Old windows have often been used to build cold frames. The frame will cool at night but the soil will retain some heat, which is beneficial. Inside the frame will be warmer than without the cold frames.

The flip side of the coin is that overheating can occur on sunny days. You will need a way to ventilate when that happens. Automatic sash openers for this purpose are sold by garden supply stores.

Jim

six old windows? your coldframe is going to be deluxe. Are they double or single pane? If you dig really deep, and somehow fill the bottom 10-12 inches with water containers (milk jugs for ex.), it would have to freeze very hard for your seedlings to suffer (in the teens). Likewise if you throw a tarp over the coldframe.

It was 103F in my low tunnel today before I could get outside to open it up... the outside temp was around 37F at the time!

Just make sure to vent it on clear days. I would think glass would heat up even more then 3 mil plastic.

If you have electricity, controlling temp. in cold frame is very easy. All you need are: a simple thermostat,and a bathroom fan. You have to provide flaps/dampers for air inlet and outlets, so when the fan is not working, they are shut.
About OP's question. Windows can make excellent cold frames. Most windows are double glass so they will lose heat much slowly.

You want to build your coldframe around your window cover dimensions. Minimize the air leaks, have something like an old sleeping bag to throw on top on really cold nights and make sure it is at least partially open on any day going above ~50 to 55 F and you'll do well.

Oooh good thread, I just picked up a similar amount of windows off of freecycle with the same intentions!

Well, that sounds encouraging that I can use it for the purpose I want it for. Lots of good ideas for how to keep it warm and not too warm.

I guess I was hoping someone might have a source of directions for how to construct one, using the windows. Maybe it would help if I post photos of the windows and the 4x4 beds to give a better idea. I can't today, but I'll try to come back tomorrow with photos.

for ideas/pictures - Just google "cold frame picture" and "cold frame project" - you will get plenty of pictures

http://www.woodworkersworkshop.com/resources/index.php?cat=388

and do search on this website "cold frame" - lots of discussions with project pictures that people have done

Thermopane glass that has the low-e coating is not ideal for plants. Most does have it. I go for single-glaze glass.

A cold frame can be super-simple or as complicated as you want. The simplest is make a box out of whatever you have, 1x or 2x boards wide as possible. Make the outside demension of the box the same as one or more sash. Make up a growing bed and place the box on it sloping somewhat toward south, 10 degrees or so, just push the south end down and prop up the north end and heap up soil and pile mulch on the north side for insulation. If you don't have hinges just set the sash on the box. On sunny days around mid-morning take the sash off, put them back on mid-afternoon. Cloudy days leave them on or put a block under the sash to ventilate it a little.

Works just fine. Take you about 30 minutes.

I found a complete sliding glass door in someones garbage. Ended up ripping the glass panes out of it and keep them.. I still haven't gotten around to taking them apart (2 panes in each panel)... Very heavy glass. Always on the lookout for free junk.

Sliding glass doors are od shaped and heavy.
PLUS, you CANNOT cut them to any size (tempered), they will just shatter. I have a few of them.

And they are undoubtedly coated with low-e.

jollyrd, I often forget to just check google first and do a GW search beyond scanning the top page. I see there are lots of information on the subject. Thanks for the woodworkers link too.

These windows are 8 panes that seem to be single pane uncoated glass. I plan on sanding and painting them black before working with them. Here are the photos of one of the windows and the 4x4 beds that I'll try to use them with....

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The electric fan ventilation mentioned above by sysonn would be a pretty easy and effective way to prevent overheating.

Jim