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I live in tropical region. Have pretty hot summers (44 degree C). The question is regarding houses built with brick and mortar (walls) and reinforced concrete flat roofs. In this forum I have come across other discussions on solutions for reducing heat absorption for roofs. But the problem is not complete just with the roof. Many of the solutions suggested for roofs can't be applied to vertical walls.

In short my question is "how to reduce heat absorption by walls of the house?" Goal is to cool down interior of the home and there by reducing AC utilisation and improve sustainability.

Assume: Techniques for roof cooling has been done (white roofing), tinted glass and light colour curtains used.

Challenges: Walls are many times painted non heat friendly colours, can't make shades on walls for high raises (plants, truss etc).

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  • RC made flat roofs? Please explain in your post. – user2451 Apr 18 '17 at 11:18
  • I assume changing wall opening sizes is out of the question? – user2451 Apr 18 '17 at 11:20
  • "RC made flat roofs? Please explain in your post.": Concrete roof (RC = reinforced concrete, and they are flat or horizontal roofs, not the inclined roofs that one see in places with a lot of rains. Hope it clarifies) – user4462 Apr 18 '17 at 11:29
  • "I assume changing wall opening sizes is out of the question? ": It is defenitely an option (and obvious one). However, normally large buildings (residential towers) go through a re-painting cycle may be once in 10 years. – user4462 Apr 18 '17 at 11:30
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A solution is to grow vegetation along your wall. Here in Europe we see some mansions with huge vines or ivy.

It can be beautiful. Not sure what kind of vines you can grow in your place, but it's worth saying, ain't it?

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  • I have seen such buildings with vegetation on walls. The building in question is a tall one and may need some creepers to do that. – user4462 Apr 24 '17 at 14:24
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Use some thermodynamics

Using the basic thermodynamics principle of black-bodies, your goal should be to change the emissivity of the surfaces of your house. Each surface has a measure of its ability to both absorb and emit EM radiation (at the temperatures we live at, this radiation is primarily in the Infra-red range).

This heat transfer is radiative, not convective or conductive and it is important to note how it works. While conductive and convection heat transfer always go from hot to cold, a cooler wall can and will emit radiative heat into warmer outdoor air.

Your house will be receiving radiative heat transfer from the sun. This will be much more significant than any heat your house could shed. Therefore, on sun facing sides, you want your house to have a very low emissivity, for example white paint or bare aluminum. This will reduce the amount that the walls and roof of your house heat up in the sun.

However, some sides of your house may never receive direct sunlight. On my house, I have a north-facing wall that never receives direct sunlight. This wall will be heated primarily by convection from the surrounding air. Convection is not affected by emissivity, but the radiation that it loses as IR is. Therefore, on this non-sun-facing wall, I would want a very high emissivity (black paint or glass).

Heat transfer on this wall could (depending on the temperature delta between inside your house and outside) be a net flow out of your house. The same principle could be extended to any other surfaces of your house that do not ever see sunlight. If you have an awning or an overhanging porch, you could install windows in that area to radiate heat away from your house while not absorbing heat into your house.

Incidentally, this principle is more usually used for the opposite purpose, keeping your cold-weather house from leaking too much radiative heat. Window panes, for example, are huge heat emitters, as you can see by looking at anyone's house with an IR camera in the winter. But this very thing you want to avoid in Chicago in the winter can be used to your advantage in Miami in the summer.

Unfortunately, this princple is probably more useful to consider when designing a new structure, but there may be some ways to implement it on the house you have. For example, painting the non-sun-facing side of your house a darker color, or covering sunward-facing windows with a white awning. You'd have to look at the specifics of your house to determine what you can do.

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  • a cooler wall can and will emit radiative heat into warmer outdoor air so energy gets transferred from the cooler wall into the warmer air? So the wall gets even cooler? Under what circumstances does this happen? – stijn Apr 20 '17 at 8:05
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    @stijn At the same time as net radiative heat transfer is going out of the wall into the environment, net convective heat transfer is going from the air back into the wall (assuming the air is hotter than the wall). At the temperatures of a normal day, the convective heat transfer would be expected to exceed the radiative, so net heat is still going into the house. However, you can't do much to stop the convective heat transfer (laws of entropy and all), but you can take action to increase radiative transfer, thus slowing net heat transfer into the house, thus reducing heat absorption. – kingledion Apr 20 '17 at 11:42
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Stainless-steel is good for reflecting heat. And stainless-steel can be used both as a roof covering and as an exterior wall siding.

Furthermore, 430 stainless-steel is good-enough for roofing and siding but costs about 40% less than 304 stainless-steel. In fact stainless-steel sheet-metal is bought on coils and by the pound or else it is fabricated and sold expensively by the foot.

If the homeowner should make their own system for attaching stainless-steel sheet-metal lengths cut off a coil, there are several details that need to be known. The most important detail is that slotted attachment holes must be cut to allow for thermal expansion. Also, any nail, screw, or flashing that touches the stainless-steel must also be stainless-steel. In fact, 430 can rust where touched by a cut-off disk but will not rust where touched by a grinding wheel. Otherwise, stainless-steel sheet-metal can be used in standard standing-seam systems.

Now, aluminum is not good at reflecting heat and copper is not good at reflecting heat.

Well, metal transfers heat but metal also reflects heat and the overall effect is that the house is cooler under the metal. There are special painted-steel roofings that reflect more heat than stainless-steel but paint is temporary while stainless-steel is permanent.

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