Australian Passive Solar Home

When talk turns to energy efficient housing, I often see people mentioning ‘resilience’. The way the word is used varies, but one example is the maintaining of safe interior house conditions in – say – a heatwave when the power fails.

Death rates rise in heatwaves. Houses relying on electric cooling (air conditioners) will have worse conditions if power is not available. The longer that power failure occurs, the greater the potential for dangerous conditions.

But here’s where it gets interesting. More and more people are assessing home energy efficiency on the basis of the external energy consumed to maintain a house at a comfortable temperature. For example, they may say that the utility bill was tiny for cooling, because on-site solar power was being used. Other people show logged interior home temperatures, never mentioning that cooling systems are operating that whole time!

So what happens in a blackout? Most solar cell systems, even those with home batteries, will not run the house. Even those that do will not usually able to run an air conditioner for long periods.

In that situation you’re basically relying on the passive abilities of the house to maintain a good interior temperature. It’s one reason I look askance at energy efficient home approaches that rely on electrical cooling as an intrinsic part of their operation in hot weather.

The most important way in which a house can maintain liveable temperatures in extreme conditions without electrical power, is to have the combination of insulation and thermal mass. Let’s take hot weather. If the house has the internal ability to absorb heat in the thermal mass, and to equally reduce the transmission into the house of external heat, then it will stay cool for quite a long period with no power. How long? 3-5 days are examples. Opening the house during the cooler nights to reduce the temperature of the thermal mass means those 3-5 days can be extended almost indefinitely.

The next time someone talks about resilience, ask how their energy efficient home design will cope in heatwaves with no electrical power. If using the air conditioner is an intrinsic part of operating the house, the answer is probably ‘not very well’.