Imagine it’s the end of a baking hot summer’s day, perhaps one of those 45 degree plus crackers. The home’s insulation, thermal mass and shade did a fantastic job keeping indoors comfortable without using a reverse cycle air conditioner. A cool change arrives and the temperature outside drops significantly. Time to take advantage of that cool air using the gentle art of natural air circulation.
What is natural air circulation?
Passive cooling (aka natural air circulation) has been around as long as Earth’s atmosphere. It’s that cool breeze we seek when outdoors on a hot day.
Your Home points out that passive cooling has the least environmental and financial impact of all the cooling options. The planet creates breezes for free so we should take advantage of that generosity. Mechanical cooling methods such as evaporative coolers or air conditioners can’t avoid costs, starting with the purchase price.
A gentle breeze flowing through the interior of a house makes the occupants feel cooler because it increases sweat evaporation. The air movement also flushes out warm air and cools the interior thermal mass. Arranging for the breeze to pass over a garden water feature before entering the house boosts the cooling effect. It’s the natural equivalent of an evaporative cooler.
Think about heating before cooling.
Thinking about how the house heats up on a summer’s day comes before considering how to naturally cool it that night. That’s because the more it heats up, the harder it is to cool with natural air circulation. Breezes are good, but let’s not make the task harder than it has to be.
The Your Home website provides a handy checklist of four items to minimise heat gain
- Shading windows & walls reduces direct solar radiation hitting the home.
- Reflect heat with a light coloured roof.
- Insulation stops heat passing through walls, roof and floor. Think of double glazed windows as a form of insulation.
- Use thermal mass to moderate the indoor temperature.
David ensured our design ticks all those boxes, along with another idea – a front entrance air lock.
Having an air lock at the entrance prevents heat gain when opening the front door. Cool air rushes out, only to be replaced by hot air.
Ways to encourage natural air circulation.
Ensuring a good flow of cooling air through a home doesn’t happen by accident. Everyone knows about water and hills. You have to respect this behaviour to get water to move from one spot to another without a pump. Think of rainwater flowing along a sloped gutter to the downpipe. Breezes have their own peccadilloes so these have to be factored into the design.
What are the design features that promote passive cooling? Houzz recently published a set of handy suggestions for boosting natural air circulation.
Start by orienting the home according to the prevailing direction for a cooling breeze. Consult the BOM wind roses website for information if unsure about the direction at your location.
Select appropriate window styles that make it easy for the cooling air flow to enter and leave the house. Cross-ventilation is essential. Putting openings on opposite sides of a room enables the air to flow through the room, from the higher to lower pressure sides. Help warm air leave the building by installing clerestory windows or roof ventilators. As the warm air departs, it draws in cooler air from lower openings. The author’s final suggestion is remembering to use these features. When the outside temperature drops, open the windows and let the house get on with the job of cooling down for the next day.
Passive cooling features in our house.
There are several features in our design that maximise natural air circulation.
Orienting our house north-south is a good starting point. After hours of unrelenting hot, northerly wind pushing the temperature higher and higher, a cool change arrives when the wind swings around to the south. Locating most of the windows on the northern and southern walls provides the cross-ventilation necessary to take advantage of this breeze.
As recommended in the Houzz article, we’ve installed high windows to assist with air circulation. Warm air trapped near the ceiling flows out the open clerestory window, drawing cool air into the house via the lower windows.
We specified hopper windows to minimise the amount of warm air that stays next to the ceiling. Aspect Windows inverted and modified a standard awning window to meet our requirements. There were some initial concerns about rain finding its way through the base of the upside down window. However, the eave above the hopper windows protects them from direct rain.
Opening and closing these high windows is a simple task using an extension rod which connects to the special winding mechanism. I can hear the window frame seal engage so I know the window closes properly.
I’d recommend installing hopper windows.
Turbo charge with evaporative cooling.
We’re looking forward to trying an experiment with natural evaporative cooling in the second bedroom.
A sashless double hung window is a feature in this bedroom. The tall, thin design allows extra light into the room with a view back down the driveway. That’s all very well, but it has a second job. Just outside the window, we’ll install a water feature. Being double hung means we can open the window at floor level to allow in a flow of air cooled by passing over the water feature.
Recent experience with using this window showed a decent flow of air. The full test happens after the landscaping is finished. No harm if it doesn’t work as well as we’d like since the bedroom gains its very own water feature garden.
Learning to use the features.
So, all the features are in place for natural air circulation. Our house isn’t clever enough (yet) to open and close windows as necessary so it’s up to us to make the most of the opportunities.
Fortunately, good house design ensures the process is not at all complicated. Each night, we check the weather forecast for the predicted overnight minimum. Sometimes the outdoor temperature drops below our interior temperature quite early in the night so I open all the windows before going to bed. At other times, I’ll wait until after getting out of bed next morning to take advantage of the coolest part of the day. There are days when the southerly change arrives in the afternoon so it’s all hands on deck as soon as the wind swings around.
Here is an example of the indoor/outdoor temperatures at the end of a warm day.
After a night of passive cooling, the interior temperature dropped to ensure the house was comfortable for the next hot day. Interesting to note the moderating effect of the thermal mass (concrete floor and Timbercrete walls). Whenever we’ve used passive cooling, the interior temperature hasn’t fallen below 21 degrees although the outdoor temperature is lower.
Remember, passive cooling is responsible for the falling indoor temperature. No mechanical cooling was needed to achieve this result.
If a cool change hasn’t arrived?
Last night’s 30 degree minimum temperature demonstrated there’s no guarantee a cool change will arrive at night. Opening the windows would release cool air trapped inside from before the heat wave started and quickly warm the interior. Hence, there’s a Plan B for getting a good night’s sleep.
Each bedroom is fitted with a ceiling fan. The fan gently and quietly stirs the air, immediately making the room feel more comfortable. A nifty feature on the fan’s remote control is a timer function. I set the fan to operate for an hour and we’re usually asleep when it automatically switches off. There’s no point in using the reverse cycle air conditioner to cool the entire home if we just need to make one room feel cooler.
And the lessons are?
Natural air circulation is a useful cooling strategy IF the house is designed to minimise heat gain AND make use of cooling breezes.
A ceiling fan is the next cab off the rank if the conditions aren’t right for passive cooling.
Turning on the high efficiency reverse cycle air conditioner is the last resort after an extended period of hot weather makes the interior uncomfortably warm.