Can you get under the base on the outside, just a wee bit? If so, you could lay out some heavy plastic, tuck it just under the base, then bring it up and around perimeter of the GH and secure it with tape, or a combination of tape and cords.

My origninal GH plans called for a wood framed structure, with the north end as a seperate, heated, insulated space, with a door leading to the actual GH. Alas, it was not to be, and while I ended up with a very nice GH, I know exactly why you want a separate, insulated space for potting, storage and general puttering, in the warm and dry in winter and possibly even AC'd in summer.

If your GH could be attached to another structure, even if it's just a wood storage shed that you could insulate and heat and perhaps, retrofit with a few small windows, you might get what you want.

But for this winter, if you can't get under your base, you can still lay down plastic, leaving enough at the top to overwrap bales of straw, to surround the GH. If you keep the straw dry through the winter, it will be good for another round, or use it to mulch your beds in spring. Sprinkle some moth repellant flakes on each bale before you wrap it though, or small critters, especially mice, will see it as an invitation to move in.

Straw bales are among the best of all foundation insulators, but they do have that drawback: they must be kept dry, or they will rot and by spring, stink, and unless defended by a replellant, they will attract small rodents, followed by their slithering predatators.

Another option would be thick foam insulation, wrapped in the plastic, then secured to the GH base. Either way; if you can get the bottom foot or two sealed and insulated, it would make a large difference in keeping the GH warm through the cold of winter.

I've no idea of the size of your greenhouse, but a heavy-duty foil-faced bubble-wrap type material works well for me in my little 6x8 HF. We installed hardware cloth around the perimeter to exclude burrowing critters; it's on the inside, covering the ground for several inches from the wall then runs up and is secured to the wood foundation. The foil-faced insulation material is installed on top of that, and all is covered with the gravel.

I have a 3 tiered grow light shelving unit in my greenhouse. I have heat mats laid on the shelves and have used clear heavy gauge plastic to make a cover for it. The door opening is generous enough that it can be pulled back to either side when not needed. But when the temps are extra low and I want to make sure my seedlings are warm and protected, I can pull the flaps shut. The heat mats provide the heat and there is more humidity in there too.
I can even move some of my less hardy cactus, tropicals in there too if I'm worried about temps in the main part of the GH overnight.

Before I had the 3 shelf unit, I used a homemade bench with a hardware cloth bottom and some flexible plastic pipe hooped on the ends to support the plastic sheeting. Just like having an extra little hoop house in your GH. A small heater was placed under the bench to keep things warm.

Where can you find the foil faced bubble wrap material? Sure appreciate all these ideas.

Teresa

Home Depot or Lowes has had before, Reflective Radiant insulation in the insulation ailes.
~tom

I ordered mine from Charley's Greenhouse Supply, but you might well find a similar product for less at HD or Lowes. It always pays to shop around ... when you have the time. Link is attached below for your reference.

Here is a link that might be useful: Reflectix Insulation

Farmtek has foil+bubble wrap reflective insulation of various types. It's called Tekfoil. I haven't priced it compared to other products or sources, but I get a lot of my GH supplies from them. Those of you with smaller houses may benefit from their remnant pieces of shade cloth and such.

I plan on enclosing a small area this winter for seed starting . I think I'll just make a tent out of clear plastic/greenhouse film and put a heater inside. Last year I had good luck with putting a piece of a floating row cover over the seed flat. (I had more flats than plastic domes, and it kept the flats from drying out prematurely).

Here is a link that might be useful: Farmtek