Australian Passive Solar Home

One thing we’re discovering with our passive solar home in summer is that the overnight temperatures inside the house are often cooler than I expected. But why is that? Wouldn’t you expect the inside temperatures to be, if anything, warmer than expected?

It’s all because of the thermal mass – and how you cool it.

As covered in detail in this blog, in summer the thermal mass absorbs heat, keeping the interior of the house cooler. In most passive solar homes, that thermal mass is provided by the concrete floor slab. To get rid of that stored heat, and so recharge the slab for the next day with ‘coolth’, the house is opened at night so that the cooler air can pass through the house.

Rather neatly, in some climates, the hotter the day, the more likely the sky will be clear that night (that’s why the day was hot – cloudless skies), giving low overnight temperatures. OK, you’re saying, that’s good – low overnight temperatures mean you can better cool the slab. That is indeed the case – but how low in temperature do you want to go inside your house on a summer night?

Currently in our summer, overnight outside minimum temps after a hot day (e.g. 35°C) are as low as 12°C. Fully opening the house means in turn that the interior temperatures drop to about 18°C. That’s cool enough that you need to put on more clothes in the evening!

Now it so happens that Georgina and I relish opening the house and having the cool air blow through after a hot day – but you may not.

Opening the house to a lesser degree, so that for example 20°C interior temperature is held all night, obviously gives a warmer interior. But if the next day is going to be hot, doing this also means the thermal mass may not be cooled sufficiently to give adequate hot-day performance.

In other words, the trade-off is lower evening and night temperatures for lower daytime peak temperatures.

Remember, this is all with zero mechanical and electric cooling systems operating. If you wished, it would be easy to let the house drop to only (say) 20°C overnight and then use the air conditioner the next day, probably for just a few hours in the late afternoon. So it’s all within the choices you make.

But as I say, we love the cool night air blowing through the house – it feels so refreshing after a hot day. We also don’t mind cooler conditions for sleeping, or even putting on a lightweight extra layer of clothing in the evenings. But that’s us – it’s something for you to keep in mind if you’re thinking of a passive solar home, especially in climates with high diurnal (night/day) temperature ranges in summer.