Just Enough Science to understand your house in a heatwave (Pt.1)
(Spoiler, if you’re in Perth its probably got a lot do with your double brick walls)
Sustainable Housing Design
Sustainable housing design can often feel quite abstract and scientific. It can mean comparing the insulation values of different construction materials, predicting life cycle costs in energy consumption in your home over the next 10 years or the thermal benefits of double glazing. However, in our part of the world, in the south-west of Western Australia, a couple of times each summer, it goes from the abstract to the undeniably present in a rather rude way. Late last year (2021) it happened between Christmas Day and New Years Day – a four day stinker of a heatwave that ranged between maximums of 43 and 38 degrees. This article will look at how your home heats up, and why it feels so much worse on day four of a heatwave. It’s a little bit technical (damn science) – but I’ll do my best to keep you awake throughout.
Growing up in Western Australia, I have so many childhood memories of sweltering through a heatwaves each summer. You know the sort of mounting heat over a few days where the house starts out as ‘this is pretty hot but if I lay still, I can breathe through it’, but by the afternoon of day four the walls and floor are hot to the touch, you’ve finishes all the frozen juiceboxes and you’ve worked through all of the $5 weekly movies with three days left before you could get some new ones.
We moved around a fair bit as kids, but our houses were nearly always your typical double brick, clay tile affair and often didn’t have any air conditioning. Regardless of the orientation of the site, the lounge room, master bedroom and garage faced the street, the laundry and bathrooms were at the back of the house, and the kid’s rooms seemed to have been arranged to fit wherever was left over. The houses were mostly rentals, so gardens were designed to be more manageable than practical – think some reticulated lawn, garden beds at the boundaries and a few short shrubby trees out near the verge, but nothing big and shady to keep the heat out.
Our houses’ defenses against these heatwaves were pretty feeble. We’d “close up” in the morning as the temperature started to rise. This meant engaging the only tactics we had – closing the windows and blinds. But it also marked a signal in the day – a kind of halt in proceedings where the character of the house would change. Lights would be turned off, the TV turned to low and all activity would slow to a crawl as we all steeled ourselves for another belter of an afternoon. Sometimes we’d make a break for it and go to the pool, but if we were around the house all day we just lazed about and waited it out. As the shadows started to stretch and the breeze picked up (I was in the Great Southern of Western Australia, so we didn’t wait on the Fremantle Doctor, but on its usually less competent and always much tardier colleague from Albany) we’d run a little sortie outside to see if it had really started to cool. If we got the all-clear, this meant “opening up” to let the breeze run through the house. We’d broken through the worst of the day and the house came alive again. It would stay this way over night until the cycle started all over again the next day.
While air conditioners are being increasingly common in our houses today, this daily routine has played out in houses for centuries. From a scientific and architectural perspective what you’re doing when you open up those windows and blinds in the evening is called ‘purging’. It’s basically removing as much of the heat the heat that has built over the course of the day. How well your house stacks up over the multi day heat wave comes down to three factors: how well your house can resist the heat during the day, how well it can purge the heat over night, and how you, it’s occupant interact with this. Before we get into this, a little background.
Designing for comfort
The background to feeling comfortable in your environment
No, I don’t mean feeling more relaxed in awkward social situations. In this part we’ll get into how the elements in your environment impact your comfort level – with a particular focus on the south west of Western Australia
I’ve broken this article into three parts. I’ll release the next few parts over the coming days
Also, this article uses the word thermal quite a bit. It basically just means heat, or relating to heat. So Thermal Comfort means comfort, in relation to heat.
What is thermal comfort
AKA is the temperature of your environment nasty or nice?
Think of thermal comfort in the same way we think of ‘room temperature’. It’s the kind of Goldilocks range of temperatures where things aren’t too hot, and they aren’t too cold. You can comfortably do most activities in this temperature range. It differs marginally between different people based on things like age, metabolism and weight, but for our purposes let’s say it’s about 23° to 26° in Summer and slightly cooler – let’s say 20° to 25° in winter. To be comfortable in our immediate environment (our home, office, car, backyard, netball court etc), we want them to be in or near this temperature range.
However, as I’ve just spent about 500 words above talking about, this doesn’t always happen – in fact, depending on where we live very often our immediate environment is outside of our thermal comfort range. There are large fluctuations over the course of a day that fall outside of this range. If it’s just by a few degrees we can make do – we respond consciously, subconsciously, and physiologically. To cool down we move our deckchair into the shade, put ice in our drink, wear shorts, laze around, jump in the pool and if the difference between our thermal comfort range and our immediate environment isn’t large often this is enough.
Psychology plays a part here too. Our thermal comfort is lower if we’re leaving a cool environment and higher leaving a warmer environment. So, it changes a little bit between summer and winter, day and night and whether we’ve just stepped out of a cool room or a warm room. Also, and I don’t have the science to back this up, but I’m pretty sure that our threshold for a really hot day is significantly lower if we’ve just copped three scorchers in a row, as opposed to a one-off hot day.
Considering your climate
AKA where you live in the world has a big impact on the type of house you should design
Unless you’re European Royalty or something, chances are you don’t have a summer house and a winter house so your place is going to have to be reasonably comfortable all year around. Most regions in Australia have large seasonal variations over the course of a year (and the course of a day) – we want our houses to be warm during winter and cool during summer, so far, so obvious. However, the extent to which we expect our buildings to perform through different times of the year will be different in different regions. Here in Western Australia the cool to mild conditions found in the state’s south, such as Albany are a world away from the warm to hot conditions of Newman in the Pilbara. While Albany will have hot days, and Newman cold days, when it comes to temperature we design for the average, not for the extreme. In the context of Perth, we have a Mediterranean climate which means we have wet and mild winters with hot and dry summers. While our winters aren’t arctic cold, they can get pretty chilly and we still want to be reasonably comfortable for those long months too. If we designed our houses for just the (on average) three or four 40° days per year then we wouldn’t have particularly enjoyable winters. But that doesn’t explain why our homes get progressively hotter over the course of a heatwave.
(Pro tip, if you’re interested in seeing other places in the world with climates that are like yours, check out the Koppen Climate Classification. It’s a great way to see how other cultures have responded to the same climate in different ways. For example, Perth is classified as a “Csa”. Other places in the world that have been dealing with the same climate for millennia include places with buildings as diverse as Beirut, Lisbon, Sacramento, Casablanca and the Vatican City.)
Day and night temperatures
The diurnal temperature range - AKA the difference between the hottest part of the day and the coolest part of the night
If you’ve been camping out in the desert and sweated through a stinking hot day, you might have been surprised when you woke up in the early hours of the morning to reach for a beanie and woollen socks. This is because deserts have a very high range between daytime high and night time low temperatures. On the other hand, the humid tropics have a much lower range, where the temperature fluctuation is less marked. This difference between the hottest and coldest time of the day is called the diurnal temperature range. Here in Perth our Mediterranean climate means we have a relatively high range between daytime and night time – and we can use this to our advantage. By doing our best to prevent heat outside coming into our homes during the day and by allowing that heat to dissipate as quickly as possible as the outside temperature drops at night we stand the best chance of avoiding the accumulative effects of three or four days of extreme heat. (more on that in Part Two)