Sleeping Warm Without Fuel: The Insulation Bound, Why the Ground Pad Decides It, and a Layered System Sized from Stated Materials

A sleeping person is a small heat source of about 80 watts, of which roughly 65 watts must pass through the insulation of the bed after breathing and the uncovered head have taken their share. This paper balances that 65 watts against conduction through a layered sleep system assembled from plainly obtainable materials — wool blankets, worn clothing, and a 12-millimetre closed-cell foam pad — for a 75-kilogram sleeper with a mean skin temperature of 33 degrees Celsius. The decisive asymmetry is underneath: body weight compresses the underside blankets to a stated 25 percent of their uncompressed loft, so the ground side of the bed, with a smaller area, conducts more than twice as readily per kelvin as the top side whenever the two carry similar material. In the sized system — four blankets over the body, two blankets plus the foam pad under it — 68.1 percent of all conducted loss goes into the ground. The system holds thermal balance down to a night temperature of 0.44 degrees Celsius and no further. Removing the foam pad while keeping every blanket collapses the balance temperature to 21.17 degrees Celsius: without the pad, the same bed is not in balance even in a mild room. Placing one extra blanket underneath raises the balance point by about 3 degrees, against about 2 degrees for the same blanket placed on top. Balance at minus 10 degrees Celsius requires a thermal resistance of 1.257 square metres kelvin per watt on both sides at once, which compressed underside loft cannot supply from blankets alone. The ground pad is not an accessory to a cold-weather bed. In the arithmetic it is most of the bed.