What a Sleeping Bag Rating Actually Buys: Insulation Thickness Required per Degree Below Freezing, and the Cost of the Mattress Pad the Rating Assumes

Sleeping bags carry comfort ratings in degrees, but the physical content of a rating is an insulation resistance, and the insulation a camper must carry grows linearly as the temperature falls. This paper converts temperature ratings into required loft thickness using a stated insulation conductivity of 0.040 watts per metre-kelvin and the sleeping heat balance. The loft needed over the body is 7.4 cm at 10 degrees, 10.8 cm at freezing, 14.1 cm at minus 10, and 17.4 cm at minus 20 — about 1.7 cm of additional loft for every five degrees. Beneath the body the bag's loft compresses nearly flat, and the paper computes the pad resistance that must replace it: at minus 10 degrees the pad must supply at least 1.04 square-metre kelvin per watt, without which the underside alone would drain 218.0 watts per square metre. A rating, in other words, is a two-part specification, and the pad is half of it. This study is simulated, not measured.