I had posted my setup last year on the citrus growers forum. I've made some minor improvements and so I figured I would give an update.
The original project was to make use of storage space above my garage. It already needed a new roof so when replacing shingles I replaced an entire face with 8mm dual-wall polycarbonate.
The southern face of 'B' section is what became the greenhouse
This is how it looked before
I added better subfloor as the existing floorboards were barely strong enough to walk on
Work in progress
Glazing complete
Painted the floor white to maximize light
At this point it was already getting late in the season and I had to move the plants in while I worked.
Insulation - fiberglass batts covered with reflective polyisocyanate foam board. Under the floor I also have fiberglass batts.
Added a door, cut to size and covered with foam insulation. Doubled up the 2x4s on the walls to add depth for insulation
Added remaining wall insulation.
A nice 1F (-17C) winter day...
...with huge growth flushes
This was a year ago. Everything has worked out beautifully. I was able to keep it heated with two 1500w space heaters with built-in thermostats. I had to use propane on below 0F days but I realized come springtime that what I thought was an on/off switch on the heaters was actually low/hi power and I think I was running at half power all winter! So hopefully I won't need propane at all this year unless there is a power outage. I try to keep it at 55F but often it drops into the 40s on cold nights. Conveniently my local power company just started offering lower off-peak electric rates so it isn't as expensive to heat as I had feared. From walking around feeling for cold spots on a freezing day it seems like the polycarbonate face is clearly where the heat is being lost. If I did it again I think I would use 10mm or more.
Condensation has not been an issue at all. I have yet to see any. I think the heaters keep it dry. The biggest problem was watering. I had set up a slanted bench for watering trees, where runoff water was directed into a pipe that drained outside. The problem was that the bench could only hold a few trees at a time, and it took hours before they stopped dripping water. This meant I could only water a few trees a day and I had to rotate them constantly on and off the bench. I now have twice as many trees so this would be even more impractical.
My solution this year was to add vinyl flooring and a floor drain. Because the floor sags very slightly, water tends to flow towards the center. I dumped a bucket of water on the floor to see where the lowest point was and installed a drain there, with a pipe flowing outside. I also added an extra support post in the center of the garage to ensure the added weight isn't an issue. Now I just leave the containers on the floor and let the water spill out and find its way to the drain.
It was getting dark out but here's a picture from today
