I only have a small growcamp greenhouse, which I use for my orchids mostly. Like what others have said, it gets same temps as outside during winter and as hot as it can during summer, so I have the plastic foil flaps removed during summer, just the screen is in place.
For this winter, since reading this thread, I added some bubblewrap too and some mylar, but I have decided to just pull in my more sensitive orchids and in their place I put my succulents that can take the cold but not the rain, since our winter is oftentimes more with rain. I have now accepted the fact that when temps of 20's come in winter, I am just deluding myself my orchids will stand it. They get cold damage even while covered by my growcamp, so I just pulled most of them in. Thankfully my collection is not that much and can go to the our rooms facing the south window. Even in our mild winter conditions, I am just asking too much of my orchids, I think if I continue to keep them outdoors. The only orchids left inside the growcamp right now is my Zygopetalum and this noid Den that really likes being cold but dry. My tillandsias will remain inside the growcamp too, they handled the winter temps well as long as inside the growcamp. My Vanda coerulea, pleurothallids and cymbidiums, they can take the winter temps nicely here, so they stay out as well, or if I change my mind, there is still space inside the growcamp for them.
But I still want to research more of other ways of passive heating..thanks for the discussion about the water jugs..makes sense..I just need more sun! The city trees and our community trees are still fully leafed so sun is blocked a bit, it gets only a couple of hours south sun.
Anyways, I saw some youtube videos where they were using inverted clay pots with some metal inside and heating it up with tealights. I have not experimented with it yet. But it looks interesting to me. Maybe I can use sterno instead of tealights. Just for those nights that we do have a forecast in the 20's which often happens in January-February. For now we have only gotten down to 39F a few nights ago, so it is still okay.
