Regulating Temperature

Post date: 06-Feb-2015 04:12:02

My aim in expanding my original tiny canopy to one 18 m diameter was to give much more growing space and also to effectively shift my climatic zone from Subtropical (Sunshine Coast) to Tropical (Cairns). I really want to be able to grow my own drinking coconuts in Maleny, which is currently impossible. (I regularly get -6°C overnight lows in winter - or used to at least) Ideally, the temperature should not get below 14 or 15 °C for coconuts to produce well.

I finished the greenhouse canopy in early July 2014 and winter in the LifeBubble was wonderful. The only issue was that condensation forms on the inside of the canopy overnight and on cold sunny days and so it would "rain" on the "garden beds" yes I mean real beds you sleep in! The heat was also too intense on sunny days leading up to summer so I made up some waterproof shade sails and put them over the beds and this did the trick. I am now making another one for my new kitchen area. I measured the temperature of a concrete path surface at 68 °C during a heat wave in November and soil surface temperatures were similar. The air temperatures were as high or higher than as the outside temperature of 42 °C and it was humid as well. It was time for some emergency modifications!

First I cut a much bigger (3.6m) vent hole in the top of the canopy and monitored for a while. Amazingly it didn't make much difference to the ventilation. Next, I cut a larger 50% shade cloth vent panels on either side of the doors giving about 16 m² of vent area down low. This made much more of a difference to the airflow and cooling than the upper hole, which really surprised me. I was sure that since hot air rises, the upper vent hole would be most influential. I'm not sure if this will need to have ETFE roll up the film on it during winter to keep the heat in better in the cold times or not. That is something I will need to monitor closely during June - August. So far it has worked really well during the summer. It also prompts the question about whether you really need that much plastic or glass directly above the LifeBubble to create a significant greenhouse effect. I remember reading an interesting article where Indian researchers demonstrated that simply providing bamboo poles with plastic or even hessian cloth to reduce wind provides many of the benefits of a greenhouse including increased CO₂.

The next measure I took to moderate the heat is a compromise. Ideally for best-growing conditions you need high light levels, but the November- December heat was too hot to be comfortable, so I rigged up some 70% white shade-cloth. White shade-cloth allows more diffuse light to pass through for the plants but has also reduced the heat extremes during the noon to 14:00 hottest part of the day on sunny days. Until more trees and my coconut palms grow and provide more natural shade I think this has been a good compromise. The papayas under the shade don't appear to be much less productive and importantly don't get sunburnt leaves now and the pineapples also seem to be benefiting from the shade.

The final step I took was also necessary for irrigation anyway, but by timing the applications for the hottest part of the day, the overhead sprinklers do a fantastic job of really quickly dropping the temperature. The fact that the shade-cloth holds quite a bit of moisture the evaporative cooling effect seems to last for an hour or so after a 5-minute application.

The most common question I get is around high humidity and mould growth. Irrigating to cool the LifeBubble would logically tend to increase mould problems. However, I'm really impressed with the lack of mould in the earth-sheltered ferro-cement dome. There just isn't any, even on timber surfaces and furniture and the bedding is also dry and comfortable. Conventional houses here in Maleny do have problems with mould growing on the plaster walls and furniture during summer but in the LifeBubble I have far less impact and this may be because the warmer surface temperatures mean less dew point condensation on the surfaces. It would be interesting to investigate the actual how and why of this counter-intuitive outcome.