I bought a thermometer yesterday, because the old one constantly showed 25°C, which I doubted.
It showed a considerably lower temperature than 25°C. I logged the temperature for some hours manually and experimented with the heating and putting a glass of hot water into the green house. The lowest temperature was 20.4°C and the highest 25.1°C. It would be best to have between 22 and 25, though.
Logging manually is quite annoying, taking the temperature every 10 mins. I wish I already had a Raspberry Pi, because it can do that for you.
Insulation with Rubber Foam
This morning, I started logging again, nonetheless. I had put some rubber foam between the window and the green house in case the windows aren’t completely airtight. I had already thought about getting a heat mat, because my impression was that the temperature was too low on average. But as my calculations showed, it was 22,7°C on average yesterday night. So maybe no need for a heat mat after all.
With the rubber foam and not using the glass of hot water, the result was a bit more even and barely left the ideal zone of between 22 and 25°C:
The lowest temperature was 21.4°C and the highest 24.1°C with an average of 22.9°C.
Bottom Insulation with Board
I have noticed that the window sill (made of some kind of stone) gets really cold when the heating isn’t very strong. So I now put a board underneath the green house to isolate it a bit better. But of course, that isolates into both directions. I would need a semi-permeable material. However, it seems that the temperature already fluctuates less:
The lowerst temperature in this setting was 21.3°C and the highest 23.3°C with an average of 22.4°C.
However, one must not forget to take in account that the sun isn’t shining today. If it was, it would heat the greenhouse even more and I would need the heating less, which heats periodically rather than being at the same temperature at all times. Which is clearly visible from the diagrams.
Using “solar” energy
I googled on how to keep a greenhouse warm, but only found tips on “proper” greenhouses, not ones for window sills. However, I found one solution, which I could modify. I will get some black film canisters, fill them with water and let them collect warmth during the day. I will check whether it is of any effect later.
Another idea was to use Christmas lights, because they emit warmth as well. But I would need electricity for that. I like the idea with the film canisters more. It’s a greener way.