So I have a single pane, tempered glass greenhouse, heated with 2 Empire 24K BTU propane heaters. We just finished the fourth winter. For additional insulation, I have done the following:

2005-2006: solar pool cover on the outside (fail)

2006-2007: solar pool cover cut in sections on the inside except over the ridge vent; 6R foamboard on the north wall

2007-2008: same

2008-2009: 4 mil PE film, 4 10x25' sheets in all, clipped to the inside, giving roughly a 2" space on walls and about a 6" space on the roof; same north wall insulation as before.

We had an average winter, we live almost exactly on the "zero" line running through the middle of Missouri: http://capitalclimate.blogspot.com/2009/03/us-2008-2009-winter-temperatures-near.html

Results? This year we used ~1000 gallons less propane than in previous years. To me, this is an indictment of the solar pool over technique. With a solar pool cover, you cannot easily hold it down on the outside in windy locations. And regardless of whether you use it inside or outside, it is difficult to seal off the dead space between the glazing and the cover, and that dead space is what really provides the good insulating properties.

Discussion welcome.

SB