I use heavy duty clear vinyl on a 3 x 4 frame hinged on the box with one batten in the middle. I do not have a snow load here. This year I doubled the vinyl to provide better insulation. Double plastic/vinyl will be fogged up most of the time on a cold frame.

My top panels are light enough to use an inexpensive vent opener. Glass, vinyl or plastic will heat the space in a hurry in direct sunshine without venting.

Shadescreen on top will reduce sun exposure. A row cover fabric can be mounted inside for additional frost protection, sun shading and to keep hungry insects out when the top is open.

Plan sufficient battens and slope for snow on top. Use a blanket on top or light bulb inside for the coldest periods.

Don't over analyze it, just do it.
Rick

I would go with glass. All of the materials have the potential to overheat on a warm sunny day, as Rick pointed out. But glass will retain more heat at night than single layer plastic.

Regardless of whatever you use as the "light" for your cold frame, it will need to be vented on sunny days. Eliot Coleman's definitely the expert in this field. His rule of thumb is basically this: When in doubt, vent.

As mentioned in many of the writings by Coleman, he suggests trying to get an automated vent system, if possible. Most people don't have the luxury of being at home to vent as needed. If this is impossible, then just vent by day and cover at night. If you can't at least do this, you probably aren't a good candidate for using a cold frame. Plants are much hardier (in general) when they are protected at least a little bit from the wind, and it is the wind AND cold temperatures that damage most plants. The cold frame can create a very nice microclimate around the plants, even with the vent open during sunny but cold days, and even on cloudy and cold days. Ultimately, you are going to have to try it, and learn as you go. There will be mistakes made, either venting too much or too little. But, usually, venting too much is harder to do than venting too little. The plants will "cook" in the frame much easier than you think. So, err on the side of too much.

My opinion: Do it with whatever you can get your hands on that is the cheapest to get some started. From there you will learn what works and what doesn't, and you can make adjustments as needed moving forward. The best advice I can give is: JUST DO IT! You will learn more that way than you will by reading about what other folks do. Making mistakes is a part of the game. But, once you've made the mistake, the remedy will be easier for you to understand.

Speaking with no experience here (other than having read Coleman's book), I would take a look at the thin Plexiglas/Lexan sheets available at Home Depot/similar. They'll be light enough that lifting the tops won't be too challenging, and the material itself is pretty sturdy.

The only thing I'm not sure about is how well it'll hold up to UV over extended periods of time - not a minor consideration.

Okay, my area has recently been moved to zone 6 from zone 5 (ah, global warming...). But you are in zone 7, and I can confidently say that you are really over-thinking (and over-engineering) this.

I have to deal with a lot of sunlight during the winter, so overheating is a major consideration. Eliot Coleman tried heavy row covers as the inside layer (high tunnel with greenhouse plastic as his outside layer) and found the heavy row cover fabric blocked light from his growing plants. He's in Maine, lots of clouds in the winter, and fruther north than I am in New Mexico. So I went with the heavy row cover fabric as my outer layer on low tunnels (10 ft pvc pipes bent over 3 ft wide rows), and light row covers over 5 ft arches of concrete re-mesh as my inner layer. At Thanksgiving I spread 6 inches of leaves over the plants (carrots, radishes, and turnips). Last week I harvested some of all 3, and although a few radish leaves at the edges were frosted, the roots were fine.

Kale (and cabbage and Brussels sprouts, if I were growing them this year) is in a bed protected from deer by re-mesh arches and mulched with 6 inches of leaves, but otherwise is not protected (deer LOVE cabbage-family plants). Likewise spinach and miner's lettuce are not under row covers, but are mulched with pine straw. When temps get down to the low teens and singles I'll pull a row cover over them, too.

I don't have any lettuce or chard in the winter garden this year, but have had it in the past. Put them in with the turnips and radishes, as they are more tender than spinach or kale. In zone 7 they should last all winter. I used to give up on lettuce around Thanksgiving, but have found that deep mulch keeps the mature lettuce around for a bit longer even in my simple tunnel. I was also advised to just pull my Thanksgiving lettuce up by the roots, cut the lettuce about an inch above the crown, and then replant the roots in a pot on my kitchen windowsill. I never have the patience to wait until it grows a whole head, but I have a few leaves to put in sandwiches every week for a couple of months.

Keep it simple, keep it cheap, keep it fun.

Catherine

A friend uses a 30 gallon tub, with glass over it to start his tomatoes 4-6 weeks early, as a vent, he put a writing pencil under the edge of the glass. He has raised tomatoes this way for over 28 years, without any problem.
There are some really good post here.
The best is Just do it!

MIne have always been recycled storm windows. They work great and the box set up isso easy anyone can build one. I forgot so say that it works great, been growing greens for several years now no heat and it works through the coldest of days

I had bad luck with the plexiglass sheets. I had to replace two of the three after the first year and another the 2nd year (2 cracked with normal opening and closing, 1 cracked as a result of my son sitting on it). I now just have them covered with thick painters plastic, but a hoping to find a more insulated solution by fall.

Here is a link that might be useful: my coldframe pictures