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I read a book about hypothermia recently : Hypothermia, Frostbite, and Other Cold Injuries: Prevention, Survival, Rescue, and Treatment :
One of the author claims that using a good space blanket (it was more a space sleeping bag) can add several degrees to a sleeping bag rating by preventing radiative heat loss. I wanted to put that claim to the test and did it last night.
Here is the experiment setup :
The night temperature ranges from -12.5 to -16.5 celsius (9.5 F to 2.3 F). I decided to use a space blanket inside my sleeping bag.
By experience I know my bag is good down to 14 F without layering too much (I never used expedition weight thermals) and good down to 6 farenheit with a VBL bag.
I started the night with the space blanket inside my sleeping bag, bu i very quickly remove half it, letting only my legs covered by it. even so I was too warm at around 6F (-14.3 celsius). I had to remove the E-blanket as I was starting to sweat quite a lot. I changed for the VBL liner and get some good sleep until around 4 AM. It was -15.1 celsius (5 F) and my right leg and shoulder where painful due to the cold. I knew I was too cold so i changed again for the space blanket. but i just covered my legs. I felt asleep and woke up in the morning.
conclusion : space blanket add some warmth. More than a mere VBL liner.
Temp rating with midweight thermals : 14 F
temp rating with midweight thermals and VBL liners : 6 F
temp rating with midweight thermals and Emergency sleeping bag : lower than 2F.
-my bag is narrow on the bottom but really roomy on top. Radiative heat loss is consequently higher on top. I expect the temp rating gain on the top to be higher but i couldn't put it to the test as I was to warm.
-the gain with the space blanket here is bigger that 12 F (6celsius) wich is remarkable. But I suspect this gain to be less with high loft sleeping bag. Cause heat radiation cannot cross thick layer of insulation.
- My space blanket was the cheapest one I could find.
-I always feel colder at the end of the night when sleeping at the limit of my sleeping bag capability. Which is normal since I suspect i'm not in a steady state condition. I have to produce more heat than normal.