I always thought that cold frames were just to get an early start not to garden all winter. Some people do heat them but I think it takes all summer to set it up.

6 mil clear poly plastic or row cover is only effective to 28 degrees. Unfortunately, heat will be needed for growing through the winter depending on where you're located.

Eliot Coleman says that's normal. Every morning when he gets into his cold house, all the veggies are frozen thigh. They unfreeze around 10AM when the sun has been hitting the house for a moment.

If you want to do this kind of setup, you need really cold hardy plants that be lightly frozen and unfrozen without too much damages, like kale, spinach, etc.

I had a cold frame similar to your last year, a lot of air could get in. In autumn it didn't help a lot to extend the season BUT in february (zone 5b) it was already unfrozen and in march I was already growing in it. For me, it was more useful in spring than autumn.

I had a cold frame for years in my youth. I seem to recall the different terminologies of 'hotbed' versus 'coldframe'. The old timers in my youth often had 'hotbeds' and started up their pepper and tomato plants outdoors. These often had a light bulb inside to heat them on cold nights or some claimed putting in fresh horse manure on the bottom gave off enough heat from the decomposition process to keep their tender plants alive in cold spring weather.

We had a cold frame made from plywood sides that angled to the south with the shorter end on the south and the taller end on the North. On top was an old screen door that had both glass storm panels in place. This door was hinged to the wood part of the frame and could be opened to various settings to regulate the heat buildup or be closed entirely. The door mechanism to regulate height open was a simple chain that was attached to the door at one end and attached to a stake in front of the coldframe that had nails pounded in it at various heights. All manual on the controls, but in those days mother did not work outside the home so she had the job of checking the thermometer inside the coldframe and adjusting the door during the day.

I recall more than once having to put a canvas tarp over the whole thing on some very cold nights or bringing in the plants for the night if we thought it would get too cold (Mother was not too happy to share the living room temporarily with the vegetable plants!).

Coldframes are ideal and should be used more. We got great stout bushy transplants ideal for our garden when grown under cool conditions. Way better quality than the leggy tomato plants one sees for sale these days that are grown in a hot greenhouse. It was a great way for a grade school boy to make some cash growing and selling vegetable transplants to the local neighbors too.