I used a double size heatsheet last summer when I fell into our chilly side of Pacific Ocean,probably about 50 something F. water temp. & a sunny but windy day about 75 F.,My teeth were chattering,my camp towel was wet,so the heatsheet was available,& it worked well enough to warm me up in the sunshine,I still had my wet clothes on.I was fortunate enough,that it was summertime,& I had a change of clothes waiting back at camp about 150 yds away,I also ruined the heatsheet on the rocks that I attempted to warm up on,it has holes with duct tape now,I also carry 2 Lg survival bags from Survival resources for now on.no affiliation,just a Happy customer!