Because some US friends in a Facebook group who have been hit by unusually cold weather were asking, I took some video footage I had and made a video of how we heat our greenhouse (without electricity) in the winter.
So did the insulation and clay pot heaters end up helping?
Our greenhouse is very small, about 5 square meters or 50 square feet. It was very cheap and isn’t well insulated, so I add a layer of bubble wrap (also fairly thin because I couldn’t source anything more substantial).
Temperature is measured by a hanging digital thermometer in a corner nowhere near the clay pots – about 30cm or a foot off the ground (above the leaf celery). Heat does still rise to the top of the greenhouse, but the thermometer is at plant level. In addition, I have a couple of gallons of water in watering cans that store some heat, but aren’t much help if temperatures drop well below freezing or it stays below freezing for too long.
I don’t put the clay pots directly next to the plants because that would be too hot.
We are theoretically frost hardiness zone 8b, but we’ve had a couple of nights with temperatures that were more like zone 7b…
When we had -11.8°C outside (10.8°F), the greenhouse was at -6.1°C (21°F)
When we had -13.1°C outside (8.4°F), the greenhouse was at -7°C (19.4°F)
So basically, the heating and insulation in the greenhouse raised the temperatures by approximately 6°C or 10°F, which may have just saved our cauliflower – which is frost hardy to an extent but would definitely not have survived the temperatures we had outside…
With only insulation, when temperatures dip to just below zero or unexpectedly dip too low and I forget the candles, the temperature in the greenhouse gets to be almost as low as outside – a maximum of 1°C / 1.8°F higher. That all depends on whether there was sun and it was able to heat up during the day.