Passive cooling is cooling without any other energy input. It includes direct blocking, preventing unwanted airflows and encouraging desirable ones. The following sections will help you discover how we keep our house cool –
As mentioned before, the best way to keep something cool is to keep the full Sun off of it.
For a house, we need to consider walls, windows, doors and footpaths.
Shade cloth is the primary defense, whether in scrap pieces or commercial shade sails, the effect is the same – they block a percentage of the Sun from getting through to the area that they protect.
You can buy shade cloth in various percentages which correspond to the amount of light they block. Common percentages are 50% 70% and 90%. It comes is a range of colours white, brown, green or black. Each colour adds a colour cast to the light that it allows through and can be used for effect. At the back where shading is important but there are many plants that still need light, we use a brown 70% shade cloth.
In extreme heat we add other pieces of shade cloth here and there to give extra coverage. In the area of the front porch we have 50% white shade cloth so that plenty of light is allowed through and it remains bright and uncoloured. We have that positioned so that there are several layers between the garden and the front door so that the area is well protected and bright.
Glass both transmits light and conducts heat. for a more detailed description of how this affects room heating, see the page ‘glass and heat‘ on this website.
Like everything, the more the heat is blocked, the less the windows are heated. One of the best ways to block heat from windows is through the use of blinds and the best place for blinds is on the outside of the window.
Blinds come in a huge range of shapes, materials, sizes and capabilities. At Ligaya Garden, we use cheap, shade cloth, pull blinds (the kind that keep getting stuck and require me to get on a chair and pull the cords straight). They were one of the first things we added to the house, even while we were renting.
Blinds work by blocking the light and heat from reaching the glass. Along with the garden shading the front of the house, they make probably the biggest difference of all techniques to keep the house cool.
If you’re stuck for cash, just hang up some blankets or sheets across your window frame. It doesn’t matter so much how they look as how they work. We added some shade cloth scraps across the front of a neighbour’s lounge windows by nailing them to the fascia and holding the bottom down with rocks. The year we did that was the first year that they could open their lounge windows in Summer.
Curtains and interior blinds are not as effective as outside blinds but still help enormously. For best effect, they should reach the floor and have a pelmet to stop thermocycling. See the convection & thermocycling page for far more details.
Insulating is crating a physical barrier to the transmission of heat. The best insulator is motionless air. It also the cheapest, provided you can find some way to trap it.
Most people think of insulation as those pink batts that you put in your roof but if you’re on a super tight budget, newspaper can do the job and it can be shredded to provide more bulk. Simply put garbage bags partially filled with newspaper and sealed into the roof cavity. At a pinch, sheets of corrugated cardboard can do the job. We’ve tried both and they are a temporary solution at best.
The roof batts are made from glass fibres and work on the concept of motionless air. When all of the fibres are packed together in a batt, they trap millions of tiny, unconnected, pockets of air in between them..
Some pundits suggest soaking your cardboard or newspaper in Borax first and letting it dry before placing it into the roof space. The Borax helps to make the paper more fire and rat resistant. As I said, paper is a temporary fix to get you over extreme weather.
Window insulation is available too. This is a plastic film that is installed over the glass. It’s quite expensive and as you can see in this post, we found Bubblewrap to be a cheap and easy to install solution.
The best way to keep something cool in Summer without great energy and water inputs is to block the direct sunlight from hitting it
The movement of air can bring a lot of heat into your home, Many older homes are riddled with gaps that can let the air in.
These could include poorly fitting doors, badly fitting and loose windows, cracks in walls and windows and the common big gap at the bottom of the door.
There are many sealers available from silicone, caulking to that fun stuff in a can that foams up uncontrollably. If you’re stuck for a buck though, or don’t want nasty chemicals around your place, simple things like rags or scrunched up newspaper can do the job. In our first year, we used plain, everyday packing tape to seals some of the larger cracks and over the joins in the window frames. It worked a treat!
For that big, under door gap, there are all kinds of elaborate things that go by the name of ‘draught stoppers’ that you can find at your hardware store.
We invested in a few of those so things would look nice but our favourites are still the home made fabric snakes. They’re an old, simple design that you can make out of scrap fabric.
If you can’t make or buy a fabric snake, rolled up clothing will scrunch into that gap.
Cross ventilation is a special way to utilise airflow. If one side of a building is cool and the other side is hot, if allowed, air will flow from the cool to the hot side. This phenomenon is also called the ‘solar chimney effect’. I’ll be doing some experimenting with that in the near future and will cover it in blog posts and on our DIY page.
For us, it means that in the morning, when the Sun’s in the East and that side of the house is getting hot, we can open all the doors on the cool Western side and a few on the hot Eastern side and air will flow through, creating a cooling breeze.
As the air temperature on both sides of the house balance out, the effect slows and stops. That’s when we close everything up.
Cross ventilation can be caused by breezes or wind blowing in the right direction as well. In perfect conditions, both the morning breeze and the cross flow that we have induced work together to cool the house.
Whirlybirds and roof vents
These are so good at creating airflow that they deserve their own paragraph. We’ve installed one at every home that we’ve had if rented and two at the houses we’ve owned.
Whirlybirds allow hot air to rise from the roof cavity and help draw warm air from the house if you have any ceiling vents. As they spin in the slightest breeze and sometimes just from the flow of hot air from the roof cavity, rain and birds can’t get into them.
You can buy non-moving vents too. They’re like a metal chimney and allow the heat to rise and disperse.
Thermal mass is an important concept when looking at passive cooling or heating. You might have noticed that large objects such as a concrete slab stays cool until late morning even though the air around it has warmed up a lot, It stays warm late into the evening once the air has chilled.
The mass of a large object absorbs and radiates heat at a rate that is much slower than the surrounding air and other, smaller, nearby objects. The this themal effect is called thermal mass and provides a kind of thermal flywheel that slows down the changes in the temperature of the immediate environment.