31 January 2015

Extractor fans, security screens and other passive cooling tricks

The key to passive cooling is ensuring the cool[er] evening/morning air gets into the house, while the hot day time air is kept out.

We often find that by about an hour after sunset the house is warmer inside than out. Our trick for getting the warm air out and the cool air in is: Extractor Fans. They are designed to remove steam and smells, but they are also brilliant for removing hot air.

We generally run them for a couple of hours at night, turn them off while we're all asleep (they are not designed to run for twelve hours at a time) then pop them on again in the morning around dawn if the house is still too warm. We also line up our pedestal fans so that they assist the overnight breeze (generally easterly in summer here).

If you've followed this blog even a little you would know that we also shade, shade, shade the house in summer. [If you're new to the blog, have a look at these posts on shading: 1 2 3 4 5 6 7 8] However, its two years since I last wrote about shading so here we go again with some more ideas we've been trying:

- pillows between glass and blinds in the lounge room window (above - NE window, guarded by Eva's scarecrow)

 - cardboard boxes between glass and blinds in the office window (NE window)

- large cardboard tray from a flat-pack furniture item, and a pile of polystyrene cooler boxes, to add insulation to the laundry door - nearly four square metres of glass that gets afternoon sun in summer (SW facing). It can only partially be shaded from the outside, given that it is a door so is not suitable for shading that can't easily be pushed aside.

- a mattress across the window in the children's bedroom (SE window).

- a cot mattress and more flat-pack boxes between glass and blinds in the bathroom (SE window)

- a former painting canvas between glass and blinds in the other bathroom (NE window). Neither bathroom has eaves, so shading these windows in other ways is particularly crucial.

All these shades are removed at night to let the night air in and the day's warmth out. This is possible because most of our windows have security screens. If we were not renters we would fit security screens to the two main living room areas immediately, as not being able to leave them wide open all night significantly hampers our overnight cooling.

After two weeks of lovely cool holiday in Victoria, we last week landed back into the peak of Perth summer. Its hot here! The temperature dipped below 20°C this morning, briefly, for the first time in six days, and maximums have been between 33°C and 40°C for a week, with a similar week forecast ahead. So far we have run our airconditioner for about five hours, on one night when it was not forecast to cool down and the air outside as full of bushfire smoke. Twice in the past ten days guests have commented on how cool our house feels, even noting that one room felt like the airconditioner was on (this at a time it was around 31°C outside).

Initial Time: All the additional shades above are daily measures that we only put up on the hottest days, and require about five minutes at each end of the day to put up and pull down. Extractor fans take seconds to flick on and off; pedestal fans take a minute or two to get into place where they are most effective for night times.

Initial Cost: Zero.

Ongoing time or cost commitment: As above.

Impact: An extractor fan uses around 25 Watts to run. We run three, for about five hours in total: 375Wh per night (about one third of a 'unit' in our system). 

Pedestal fans use around 40-50 Watts. We run four, all night, a total of around 1440-1800Wh. However, we also run them with the airconditioner if its on, so that we can direct the cool air up the passage and into the bedroom, due to the awkward placement of the airconditioner unit in our living/kitchen area.

Our airconditioner uses (roughly) between 1.5 and 2kWh (units of energy) per hour, depending on what temperature we set it to, what the temperature is outside, what the temperature is inside, etc. The night this week when we didn't open up the house, we ran the airconditioner for five hours (and pedestal fans for about twelve hours) to bring the inside temperature down to a bearable level ready for the next hot day - that is, to achieve the same effect that running fans and opening the house usually manages. Overnight cooling by airconditioner (and fans), then, uses about 9-12kWh per day, while overnight cooling by using the evening breeze assisted by fans uses around 1.8-2.2kWh.


  1. How do the pillows etc in between the blinds help?

    1. They create an extra layer of insulation, and keep out a bit more sun. The lounge windows are mostly shaded but they have direct sun along the bottom edge for half the day. When I feel the pillows after that the outside is hot to touch but the inside is cool, indicating that quite a lot of heat has been absorbed by the pillows rather than entering the room.