Hey, I recently wrapped my GH in some greyish (more grey than transparent) 4 Mil plastic... This against my polycarbonate panels created an air layer that is proving to slow escaping heat... My GH like yours stays about 10° warmer than outside BUT now I'm at least 15-25° warmer after I wrapped!!
Got plastic from Lowes 20'-100' (I have enough for a second layer, just incase) I really wanted this much bubble wrap but oh well.
It got down to 25° last night and my GH stayed above 48° but I also have a heat mat... TODAY at 10am it was 38° outside but my GH was already 98° (my heat mat turns off 8am) Also, my GH is surrounded with more than 1' of mulch
Last year was bad...this year yippy.
Good luck.