Some (don’t) Like it Hot – stop your house turning into an oven this summer

Its stinking hot. Your house is like an oven and you wonder if there is any way to keep it cool. Well your t(r)usty techno-nerd has poked around to see what works.

You may have seen the ads in the Sunday papers for roof whirligigs promising heat escape miracles, people knocking on your door wanting to install insulation in your roof to make it ‘much cooler’. But really, are they it worth it or are they just tricks to help you part with your money?

Taking the temperature

I’ve chosen an almost 40c day to test some ideas. Armed with a digital infrared thermometer (you point it at something and it reads the temp of what its reflecting or radiating), I’ve gone around the house pointing it at walls, windows, ceilings and floors to get an idea of where the heat is coming from.

digital thermometer

digital thermometer

Initial tests

Test unit is double brick with largish windows. The roof space is not insulated and the courtyard is concreted.

  • Outdoor ambient temperature: 38.8c
  • Indoor ambient temperature: 29.5c

Indoor temperatures:

  • Ambient temperature 29.5c
  • Ceilings 31 – 33c
  • Windows in sunlight 31 – 32c (shaded by tree 30c)
  • Walls 29c
  • Floors (concrete) 26.6c

Notes:

The uninsulated ceiling is only marginally warmer than the windows. Both these are primary heat sources for these rooms.

Window temperature was the same regardless of blind or curtain type; white plastic venetians, blockout wooden venetians, blockout curtains.

Window shading trial

Over two days I tried different external window covers for windows, using matchstick blinds. Here are some interesting results:

  • with lightweight internal blind only – 30.6c
  • with matchstick blind on outside of window – 27.6
  • with shaded bamboo slat blind on outside of window – 26.8

The best result was to put a thick slat bamboo blind over the windows from the outside, then shade the bamboo blinds using other matchstick blinds strung over the yard like a shadecloth. The reason this works best is if you just use one blind on the outside of the window, the direct sun will just heat this blind up and the heat will be transferred through the window.

Using a thick bamboo slat blind reduces the heat transfer from the hot air through the window, forming a sort of barrier.

Bamboo Slat Blinds

Bamboo Slat Blinds

Using bamboo matchstick blinds like a shadecloth reduces the sun on the window blind and also on the concrete and nearby walls, which usually reflect strongly in through the windows.

Matchstick Blinds

Matchstick Blinds

The combination of the two blinds makes the window quite dark, but still with a slight glow.

Conclusion

The main source of heat penetration for most rooms is windows that are not shaded from the outside. Glass is not an insulator, so the heat just radiates straight through. Inside curtains do little more than trap the heat. So, regardless of window covering, windows become heaters for the rooms.

On very hot days, outdoor shading away from windows, eg. trees, reduces temperature by about 1c, because even though they shade the window, the heat of the air still radiates through the windows. Almost identical amounts of heat enters the windows regardless of whether they face north or south.

Heat through windows is the result of direct sun, hot air against the window, or heat and light reflected off concrete, grass or nearby walls.

Radiant heat on unshaded window

Radiant heat on unshaded window

Blocking this heat requires blocking the window from the outside. But also blocking heat from heating up external window shades.

Blocking radiant heat using  external bamboo blind

Blocking radiant heat using external bamboo blind

For patios, a combination of an external window-blocking blind plus an overhead matchstick blind is the best result. Avoid ordinary nylon overhead shadecloth as this does nothing to reduce radiant heat.

Combination shade blind and window blind

Combination shade blind and window blind

For unit balconies, try having the shading matchstick angling out to the balcony rails or halfway. But be careful not to allow sun to reflect off the concrete balcony floor.

A shaded external blind is best

A shaded external blind is best

These solutions are also very cheap, even for a student-renter. Outdoor bamboo slat blinds from Bunnings are around $20-25. Matchstick blinds (actually just suspended bamboo fence screens), around $60. This is in comparison to indoor blinds that can be around $100 for similar sizes.

So, before dishing out on roof insulation and whirligigs, or wandering around your sharehouse naked, get your windows covered – and shaded – from the outside.

Update: On a second 40c day, with double blinds in place and sun shining on test windows all morning, by 1pm outside ambient temperature is an oven-like 40c, indoor ambient, 27c. Proof positive this system works.

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