Australian Passive Solar Home

Thermal mass in a passive solar home absorbs interior heat during a hot summer’s day, keeping the home cooler. At night, the house is opened to get rid of this stored heat – a process called night flush cooling. But how does night flush cooling work in practice?

In our first summer of using night flush cooling, we learned quite a lot. What we learned has implications for both home design and operation.  

• Time of house opening

During hot spells, we watched the temperature display and opened the house at night only when the outside temperature dropped below the inside temperature.

In the 2025-2026 summer, the time this occurred varied from 7.30pm until 11pm (daylight saving time), the latter occurring twice. Opening the house at 8.30 – 9pm was most common. These times are later than we experienced in our old house in the same location, as the old house was typically warmer inside, and so outside temperature equalled inside temperature earlier in the evening.

To put this another way, the cooler a house stays in summer, the later it will need to be opened as night temperature falls. If you are someone who always goes to bed early, keep this in mind!

• Degree of openness

Depending on the conditions, different ventilation strategies were employed. The table below shows the approaches.

Airflow amount	Strategy
From minimum (top) to maximum (bottom)	Windows open
	Windows and ceiling hatches open
	Windows, sliding doors and ceiling hatches open
	Windows, sliding doors and ceiling hatches open, ventilator set to half power
	Windows, doors and ceiling hatches open, ventilator set to full power

In addition, the amount of airflow through the house heavily depended on wind speed. The higher the outside wind speed, the less the house needed to be opened for the same air throughput.

• Duration of exposure

If the outside temperature cooled earlier in the evening, and so the house was able to be opened earlier, the number of hours the thermal mass was exposed to low temperature airflow increased. In this situation, the house could be opened a reduced amount (e.g. only the windows opened, not the doors and windows) to achieve the same reduction in thermal mass temperature.

This effect is significant: the duration that the thermal mass was exposed to cooling airflow could vary by more than 35 per cent, for example from 8 hours (11pm to 7am) to 11 hours (8pm – 7am).

• When to close-up

As stated, the house was opened when night outside temperature fell below inside temperature. But when should the house be closed in the morning?

We chose to close the house a little earlier than you would if basing the decision solely on temperatures. This is because for a short period very early in the morning, the eastern sliding door was exposed to summer sun. Closing this door allowed the net curtains to be drawn, so decreasing direct solar gain. Rather than close this door first, and then only later close the rest of the house, we chose to do it all at the same time.    

• Doors versus windows

The effectiveness of the night flush cooling was much greater with open sliding doors versus open awning windows. This is because of the doors’ greater opening area, and because the airflow through the doors better ‘wetted’ (flowed over) the floor.

In our house I don’t think that the thermal mass would have been able to sufficiently cooled with open windows alone – something to keep in mind when you are designing your own home.  

• Different rooms

One aspect that hadn’t occurred to me until we operated the house in summer was the fact that not all of the house needed to be opened at the same time for night flush cooling.

For example, if we were in the loungeroom, and a gusty evening cool change occurred, the lounge room doors and windows could stay closed while other rooms were opened. That left the warm ‘snug’ in the lounge room while allowing the slab to be cooled elsewhere. (The same could be employed in a bedroom, for example, where it was wished to be kept warmer.) In our case, when we went to bed, the lounge room windows and sliding doors were then opened. 

Obviously, the larger the area of the slab (and other thermal mass) exposed to cooling airflow, the better from a summer energy perspective. However, through conduction, all parts of the slab will lower in temperature over a period of hours (even those parts not exposed to the airflow), so cooling any part of the slab is still advantageous.  

• Water tank cooling

The internal water tanks were much more effectively cooled if there were gusty breezes through the house. While the slab, too, responded well to these conditions, the water tanks were much slower to decrease in temperature when there was only slow air movement around them.

• Source air for night flush cooling

The source air for night flush cooling may vary in temperature. For example, the air entering the house through our southern windows was measurably cooler (e.g. 1-2°C) than that entering through northern windows. I assume that this is because the southern side of the house was better shaded than the north, and so the ground was cooler. Therefore, when maximum cooling of thermal mass is needed, it may be worthwhile opening southern windows in a house first.

Certainly, being able to source night cooling air from the south is important, so keep this in mind when planning your house.

• Night minima are very important

As the summer progressed, a pattern could be observed in our house performance. If the overnight minimum was 18°C or lower for night flush cooling, the house interior temperature did not exceed 25°C the next day, irrespective of how high outside temperature rose (even 40+°C). However, any overnight minimum above 18° – even 19°C – meant that this was not achievable.

This specific temperature relationship applies to this house and may well not apply to others. However, what is clearly important is that when designing a passive solar house, data for overnight minima in summer should be carefully analysed. Looking at the highest overnight minima, and how often that occurs, will show you when a house is likely to be challenged in night flush cooling summer performance.

• Time and effort

If you wish to operate the house at optimal levels in summer, time and effort are needed. Opening the windows too early in the evening will cause interior temperatures to initially rise rather than fall. Not opening the windows at all for night flush cooling (e.g. because you’re away that night) will impact house performance over the next few days. Closing the house early (e.g. because you have an early morning appointment) will reduce the amount of night flush cooling the thermal mass receives.

A few points about this.

First, unless the conditions are extreme, doing these things will make little difference: a well-designed passive solar home is resilient enough to cope with these actions for the great majority of the time.

Second, summer is the season when you, as the homeowner and operator, will need to do the most. In other seasons, the extra workload of operating a passive solar is essentially zero – you’ll just need to open and shut curtains daily.

Finally, opening and closing the house comprises some of the philosophical investment in working with the house and climate – of being part of the outside world rather than simply excluding it. If you feel rewarded by watching the house work with the climate, it will be worth it. But if you feel it’s all a bloody chore, a passive solar home is probably not for you!