those won't hold heat in the winter.
Um... Well this greenhouse is kinda tricky.
It holds heat. It actually heats up really well... As long as its in the sun. If you want it to be warm in the snow, you'll want it in 100% sunlight for as long as possible. It doesn't hold heat from the day at night, but it does hold heat if you heat it from the inside with an electric heater... That's what I did. With a heater, swamp cooler, the greenhouse, the mods, ect... Basically just the equipment to run it... It's going to be around $700-$800. I had to buy the greenhouse with christmas money, then install heating/cooling with Easter money, ect. Now I have to sell off my other hobby to have enough money to buy plants lol. If you don't like the costs (I didn't know what I was getting myself into) then you may just want to go with a huge grow rack or a terrarium...
With my climate, the greenhouse works really well as highland intermediate for 90% of the year. There's a few days, plus a good amount of summer, that are lowland, but highlanders will survive it. My local nep store, which has a very good reputation and deals mostly on the internet, has talked to be a lot about temperatures and humidity. They said that sometimes the humidity gets down to 40% in the greenhouse and no big deal. They also told me that one day when it was 90F outside (this was in March) that the greenhouse was over 105F. All of the highlanders looked great. They don't keep many ultrahighlanders, but I've seen densiflora and singalana there. I actually bought a singalana from them and it withstood 92F + 30%- humidity.
If you live close to the ocean in MD, the temperature won't change as much as it would inland... Never been to MD, I have no clue what its like. I just looked it up and it can be down to 0F in some parts and 10-15F in other parts... I would hate living in that part of the country because heating/cooling would be a nightmare lol... It gets like 10000F in the summer and like -1000F in the winter.
If you want to know any specifics about the greenhouse, let me know. It's a great greenhouse for the price, and much better than any you'll find on the market. It's not necessarily the greenhouse that is the problem, its the outside temperature.
Oh, and in winter when it was like 70F outside, the greenhouse was over 105F in the sun. When I installed the shade cloth, it doesn't go over 90F in the full sun on a 70-80F day. Cover the long sides and roof in 50% shade cloth and you're good.