The new greenhouse is progressing nicely. Two layers of plastic are up on the hoops. This is a change from the last one which only uses a single layer. The advantage of doubling the plastic is that the space between the layers can be inflated with a small fan. This creates an insulating air space and also maintains tension on the plastic which (in theory) reduces stress on it from wind and other weather.
The south end wall is framed and is now glazed with triple layer polycarbonate, which has decent insulating properties while still letting a good amount of light through. Here we are working on a ventilation window which is now covered with polycarbonate. Overheating is a significant concern and I've decided to skip the roll up sides for this one so it's important to be able to move air through the ends
The north end wall is framed and sheathed with plywood. I'll probably add some rigid foam insulation on the inside here. It's quicker to cover the end with clear plastic but very little light comes in from the north so a semi-insulated wall will provide more frost protection without much of a downside.
I hooked up a fan through a length of dryer ducting to test out the system. Because I'm planning on using a solar panel to power everything, I used a 12 volt computer fan as the inflator instead of a specialized greenhouse fan. It's not perfectly suited to the job but it seems to work just fine. I was able to measure the current draw during my test (about 280 milliamps, or about 3.5 watts) as well, which will help me size the solar system.
Here's the view from inside with the fan running and the polycarbonate in place. The inner layer balloons inwards a bit and the outer layer curves out. It took a minute to inflate enough to be noticeable so at first I thought it wasn't working but once I let it run for a bit it looked great.
Stay tuned in the weeks to come for more updates about the solar panel setup, a passive solar heat absorber, and my thermal battery seedling bench.