[bobberman]

Very nice set up. My small greenhouse is very similar to yours but my north wall is insulated in the winter but has a clear wall in the summer where I grow climbing beans or peas in the summer. Even thought you have a nice strong film covering the greenhouse a inside plastic attached to the 2 by 4 will add warmth to the greenhouse. A thin clear film 2 mills is all you need just for the winter . Just staple it on with a small piece of cardboard stapled to the plastic so it does not tear or double the edge of the plastic where you staple it for strength! I would say 20 % more heat will stay in the greenhouse. a 2 mill 9 by 12 clear drop cloth plastic will do the trick and only cost about $3.