HelpfulGardener.com

I think what people miss in a greenhouse to stay over winter is the second layer of plastic. With only one layer you better use alot of heat. The sewond layer will cut the heat down by over 50% I use no heat in mine only water storage and some brick storage! I will use a little 250 volt heater in my sweat chamber in feb when I start my seeds!
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I have lots of flowers and spikes in the greenhouse now to keep alive over winter with no heat only water storage and double plastic. I have a fan that kicks on for a 1/2 hourat a time noon 1- 2- and 3 pm that blows the warmer noon air against the water storage! The fans only heat the drums up a few degrees but it helps with the night temp!
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I put the plastic over the bean vines I had growing over the walls of one greenhouse and it kept them alive for a extra month with some still growing! Its fun all the things you can do with common sense. I even started calling my fishing bobber the common sense bobber ! I am always interested in any ideas to add heat using only the sun or compost so keep them coming!

Wow three greenhouses... what do you do with all those plants, are you a commercial grower?

Re the heat. Simple place to start is to be sure the floor is thermal mass. Any floor space not used for growing things should be dark colored brick or stone/ slate etc, several inches thick. It will also absorb heat in the daytime and give it back at night. If you are laying down something like that, insulate under it first to separate it from cold ground.

Have you weatherstripped around all the doors, vents, etc?

Have you insulated any non-transparent knee walls, sidewalls ?

You can get thermal blanket to pull down like a curtain after dark, raise during sunlight.

If you want to use a little bit of power (and have an outlet in your greenhouse), you could put an aquarium in with heater. The aquarium water would also be thermal mass, but would be power heated as well so would give off some heat. The used aquarium water then has fish fertilizer in it to give to your plants.

I don't have a greenhouse, except a mini one I use for seedlings in the spring, so this is just ideas off the top of my head....

Got the aqarium ready..