problem, and is a standard strategy in reducing A/C load.

While you're at it, paint the roof white. Same reason.

Evaporative cooling will work, and you don't even need to stop the flow periodically to allow evaporation - it will evaporate perfectly well mid-flow. In fact, I would make a point to not let it dry out, because if you do, you will get mineral buildup like crazy and very quickly. Ideally, use demineralized water, but that will require a solar still or collecting rainwater. Separate from minerals, you will have a problem with the water damaging everything, which is the reason not to do it.

Another thing that would work is a screen that places the wall in shade.

  • Thanks for the comments. Water damage is the primary concern I have. What are your thoughts on if, instead of painting the roof white, I Spray Cooled the roof? The patent I linked had several other related patents created later. This seemed to be a more accepted pattern, and the roof is better designed for this. Do you think this would help cool the entire house better anyways? – Peter May 10 '19 at 19:36
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Have you gotten insulation set up yet? I say this because the wall in question is a garage wall. Garages usually get no or minimal insulation because they are not intended for people to live in. However, you have a room above the garage, which (especially if it was added after the fact) makes this assumption invalid.

Don't forget about the garage door, either. There are both insulated doors, as well as retrofit kits to assist with this. If the garage gets hotter than outside during the summer, adding automatic ventilation could also help.

  • The room above the garage was built with the house, but it lives kind of in isolation. The room is wrapped in insulation (like the kind you have under your floors in the crawl space), but there is no insulation over the attic portions to the sides of the bonus room. The garage doors are not insulated either. These are good ideas, and relatively easy to do, better than trying to grow a tree in my driveway :) – Peter May 10 '19 at 19:42

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