Preventing overheated homes

A warm winter home is no good if it is stiflingly hot in summer – good design and simple changes can make homes much more comfortable in the summer heat.

At the same time as homes are designed and built to be warm and healthy over winter, they also need to be kept cool during the summer. Overheating, especially in today’s well-insulated, airtight homes, is becoming an increasing problem.

Homes are often shut up during the day, allowing them to heat up. Once the house has become too hot, it’s much harder to cool down without using electricity. The key is designing the home to prevent overheating in the first place, by keeping the sun out during the hot parts of a summer’s day and creating air movement through the house.

If you’re the owner’s first point of contact, ensure they’re aware of the importance of this; if the design doesn’t include these features, ask the designer why they’re missing.

Keeping the sun out

Keeping the sun out during summer is all about angles and orientation. You want the lower winter sun to come inside north-facing rooms but to keep the higher summer sun out at its peak. Later in the day, the summer sun will be lower and will shine in western windows – west-facing rooms often overheat in summer.

Design solutions which keep the summer sun out include:

• Deep eaves on the northern side – carefully calculated to avoid summer overheating, but allowing winter sun to penetrate and warm the house.
• Minimise western windows, because the low angle of the summer afternoon sun means eaves won’t stop the home from overheating.
• Shade western windows – this is ideal if west-facing windows are required (eg to access views or outdoor living spaces). Consider deep verandas, awnings, movable shades or tinted glass.

Simple changes to make:

• Plant deciduous trees near the house to shade northern or western windows. Deciduous trees will shade the windows in summer, but with no leaves in winter, will then let the sun in.
• If it’s not possible to shade from the outside, the next best option is for the homeowner to install thick, lined curtains or blinds – as well as keeping the home warm in winter, they help to stop the sun getting in and overheating the house.

Creating air movement

Air movement increases cooling by raising evaporation rates. It can be created through cross and stack ventilation. Cross ventilation happens when cool air from outside enters a building and forces warm air out through an opening such as a window or door. Stack ventilation happens when inside air warms, rises and escapes through any gaps at the top of the house, drawing in cool air from gaps at the bottom of the house. This pull is called the stack effect because it is the same process that draws smoke up a chimney. Using the stack effect to ventilate homes is the most effective way of keeping a house cool in summer.

Design solutions to create air movement include:

• Larger windows opening to the breeze and smaller, higher windows (eg clerestory windows) on the opposite side of the house, which can be left open during summer to take advantage of the stack effect.
• Ensuring windows can be opened on all sides of the house and that there are no barriers to airflow within the house. This could mean including an air grill above internal doors, so that cross ventilation can still occur even if doors are shut. Bedrooms with only one window are frequent victims of overheating – once the bedroom door is shut, there is no pressure gradient to pull cool air through the room.
• Use of side-opening windows rather than awning-opening windows. Side-opening windows are much better at pulling breezes into the house. In-window passive vent systems, such as louvres and trickle vents. These are secure and can be left open when the house is unoccupied. BRANZ recommends 600 mm width of trickle vent for each average-sized room.

• Solar or wind-powered ventilation systems – these are roof-located ventilation systems, which can either run all the time, or be manually controlled. They are best located at a high point in a home, combined with lower opening windows.

“More and more building companies are providing design-build options and, even if you aren’t it’s useful to know why you’re building the way you are

Simple changes to make:

• One of the most effective ways to cool a home in summer is to leave windows open overnight. If there are concerns about security, easy-to-install security stays are available from your local PlaceMakers store.
• One of the most energy-efficient options for air movement is to use fans – either ceiling-mounted or portable fans that can be moved around the house as needed. Running a fan makes the air feel three degrees cooler.

Design is the key to summer cooling, as it is much easier to prevent overheating at an early stage than to rectify it later. More and more building companies are providing design-build options and, even if you aren’t, it’s useful to know why you’re building the way you are. Remember, with good design, homes can be winter warm and summer cool!

For general information: www.yourhome.gov.au/technical/fs46.html

Find out how to calculate eave depth: www.ecowho.com/tools/passive_solar_eaves_calculator.php 

About Beacon Pathway

Beacon Pathway is a group committed to transforming New Zealand’s homes and neighbourhoods through research and demonstration projects that show how to make homes more resource efficient, healthier to live in, adaptable, resilient and affordable.

For further information about Beacon Pathway visit www.beaconpathway.co.nz