Here in Iowa (Zone 5, with temp. dips to -20 F) I need some heat to keep my cold frame going in winter. My frame has plywood walls, and a plastic covered frame of 1x3s for the top. (Details in my article here: http://www.bhg.com/gardening/landscaping-projects/garden-structures/building-a-simple-cold-frame/) The cold frame sits on my deck, next to the back door. I used Styrofoam insulation on the sides and bottom, and made an insulating cover from old vinyl tablecloths and more Styrofoam. For heat, I use holiday lights -- three 100-bulb strings. With the insulated cover in place, the lights keep the temperature in the frame about 40 degrees above outside temperature at night. One point that's critical for newbys when it comes to cold frames: you need to learn how your frame reacts to changes in temperature, sunshine, wind, etc. And without an automatic venting system (or someone to check it regularly), it's very easy to fry a whole frameful of seedlings in about 30 minutes. Believe me, I've done it.