Security screens

Summer in Perth... time to talk again about passive cooling.

We have been renting this house for eight and a half years (a rental duration miracle). The balance between renters and landlords in Perth has tipped a bit in the past year. Landlords are currently keen to retain good tenants, as vacancy rates are up, so when we got a new property manager a few months back we thought it was worth asking for a few non-essential-but-desirable things around the house. Second on our short list (after 'permission to have a pet') was security screens for the four windows that did not already have them.

The new property manger was very supportive, and the owners agreed to pay for screens. They were installed at the start of January, just in time for our hottest summer months. A big thank you to both owners and agent for arranging this!

Although we could secure the windows previously, this required having them almost completely closed. In summer, a key part of our passive cooling approach is ventilating the house at night. For maximum air flow we need the windows wide open while we sleep, but in our neighbourhood that is not really an option without security screens.  

The new screens for the large lounge and kitchen windows in particular make a big difference to how much hot air we can remove from the house through the night. We are now leaving all our windows open, and, as described previously, using extractor fans and pedestal fans to assist in removing the day's heat after dark.

Initial Time: five minutes conversation with our property manager, then quite a lot of weeks of waiting.

Initial Cost: The owners have covered the cost. However, the guy installing them told us the big windows cost $160 and the narrow bathroom window was $110. I'm not sure whether this included his time to install them also (Installation took less than an hour).

Ongoing time or cost commitment: Two minutes at the end of each day to open all the windows; two minutes in the morning before the day heats up to whip around and close them all.

Impact: Our airconditioner uses approximately 1.5-2kWh (units) per hour, so it doesn't take long to gobble up plenty of energy cooling the house. Our average daily electricity usage is 6-7 units per day, so the airconditioner can double our usage in under four hours. We've had one really hot spell since the screens went in and I felt the living areas were cooler in the mornings that they had been pre-screen, meaning we lasted longer before using electricity to cool the house. My post last January about extractor fans has detailed calculations of energy use to cool overnight with our airconditioner or with fans and open windows. 

And finally (totally unrelated to passive cooling but also a result of asking the landlord for a few things): Here are the newest additions to our family, the much-loved baby guinea pigs 'Smudge' and 'Excavator'.

 

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.