Tonitime,
I recently built two coldframes and also lined them with 3/4" foamboard and used flat latex pure white. According to several references on reflecting surfaces, it returns roughly 90 % of the light. Glossy white paint was not as good! It's readily available, cheap, and easy to apply. I would fasten it with large headed nails such as roofing nails as mine have heaved up (Ijust wedged them in snugly against each other) above the frame interferring with the lid closing. I constructed the sides out of 2x8 cedar boards i removed from an old deck. The lid is an old double paned patio door I also removed from this house I remodelled. I added angle iron with weights on the end to help counter-balance the heavy door. I can open them with one pinky! I started my second crop of spinach in one of them but no seedlings yet. Salads and radishes next!
Herbert

thanks for your reply, Herbert! I appreciate hearing of fastening the foamboard down .....i'd not have thought of the freeze heaving the insulation ( not being native to a colder climate).
I am thinking of making my frame out of cinderblocks....am wondering what sort of adhesive i could use in lieu of nails??
If anyone has an idea of this, i would love to hear it also!
Toni

Toni,
The blocks sound good - lo-tech and cheap. And easily constructed. I like your ideas! To fasten the foamboards, use construction adhesive that fits in caulking guns and glob on dabs every 24 inches or so. Push the foam against the block squishing the adhesive. Don't put too much adhesive on in case you want to dismantle it and move everything. I have a problem with moles and voles tunneling under the frame and setting up house inside what they surely percieve to be the new heated mall and supermarket in the neighborhood. I got some hardware cloth (heavy screening that comes in various sizes) and attempted to block them from getting in but got sloppy and they've set up shop anyway. When I redo my soil in the frames I'll form a barrier that hopefully will stop them. Just a reminder, temps will soar when you leave the lid down during a sunny day and plummet when you forget to close it at night. I'm working on an automatic opener similar to what greenhouses use to vent. I've already ruined several planthings thru my neglect. I have too many pursuits to keep track of. What are you planting? I'm ordering some weird stuff like tatsoi and mustards to try. Having edible stuff in the cold weather is the best!
Herbert