So, I was having trouble with the greenhouse recently. After finishing construction, I set up a thermometer in there to get readings on temperature. The following week, we had some temperatures between 15-24°C and the greenhouse (before the vents and door were installed) was getting up to about 30-35° with the door space and vent spaces completely wide open.
Since installing doors and windows, I was checking for overnight temperatures. They were dropping to match outdoor temperatures. I put in the weatherstripping to hopefully get a better seal and it was still dropping to outdoor overnight temperatures. (I did get the greenhouse up to 38° in daytime when outdoor temperature was about 8 on a sunny day. So the greenhouse certainly has what it takes to get warm.
Finally I realized that I have to get the greenhouse full of stuff that will absorb the heat. Air convection is an inefficient heating method, and the poly greenhouse cover is not going to sufficiently insulate the indoors and buffer the inner atmosphere from the outdoors at night. Unfortunately, I have been at work from 8:00am until about 9:00pm this week, so I don't have time or daylight to get anything done out there. I usually cram in about 10 minutes of tinkering in the morning before leaving.
But thanks to the long weekend, I was able to get some time this afternoon after taking off from work early to put a couple hours into adjusted the hinges on the windows, and I spent some time planting some of my perennials into pots. The pots then went into the greenhouse. I also took some bags of grass clippings I collected this week, dumped them into giant tree containers I have and created 3 or 4 mini compost bins in the greenhouse. I covered them with old compost to give them a rich black surface. I also put in a container full of water.
The decomposition of the grass, thatch and leaves will cause a lot of microbial activity that will heat the bins up to between 45-70°C (topping out around 160°F). The bulk of soil will also absorb heat and hold it in the atmosphere of the greenhouse overnight, I hope. Same with the perennials potted. The should soak up lots of sunlight in the dark black pots and rich dark soil, and hopefully maintain some warmth 24-hours.
The water should smooth out rough edges. As the greenhouse heats up over 30° the water will be drawn into the air and the humidity will help refract some light, protect and moisten plant leaves, and slow down further heating up. As the greenhouse cools, the bulk of water should take considerable time to condense out of the air, and it will coat the cover to further insulate it. This is in theory, at least.
