it was ok at 50+F, for 2 hours, with just a t shirt and jeans. Tonight I tested it for 1.5 hours, at 47F dropping to 42F, with a heavy duty space blanket just folded around the modified SOL escape bivvy, with a tyvek bag around the total. Once the wife gets the sewing machine set up to her liking, i'll get the velcro seams on the space blanket, make the other tyvek bag (reflectorized) for the outside layer. Given the right clothing, it's going to work just fine down to 20F, perhaps even to 10F. It will reach 10F if you can stuff dry debris between the layers, dead rock certain. yet there's no stuffing to be lost, no zippers to jam or break, geting it all wet will have no effect on its efficiency. it can be set up as a tarp, as a hammock, or on the ground, with the gillnet hammock, bubble wrap and bug net bag acting as padding. pretty cool, if i do say so myself. it IS necessary to have the tyvek layer between the breathable mylar and the heavy duty mylar, cause even tho the breathable layer is not permeable, there's a sense of clamminess. the tyvek prevents that from occurring.