Sacha's comments regarding the difference in temperature 'behind curtains' reminded me of something which I found very interesting ?? We were making a video about houseplants and I asked the cameraman to go back from a very large well lit window about 10ft into the room but without changing the settings on the camera and to my amazement, there was 60% less light there than at the window !! Now to my eyes, it looked very similar because of the way our eyes adjust to various light levels, something plants cant do !! So be aware of just how much natural light is lost (especially in winter when levels are naturally low) when using polythene, fleece or bubble insulation etc. Levels that to us dont look that much different because of the way the irises in our eyes work !!
I'm sure that serious photographers with light meters etc. are fully aware of all this ??
Lannerman.