Hi
Geoff Hamilton done a trail with bubble wrap several years ago, he sub divided a greenhouse into two parts, one part bubble wrapped and the other not (bubble wrapped). He measured the temperature in the non bubble wrapped part of the GH and when it got to 32 degrees F (freezing point), he then measured the temperature in the bubble wrapped part of the greenhouse this read 48 degrees F. I think that works out to roughly to 9 degrees C. Shows its worth bubble wrapping.
Trouble is I don't remember the rest, how long did it stay at a higher temp etc. Re-reading I'm not sure if this is of any help at all, but I tried.
MikeB