Best floor for a greenhouse?

I just scored 13 windows which will allow me to extend my covered porch into a greenhouse/sunroom with raised beds (no gardening on the ground for me). My question is what do I do about a floor? My concerns are as follows:

1. I want to do this as low-impact and cheap as possible while achieving my objectives (not interested in portland unless it's a small amount)

2. The ground beyond the porch slab is relatively level but not flat so I want to create a more flat/level floor and eliminate low spots where water can collect

3. It gets really cold here and I want to insulate the greenhouse from the cold ground in hopes of needing less energy to heat it since we want to raise microgreens in cold weather

4. We're considering housing our chickens in one end in winter and don't want predators to burrow under

5. Our property is underlain by shale so no great amount of excavation can be done, though I do believe I can dig down to the shale and sink anchor bolts into it for post/pier if necessary

If anyone has ideas that take all my requirements into consideration, I'd really appreciate some input.

I wouldn't want any floor except the ground in a greenhouse.  If you are concerned about insulating from ground temperatures and stopping burrowing predators, I would dig a foundation 3 or 4 feet deep, or better yet, until you hit that shale, put cement or rocks or metal roofing to keep the borrowers out, and insulate up and down next to my predator stopping foundation, rather than insulating the floor.  

The floor of my greenhouse is decomposed granite (non-graded sand). I needed to make my greenhouse ground squirrel proof, so I put down hardware cloth. But the weeds started growing through the hardware cloth, so I put down cardboard. And then, I kept tripping over the cardboard and hardware cloth, so I topped the whole thing with decomposed granite, so much nicer.

i used gravel from the local quarry. it drains well. retains some heat for over the night. i put in about 6" but at $20 a ton its pretty affordable.