A home that breathes well can feel cooler on a hot afternoon without anyone touching the air conditioner. The trick is in the design. Where windows sit, how they open and how deep the rooms are all change the way air moves through the house.
Australia’s official guide to sustainable homes, Your Home, calls air movement the most important element of passive cooling. That makes it worth planning at the drawing stage. Changing window positions later is costly and often not possible.
Why moving air cools you
Moving air carries heat away from the skin. According to the Your Home guide to passive cooling, air moving at 0.5 metres per second gives about a 3 degree cooling effect at 50% relative humidity. Speeds up to one metre per second help when humidity is higher. Above that, most people start to feel uncomfortable rather than cool.
This is why a gentle breeze through a living room feels so pleasant. It is also why a house that traps still air feels stuffy even on a mild day. Good design aims for steady, gentle airflow through the rooms people use most.
Know where your breezes come from
Cool breezes usually arrive in the late afternoon or early evening. That is often when a house needs cooling most. On the coast they tend to blow onshore, like the south west breeze known as the Fremantle Doctor in Perth. In hilly areas, cool air often flows down slopes late in the evening and early in the morning.
Inland, cool night air is the more reliable source of relief. Your Home notes this works well where the daily temperature swing is usually more than 6 to 8 degrees. Knowing your local pattern helps you decide which side of the house should open up. Perth readers can pair this with our guide to choosing air conditioner sizes for Perth homes, since airflow and a well sized system both play a part in summer comfort.
Window types and where to put them
Your Home says window design matters more than orientation for collecting breezes. Sliding windows only open up to half of their area. Louvres can open up to 95%. Casement windows can catch and deflect breezes arriving from different angles.
Placement is just as important as type. High windows on both sides of a room send air across the ceiling. Low windows on both sides send it across the floor. Set windows so the airflow passes through the spots where people actually sit and sleep.
Sizing openings for better flow
A simple rule helps here. Use smaller openings on the windward side and larger openings on the downwind side. A louvre window above a door lets breezes pass between rooms while keeping privacy and security. Where true cross ventilation is not possible, fins projecting on the windward side can help draw breezes into the room.
Windows for hot humid climates
In hot humid parts of the country, Your Home suggests windows with the largest possible opening area. Louvres and casements suit this, as long as they seal tightly when closed. Fixed glass panels are best avoided in these climates. Full height double hung windows are also described as ideal for cooling with night air.
Room depth and floor plans
Breezes struggle to get through deep floor plans and houses broken into lots of small rooms. They work best in narrow layouts or open plan spaces. In hot humid climates, Your Home recommends single room depths where possible. In temperate climates, a moderate depth of about two rooms is preferred.
Hot arid areas with cool winters are different. There the advice is a compact form with more building depth. This shows why one design does not suit the whole country. A good designer will start with your climate zone before drawing a single wall.
Using rising warm air
Not every day brings a breeze. Convective ventilation, often called stack ventilation, works on still days too. Warm air rises and leaves through high outlets, which draws cooler air in at lower levels. Clerestory windows, roof ventilators, vented ridges, eaves and gables can all help this happen.
Your Home points out that convection can cool a building but usually does not move air fast enough to cool people. Solar chimneys add height and air passages to strengthen the effect. Think of stack ventilation as a way to flush heat out of the house. Fans and breezes then do the work of cooling the people inside.
Ceiling fans as part of the design
Ceiling fans are simple to plan for in a new build or renovation. Your Home suggests placing them centrally in each space, with one fan for each furniture group. A long lounge and dining area may need two. In bedrooms, the fan should sit close to the centre of the bed.
Air speed drops the further you are from a fan. So fans belong over the places where people spend most of their time. In a lightweight home in a warm temperate climate, fans in bedrooms and living areas can significantly reduce cooling energy use. If running costs are on your mind, our article on residential solar energy technology looks at another side of summer power use.
Planting and outdoor design
The garden can guide breezes too. Planting, fences and outbuildings can be placed to funnel cool breezes into the house and filter stronger winds. Tall spreading trees can shade the house. Smaller trees and shrubs beside the house should filter breezes rather than block them.
In hot humid climates, planting also lowers the ground temperature around the home. This is worth keeping in mind when planning garden beds and outdoor areas. Our landscape design tips for property values look at the garden from a buyer’s point of view. If you have a pool, the same thinking applies when choosing plants around pools so they shade the area without blocking a cooling breeze.
Thermal mass and night cooling
Thermal mass is the ability of heavy materials to store warmth and coolness. How much you need depends on your climate. High mass suits inland areas with big daily temperature swings. Low mass is generally preferred where there is little or no need for heating. As a rule of thumb, Your Home suggests limiting thermal mass in upstairs sleeping areas.
Where nights are cool, opening the house in the evening lets that cool air carry away the heat of the day. A whole of house fan can help with this. Run at low speed overnight with doors open, it can cool beds near open windows. Planning which windows can safely stay open at night is part of good design.
Frequently asked questions
Is cross ventilation enough to cool a home in summer?
It depends on your climate and the design of the house. Air moving at around 0.5 metres per second can feel about 3 degrees cooler at moderate humidity. Many homes combine breezes, ceiling fans and shading with some air conditioning on the hottest days.
What type of window lets in the most breeze?
Louvres can open up to 95% of their area, while sliding windows open up to half. Casement windows are good at catching breezes from different angles. In hot humid climates, choose windows that open wide and seal well.
Do ceiling fans cool the house or the people?
Ceiling fans mainly cool people by moving air over the skin. They do not cool the building itself very well. That is why they work best placed over beds, sofas and dining tables.
Does house depth affect airflow?
Yes, it makes a real difference. Breezes move more easily through narrow or open plan homes than through deep plans with many small rooms. The ideal depth changes with the climate zone.
Every block and climate is different. A building designer or architect who knows your area can help you plan windows, room layout and fans that suit your site.
