My greenhouse and sunroom both heat up 50F or more above ambient on sunny winter days. Yours sound much smaller so many not heat up that much. But with sunshine anything above freezing could mean a warm structure.

Also add 50F to a warm day say 70F and you are at 120F. Do you have a plan when that happens?

I use a combination of cold frame and greenhouse now. I started out with cold frames which were basicly a frame covered with heavy guage plastic. I can tell you that it can be 2 degrees below zero and be 80 in the greenhouse if the sun is shining and I'll have my vents open. I have 2 mini-max thermometers in my greenhouse.

It doesn't matter what the temperature is outside. It will vary according to how much sunshine you have. I can tell you though that even on a cloudy day the sun can heat a small space very fast and very hot. The best way is to put a thermometer in a box open on one side and facing north and put it in there. The thermometer will be shaded from direct sun and give you a true reading of the temperature in there.

I am not a big fan of those upright shelves with cover. The cover does not come right to and lay on the ground so cold air goes up them at night and they are very tippy. My son bought one and after oit blew over for the third time he ditched it. I suggest you put it in a protected place if you have wind at all or somehow secure it so it doesn't blow over. My cold frame is a pop-up cold frame that sits low to the ground and I leave my plants in it at night as long as the temperature is close to freezing or above. But then I get the benefit of the ground heated in the day giving off heat at night.

I have done a bit of research in regards to the greenhouse and based on reviews I will definitely be securing the greenhouses to my chain link fence. My plan is to bring them in at night and line them up outside during the day. I also plan on cutting some holes in the plastic cover so that it can vent better. It has a zipped front door that I can roll up so that it is completely vented if needed. Thank you for the info on the min max thermometer, never even heard of one before so now I will pick one up!