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I use a 30-gallon black plastic garbage can with holes drilled in it every 5 to 8 inches. Often it will only fill up half way and I run a bit hotter mixture (beer grain, shredded paper, coffee grounds) but I found it will turn around in 4 to 6 weeks, sometimes less. It is on the south side of my garage in full all day sun, so the heat from the plastic also helps.

Have you thought about getting some red wigglers and doing a worm bin? Those don't have to be very big, can have amazing results, and if you keep them in doors they can go year round. I have been kicking around the idea of doing one in my house, but have not gotten around to it yet.

Re: Compost for Heat
Probably 30 yrs. ago I read something about burying (fresh) manure under your cold frame to provide warmth to extend the season. I think the British were doing this a long time ago as their winters are not so harsh as the northern US. So, nothing new under the sun!

Re: Toxicity of Polystyrene
No argument here that styrene monomer and benzene have some toxicity. However, the actual exposure (amount over time) is going to be pretty small using foam sheets in a greenhouse. How long does a person actually spend in their greenhouse? And you have to consider other contributions. If my whole house is covered with the stuff, theoretically I'm breathing it all the time. Of course, any residual in the foam will outgas over time and drop off to much lower levels. And, the stuff would not be on the market if levels of styrene known to be toxic were coming off it. We don't know everything yet, but there are people watching this stuff. CCA lumber, formaldehyde, asbestos, the list goes on of things that were taken care of.

Having said that, I try to avoid using plastics for food, especially hot food, when reasonably possible just to cut down on all sorts of exposures - plasticizers are starting to worry me with respect to endocrine disruption.

It wouldn't bother me in the greenhouse though. I actually built a cold frame out of 2" thick foil covered polystyrene sheets. Grew some great veggies in that.