Why is ventilation important?

Heat from the sun enters houses in four ways:

1. Radiant heat transfer from roofing material to attic air, which in turn can re-radiate through the ceiling.
2. Entry of certain wavelengths of light through glass windows.
3. Radiant heat transfer from the building materials used for walls.
4. Hot air simply entering through open windows, doors, cracks etc.

How you lay out our cooling can make all the difference.

Once heat enters your home, it can be slow to dissipate if there's no way for air to escape. Even with evening temperatures dropping by 10ºC below the temperature inside a house, it can take ages for the house to cool. If the temperature inside remains high during the evening, the house will quickly heat up the next day too.

How ventilation works

Ventilation is particularly important for the purpose of cooling. The best ventilation systems work by taking warm air out of the home between 6pm and 3am, and replacing it with cooler air. This is called an ‘air change’. As the temperature heats up outside during the day, you then lock up your house to stop heat from coming in. This can be achieved by shading using awnings, curtains and blinds.

Air changes per hour

The number of air changes you need per hour depends on the volume, position and surroundings of your home. The amount you can achieve depends on the flow rate of your ventilators. In a typical brick-veneer home that features eaves and has trees near the windows, around two to three air changes per hour should be enough. This can usually be achieved with four or five wind-driven whirlybird ventilators, or two powered ventilators, which can move up to 2,400 cubic metres of air an hour.

In an older style home with no eaves or trees, you may need up to five changes an hour. The higher the ventilation rate, the better the conditions. But there is a limit. After a certain point, regardless of how much money you spend, there is an optimal level of ventilation for every home. This is determined by the volume of air in the home, shading, trees, and insulation.

What you need to know

Before you plan a ventilation system for your home, you need to know the volume of air in your home and compare that to the capacity of a ventilator. That is, you need to ask how many cubic metres of air needs to be moved each hour, and what sort of mechanisms will allow two to three air changes an hour in your home.

These are the vital calculations. Keep in mind though that when it comes to ventilation it really is a case of getting what you pay for. Whirlybird vents, for example, can be bought for around $50, but you shouldn’t expect to get 'air conditioning' type results from them. It just doesn’t happen. Some newer hybrid ventilation systems can move up to 2,400 cubic metres of air per hour while only consuming 40W of power. This, however, can easily cost a couple of thousand dollars. The trick is to find a cost-effective balance.

If you're considering building a passive house, you need to keep in mind that because of the fact that your house will be more or less airtight, you will require carefully planned mechanical ventilation.

Ventilation and air conditioning

Air conditioning in general is a far better manager of climate control than ventilation. However, if you team the two together, controlled ventilation can vastly improve the operational efficiency of air conditioners. Properly planned ventilation is particularly useful for ducted systems, as it can lower the temperature in ceiling cavities by up to 10°C, thus reducing the heat load on the ducts. Without air conditioning, a ventilation system in partnership with good insulation can reduce your home’s temperature by as much as 9 to 10°C. While this may not be good enough in extreme climates, it could prove adequate in areas where cooling is only necessary for a few days of the year.

Thermal mass

When planning how your home will be ventilated, it is important to consider the materials it's made from. In particular, you need to consider thermal mass.

Thermal mass is the ability of a material (such as timber, concrete or brick) to store heat. During summer it can absorb heat, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm. High density materials like concrete, bricks and tiles have a high thermal mass because of their ability to store more heat. Lightweight materials such as timber have low thermal mass because they store less heat and require less energy to change temperature.