Hi...I noticed that no one has been able to give you an answer so far. I use a plastic-covered 2-shelf 'grow rack' on my somewhat enclosed porch, which boasts a VERY cold tile floor (with a concrete slab underneath). To try to minimize the amount of cold that the plants are subjected to, I stack up a couple of old fleece dog mats (donated by my canine) on top of a heavy rubber doormat. (I also use sand-filled trays and a heating cable for one shelf of this set-up.)

My cuttings and seedlings are in plastic trays, so leaking/drainage isn't an issue. Sounds like yours, however, would have drainage holes. Fleece, therefore, probably isn't the best option. Have you considered a good bedding of straw and fall leaves? Maybe 4-6" thick when compressed by your plant trays/pots? Last year (in addition to the porch set-up) I had a couple of very primitive, small 'cold frames' outside (on ground) that I used in late winter to start different kinds of lettuce. I stuffed them with leaves and nestled my pots into the leaves. They thrived, so I guess the system works, at least for plants that don't mind a bit of chill!

If that seems too messy, perhaps you could find one or two wooden pallets for free and use them as a platform above the straw/leaf insulation? Or some other, similar 'platform,' built to fit the size of your cold frame? Just a thought...!

Good luck!

Thank you, Cathi. This is good information. I have completed one of my window well cover cold frames, and it looks great. I think I have decided to put them out on the ground over a hole filled with leaves and grrass cuttings and covered with landscape fabric, instead of putting them on the concrete patio. I think my plants will have a better survival rate that way! Thanks again.

You're welcome! Good luck! One other thought: I wonder if there's any value to using the double-sided silver insulation stuff that comes in rolls from your local big box hardware store? (In my house, I used it to insulate the space behind the pipes of my kitchen sink, to keep them from freezing. Works like a charm.) It's not cheap, but it IS reusable...........You might also post your question on the greenhouse forum. Those folks are passionate about finding inexpensive ways to keep their wintering-over set-ups toasty...........Have fun!

I feel mineral wool insulation is perfect for insulating against cold concrete its resistant to moisture it has a lot of body and will keep cold from creeping in.