The room is at south-west on the second floor, so a lot of sun exposure. The temperature got unbearable up to 36.5°C (97.7°F) yesterday. The outside was 37.5°C (99.5°F). I live in a hot and humid country. There are 3 windows in the room. I've already blocked all the windows with cardboard, which helped a little but it is still too hot. Is there any way to reduce the heat without air conditioning? We are trying to reduce the electricity bill.

The room is shaded in red, and the blue is windows.

  • 12
    Hanging a shade outside from the roof edge to block sunlight shining on the wall can help.
    – Armand
    Apr 15, 2023 at 5:10
  • 4
    What's the roof like, flat or pitched? Do you have access?
    – Chris H
    Apr 15, 2023 at 12:06
  • Modern window units are amazingly efficient. Nothing beats an air conditioner. Apr 15, 2023 at 23:36
  • I would try covering the outside of the windows with a plastic mirror film or the like.
    – Huesmann
    Apr 16, 2023 at 12:55
  • @Armand That, outdoor blinds/shutters or an awning.
    – JimmyJames
    Apr 17, 2023 at 16:51

8 Answers 8


First of all cardboard is not a good reflective insulator.

Second, hanging cardboard on the inside will not help much since the heat has already entered the room through the glass.

Get some aluminum cooking foil that will reflect the heat and attach it to the window from outside.

  • Can I attach it from the inside? since the windows is quite high, and we do not have a ladder high enough to reach it. what about those reflective film?
    – user165673
    Apr 15, 2023 at 7:49
  • 4
    Better to attach it to the outside - how does the window open? Can you reach the outside that way. If on the inside, get the reflector right up against the glass, but it still won't be as good.
    – Chris H
    Apr 15, 2023 at 11:56
  • Cardboard is a pretty darn good insulator, actually, but it would only work if installed tight to the glass.
    – isherwood
    Apr 17, 2023 at 0:23
  • Perhaps you could wrap the cardboard with aluminum foil for easier installation. If the aluminum-wrapped cardboard is folded in half, perhaps you could lean out the window (safely, and with someone present to assist) and staple/screw the top half in place, then use a string attached to the bottom-half to pull it tight against the wall, and close the window onto the string, holding it in place. (I'm picturing the string being poked through the cardboard, tied onto something flat like a large shirt button, so it doesn't come back out)
    – Jamin Grey
    Apr 17, 2023 at 20:42
  • I'm assuming a vertically-closing window; horizontally closing would be a trickier problem. Some vertical close-windows are even easier, with the bottom half of the window folding-down for cleaning. Those are less-common, though.
    – Jamin Grey
    Apr 17, 2023 at 20:42

We have used two techniques on one of our properties.

  1. Retrofit Double glazing (Secondary Glazing) - Where another window is installed on the inside of the existing one. The outside were sash windows, the internal ones are slides.

  2. Window Film - this is an adhesive film applied to the interior of the windows and reflects some of the heat back. We used this on windows where it was not possible to install the second glazing.

We got a company to do the work, but you should be able to apply the film yourself. We are happy with the results. Obviously, these are not as good as replacing the joinery with double or triple glazing, but they worked for us. We elected to do this after we saw the result on a demo setup at a home show.

Example of Glazing

Retain your existing joinery and the natural character of your home with our retrofit double glazing solutions. Your existing window frames, aluminium or timber, can be retro-fitted with new, genuine double glazing. By choosing The Double Glazing Company’s retroGLAZE® system, you will save significantly and increase the health & comfort of your home.

From Retrofit Double Glazing

Example of film

Virtually invisible insulation. Enhance comfort and efficiency year round. This low emissivity window film helps improve the insulation value of a typical single-pane window close to that of a double-pane window, and of a double-pane close to that of a triple-pane.

From 3M All Season Window Film

I am not associated with either company. I have deliberately linked to a different company's site than the one we used. The links are where the quotes are from.

  • We've decided to try installing windows film, is there any big difference between the film? The one I saw is this shopee.com.my/…
    – user165673
    Apr 16, 2023 at 8:35
  • It is hard to know, how effective it is. I just went for a brand name 3M, myself. But shopify are saying 88-90% infrared cut, so it should do something. Apr 16, 2023 at 10:55
  • @user165673, yes, there can be differences in films. I live in Las Vegas, NV, so I used "3-in-1" film on all my windows and doors, which blocks IR, UV, and visible light. The difference was considerable and immediate. My windows went from nearly searing to the touch to mildly warm even in 122F temps. Reading the Product Description you linked to, it does the same, so it should work well. Side note, any indoor plants won't get enough UV to survive. I learned that the hard way. Apr 17, 2023 at 16:32

You need to do your best to cool the whole building overnight, then keep it cool with shade during the day. External shading of windows (and walls if not well insulated) is better than internal, but the latter can still help.

Stopping solar gain in the rooms below is important because the hot air will convect into the room where you're suffering. Summers here aren't regularly all that hot, so we don't have air conditioning - in a heatwave I shade the downstairs south windows with large parasols, but upstairs have to make do with silvered blackout blinds on the inside.

You can also look into reducing solar gain from the roof. I've lined my attic with foil emergency blankets partly to keep it cooler in summer (also so I'm not always in my own light moving around up there). Again, anything you can do externally will help a lot.

  • That 'silvered' film does give some slight relief on the inside where you can't get to the outside to install it. I put some in last year & there is noticeable, if small difference. It definitely eliminates that moving patch of hot floor as the sun goes round.
    – Tetsujin
    Apr 16, 2023 at 16:50
  • @Tetsujin yes, right up against the glass especially, it will help.
    – Chris H
    Apr 16, 2023 at 18:50
  • For a more designed-for-purpose alternative to the emergency blankets, look into a radiant barrier product. They make rolls of foil you can install between the rafters, and even a paint that you can spray onto the underside of your roof.
    – bta
    Apr 17, 2023 at 22:34
  • @bta yes, and in my climate they'd do a better job of keeping heat in in winter too. But they're much more expensive and I got the impression this was on a budget
    – Chris H
    Apr 18, 2023 at 5:51

If you have access to lower floors a good solution is to draw cooler air from there:

  • Open windows on lower floors
  • Close curtains on lower floors and in this room to minimize sunlight, but make sure to allow air flow through the open windows.
  • Open all doors between this room and lower floors
  • Open one window in this room and place a fan blowing out in the window. This will bring air from lower floors into the room, and fresh air from outside into the lower floors.

If you do this very well, you can get the room down to the outside temperature. You can't do much better than that.

This will also bring a lot of dust into the house.

  • 2
    "you can get the room down to the outside temperature." The room is already lower temperature than the outside "37.5°C(99.5°F)" temperature.
    – RonJohn
    Apr 16, 2023 at 2:31
  • @RonJohn ya that's intriguing right? Probably measured in the sun, or with a different thermometer, or around 10am when the house is trailing the outside in heating up. Still cool from the night and the greenhouse effect had only just begun . You need to apply my method with some intelligence. It's best effect is early evening when the outside is cooling rapidly, and its worst is mid morning when it's counterproductive. During the afternoon with the sun baking the house is when you hope to maintain at best the outside temp.
    – jay613
    Apr 16, 2023 at 10:35
  • It was not measured in the sun but in shade, it actually got that hot. Since we're experiencing heat wave and haze. Moreover, I used the same thermometer, but interesting thing is that because the temperature got so high to the point it is in the range of my digital medical thermometer which have a range of 35°C to 40°C, I thought to try it out and it read 37.6°C outside in the shade.
    – user165673
    Apr 17, 2023 at 6:29
  • 1
    @jay613 with care, if I'm around during the day, I can keep the inside of my house a few °C below the daytime peak in a heatwave. That's just using night ventilation and shade. The internal peak temp is later than the external as well as lower, but inside is below outside from shortly after sunrise to around sunset. The margin between inside and outside max does reduce after a few days, but it's always there.
    – Chris H
    Apr 17, 2023 at 10:34
  • @ChrisH that's cool. (Literally). I can't.
    – jay613
    Apr 17, 2023 at 13:21

White paper (for reflectance/radiance) over cardboard (for insulation) should work better than either alone. I have thermal drapes which have a white plastic layer in the side facing the window; sane principle.

But you may be getting heating if)through the whole wall.

If the air temperature outside is lower than inside, a fan in a window can help a lot. It outside is hot too, running a fan at night when it's cooler can at least help start the room at a more reasonable temperature. Opening a window elsewhere and setting the fan to blow outward can help make that more effective, or using a pair of fans in that room with one blowing out and the other in. Fans will also make people feel cooler by evaporating sweat more quickly.

Note that how much power an AC will demand depends on how low you try to push the temperature. If you're willing to settle for bringing the room down to 28 degrees, that should burn less power than trying to bring it down to 20... assuming the AC is capable of doing either against this load. You can also consider running AC only during hours when the room is in use; I typically start my bedroom AC an hour before bedtime and turn it off around the time I wake up.

  • It's described as a humid place so a swamp cooler probably won't be much help
    – Chris H
    Apr 15, 2023 at 11:55
  • 1
    True; missed that. (Misted that?)
    – keshlam
    Apr 15, 2023 at 14:02
  • Even better than white paper might be something like aluminum foil, or something else with a metallic sheen. They make sunshades for cars that are metallic on one side for just this reason. You might find similar for house windows, or even just buy the ones they make for cars (they're generally pretty cheap) and hang them up over the windows. Might not be a perfect fit, but better than nothing. Apr 17, 2023 at 16:31
  • 1
    Surface reflectance vs. absorbancy are somewhat independent factors. There's some related handwaving in physics.stackexchange.com/questions/104038/…
    – keshlam
    Apr 17, 2023 at 16:36

Reducing the amount of sunshine that reaches the facade would help keep the walls cool. Here in Berlin I noticed that even a thick stone wall facing south was warm to the touch after a few days of intense sunshine in the summer (days are long here in summer).

Installing an awning or some sort of reflective insulation on the outside would help (although I realize that that may be impossible or too elaborate). It is important that it be on the outside: The wall is a buffer, averaging night and day temperatures. It cools the room during the day, while it will absorb some of the day's heat. It is essential to minimize this absorption, but you want to expose the wall to the inside for day cooling, so no inside insulation.

If there is not too much wind you may be able to rig something up with a sail-like fabric attached to ropes from windows and the roof etc. A commercial awning like this or this could be a permanent solution within reach, depending on your budget or DIY skills and your landlord's cooperation.

Of course, water is a very effective cooling method. Even in a humid climate, water on a hot facade will readily evaporate and cool the surface. Traditionally, water is sprinkled on streets and gardens for coooling, as in the Japanese tradition of Uchimizu. There are commercial solutions using evaporative cooling of facades, and evaporation is what keeps gardens and parks cooler than cities. Regularly spraying the wall of your apartment from the outside during the hottest hours should have a strong effect.


As you're keeping the room very slightly cooler than outside, you're already doing well, considering the sun is shining on it.

I live in temperate climes, where 30+ degrees is fairly rare & cooling was not a consideration at the time the house was built. My most exposed room, however, if exposed to direct sunlight all day [two outside walls and the roof directly above], can easily reach 38° whilst outside is still <25°.

Various answers have raised methods of shading it further from the sun, either externally or internally. All positive ideas.

To take another tack entirely, what temperature does it drop to overnight outdoors and indoors? This could materially affect what you need to do to keep it cooler during the day.

As the building seems reasonably effective at keeping the heat out, it's also likely to be good at keeping the heat in.
This would mean your optimum time to cool it would be overnight, by a form of air exchange. Residual heat stored in the walls & roof is going to slow this process down but drawing in cooler air whilst expelling warmer may reduce your internal temperatures towards those outside at night; at least making the whole heat cycle have to start again each morning, as you then close down this airflow.

At minimum this could be by simply opening all the windows, or better by channeling simple fans to provide a distinct through-flow of air, like inside a computer case - cool in the bottom at one end, hot out the top at the other.

Nothing other than 'true' air conditioning, of course, is going to be able to do anything about the humidity.

  • the outside can get as cool as 25°C during the night, I've even seen it drop down to 22°C Currently i have a exhaust fan that I "installed", imgur.com/a/uW9CEp7 I used to run another table fan for that push pull configuration, but that introduced a bunch of unwanted insect. So I end up just running the exhaust fan from around 8pm to 6am. It was quite effective when i was running another table fan pulling in fresh air during the night. but i still cannot think of good way to prevent pulling in the insect too. the exhaust fan alone still able to get the room down to about 28°C.
    – user165673
    Apr 17, 2023 at 6:42
  • 1
    You need bug screens over the openings...
    – Tetsujin
    Apr 17, 2023 at 6:45
  • Actually mosquito net is installed, but when pulling in air somehow some very very tiny insect still manage to get pulled in even with the net, i think it is because the net have too big of a hole imgur.com/a/SX9i9SK
    – user165673
    Apr 17, 2023 at 7:06

Get an external share or blind. Blocking the sun with cardboard from the inside will not help much, because cardboard will dissipate the heat to the air in the room.

However, blocking heat from outside, with automatic, solar-powered screens, will make your house cool without using any electricity.

Highly recommend this company. https://www.youtube.com/watch?v=w1ATAvjX-cM

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