I do not have a greenhouse, but I am looking at 6mm polycore as a possible material. Clear glass does not diffuse the light as do the synthetics.

Anything that lets in good light and keeps out cold is good for a greenhouse. If you have salvedged wondows(doulbe paned is best)use them, or put plastic over them to create a dead air space.

If however you are buying stuff for a greenhouse go with polycarbonite panels. They have a Uv film on them and are made for greenhouses. Don't forget to add closable vents

Hi I used to work several years at a greenhouse was the R&D guy /grower for about 50,000 sqft plastic and 2 large glass houses..which were changed to lexan.
Yes the double poly is best and you could dig down 2 feet or so and put ridged insulation around the perimeter. A lot of heat loss comes from there. Inflate with outside air. Some put an inside ceiling over the plants at night or a thermal covering over the entire greenhouse at night. During the day if it is getting to hot shade it from the outside..that works best. Also note that small greenhouses are not the best cause the air change is to fast. So be aware of that and regulate accordingly. Hope this helps you out : )