30 December, 2020
30 December, 2020
Cool homes
Passive cooling
Another summer, another heatwave due to climate change. It could just be uncomfortably hot, but when the power blacks out and there's no air-conditioning, the heat's a health risk. And in high levels of humidity, the heat becomes deadly.
But there are ways to keep your home cool, with passive cooling. This is a way to keep your home cool without relying on electrical appliances such as air-conditioners. It could be a way to survive the hot temperatures to come later this century.
Passive cool design, Los Angeles. Jeremy Levine, CC BY 2.0
According to computer models, passive cooling could keep houses cool and comfortable, whether it's in hot, dry Spain in the year 2050, or hot, humid Bangkok at the end of the century. If passive cooling works in these hot locations, it should work in many other places too.
There are a lot of passive cooling solutions to choose from, but they can all be boiled down to two steps. Stop the heat from entering your house. Then, if your house is still hot, remove heat from your house.
Shade
The sun can be brutally hot. According to the Australian government's guide to sustainable homes, "Direct sun can generate the same heat as a single bar radiator over each square metre of a surface, but effective shading can block up to 90% of this heat".
To use shade effectively, you need to know where the sun is. It's called the "aspect" — the angle at which the sun shines onto your house. In summer, it's high in the sky. In winter, it's from the south for those in the northern hemisphere, and from the north if you are in the southern hemisphere. But any time of year, the late afternoon sun is going to be low in the sky and shining right through your windows.
Foliage creates a cool microclimate and provides shade.Silvia Konrath, Pixabay
There's a lot of options for shade. Before building, orient your house away from the direct sun. If you can't do that, trees and greenery can still block sunlight, and also affect temperatures by creating a microclimate. Covered areas such as verandahs extend shady areas around the house, as do large overhanging eaves.
The last chance for shade is when covering the windows. Louvres, awnings and shutters are traditional solutions and folding shutters can even be angled to direct the breeze inside. Curtains or blinds are no replacement though. They still allow the heat to come inside.
Roofs and walls
Just as it's a lot cooler to wear white on a hot day rather than black, a white building exterior can also keep a building cool
It works by reflecting away the sun's heat. The white colour is so effective that it could cool a building by up to 23 °C (42 °F), according to NASA.
Whitewash keeping houses cool, Skopelos Greece. Dimitris Vetsikas, Pixabay
While white paint works well in keeping houses cool, it can create a glare.
Planting greenery on rooftops, and even up walls, is another way to shade your house from the sun, while being much easier to look at.
Green roofs are no less effective than white roofs. German research shows green roofs sit at temperatures of around 20 to 25 °C (36 to 45 °F), while bitumen and gravel roofs might go up to 80 °C (65 °F).
Insulation
If you can't make changes to the outside of your house, the next line of defence against the heat is insulation. There are two main kinds — reflective and bulk.
Reflective insulation is simply foil which reflects away the heat which manages to get through the roof or walls. It's an easy DIY project to hang it in your attic, under the roof. Laying radiant insulation in the walls requires a bit more construction. On a cold day it may not make much difference in temperature, but on a hot day it can reflect 95% of the heat. It just needs an airgap, and to be kept clear of dust.
The other kind of insulation is called bulk insulation, which provides a gap between the heatwave going on outside, and the cooler interior temperatures. The higher the R-value of the insulation, the more effective it is.
One other option is no insulation at all, at least between the ground and the floor, to make use of much cooler temperatures underground. This building technique could become a climate change life-saver.
In a study of how to keep houses cool in the hot Spanish climate in the year 2050, scientists found that an uninsulated floor had the largest impact of all techniques tested. This ground cooling effect could bring down inside temperatures by 4.5 °C (8 °F). Plus there's the added benefit of leaving more insulation for the roof and walls.
Thermal mass
If you've ever wondered how traditional cathedrals or old train stations stay so cool inside, the answer lies in something called thermal mass. Thick granite walls and marble floors take a long time to heat up during the day, and then at night they cool down again. Overall, the rock mass evens out the extremes. In winter, it holds the heat during the night. In summer, it stays cool during the day.
Materials that work as thermal mass are usually heavy materials such as rock or concrete which take a long time to cool down or heat up. Phase change materials are a recent development, and work the same as an ice cube in warm water, by absorbing heat until they suddenly melt.
The rock wall and the desert climate work in concert to passively cool the house. Jeremy Levine, CC BY 2.0
Phase change materials are a recent development, and work the same as an ice cube in warm water, by absorbing heat until they suddenly melt.
Even water works as thermal mass. Compared to concrete, water can absorb twice as much of the sun's energy without warming the house. Water containers can even be moved around the house, or poured out if no longer needed.
To cool the house even more, place thermal mass in contact with the ground underneath the house. The heat then dissipates through the earth.
Regardless of what the thermal mass is made of, it needs to be correctly located in your house to work properly. In cold months, thermal mass placed near windows, in the direct sun, helps to maintain warmth in the home. But to stay cool during warmer months, the thermal mass needs to be located in the shade, with access to cooling nighttime breezes.
Thermal mass doesn't work everywhere. It needs a big difference between day and night temperatures — about 10 °C (18 °F) or more for the mass to cool down overnight and to stay cool during the day. This is why thermal mass works well in desert or temperate climates, but is not recommended for tropical climates.
Unfortunately, with climate change, nights are getting hotter, which means thermal mass is going to become less effective with climate change. For a typical Spanish house in the summer of 2050, scientists found that thermal mass would "barely cause a change in the average indoor temperature".
Water
The prospect of unbearably hot nights later this century is something that nobody should be looking forward to. Fortunately Thai researchers may have found a traditional passive cooling solution, a waterfall inside your house.
The waterfall cools the interior of the house by evaporative cooling, which is what happens when water evaporates and takes the heat with it, like sweating. Nighttime air enters the house and blows a breeze over the waterfall. Through evaporation, the area around the waterfall cools, bringing nighttime temperatures down to a comfortable level.
An indoor waterfall and nighttime breezes cool the house at night.likeaduck, CC BY-NC-ND 2.0
It can also work together with thermal mass, even in the hot Thai nights predicted for 2050. The computer model developed by Thai researchers showed water cooling was enough to cool down a heavy thermal mass, such as the stone or water surrounding the waterfall, and keep the house cool the following day.
Breezes
The sheer cooling power of a breeze is not to be underestimated. An air speed of 0.5 m/s (1.6 ft/s) provides a 3 °C (5.4 °F) drop in interior temperature — all that's required is an open window.
So, if you're in a position to plan a house, or are choosing one to move into, it might be worth considering how a breeze moves around a house.
Breeze through an open window. Smilla4, CC BY-NC 2.0
With the house lifted above ground, breezes can pass underneath, cooling the floor. Separating parts of the house with walkways between increases the surface area that breezes can access, and cools down more of the house.
Inside, it's long, thin houses with windows on both sides that allow the best access for outside breezes. Check for points where inside airflow can be blocked, such as closed doors or windows that won't open.
To really flush out heat from a house, breezes need to head upwards through the house. Since hot air rises, it's going to sit in the top parts of the house, near the ceiling, or in the attic. Vents or fans in this area, or even solar chimneys can extract the heat, with the happy effect of drawing in cooler air from below.
The best time to cool this part of your house is at nighttime. If a house can cool down at night, it's easier to sleep and the house is cooler the next day. When predicting the most effective passive cooling solutions for the Spanish climate in the year 2050, research showed that without nighttime ventilation, temperatures rose on average 3.5 °C (6.3 °F) at night and stayed 2.5 °C (4.5 °F) hotter during the day.
Natural air-conditioning
At ground level, not every house is going to enjoy a steady breeze. Higher up, the rooftop is usually exposed to wind from all directions. That's why for centuries, home-owners in the Middle East have been using something called a windcatcher. This device catches the wind as it passes overhead and funnels it through pipes, taking the air underground where colder temperatures and water reservoirs cool the air down before it enters the building.
This combination of windcatchers and ground cooling worked so well that, in a desert environment with an outside temperature of 45 °C (113 °F), they could still keep inside temperatures at a comfortable 27 °C (80 °F).
Wind catchers at Madinat Jumeirah. Dima Soufi, CC BY-NC 2.0
There's still the opportunity to re-create the technology of the ancient Persians in your own backyard. A more modern version of the windcatcher is in production, called the Monodraught, which fits unobtrusively on top of a modern building.
To complete this experiment in desert-strength passive cooling you'll need a ground cooled ventilation system. These days they are called earth tubes. These are tubes placed underground, running from your house either in a closed loop, or to an opening outside where they collect fresh air. In either case, hot air is cooled as it passes underground and is then drawn into your house, either by fans or just convection.
Earth tubes work if they are laid correctly. A few metres below ground temperatures become stable year round, which makes it perfect for airconditioning your home. This depth varies according to location so it's worth finding out what it is before digging. Pipes should be thin, about 0.5 metres wide, and long, between 30 to 90 metres, according to different studies. To reduce humidity, angle pipes downward so condensation can flow out into the ground.
In case that doesn't cool down your home, there are still plenty of other solutions, such as hydrogel walls, or a terracotta 'beehive'.
But that should be enough. Using just a few passive cooling techniques, it should be possible to design a house to handle the heat in the coming decades, and all without an air-conditioner.
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