Wearing layers of clothing in cold weather is often recommended because dead-air spaces between the layers keep the body warm. The explanation for this is that the heat loss from the body is less. Compare the rate of heat loss for single 3/4-in-thick layer of wool [k = 0.020 Btu/(hr ft °F)] with three 1/4-in layers separated by 1/16-in air gaps. The thermal conductivity of air is 0.014 Btu/(hr ft °F).

GIVEN

Wool insulation

Thermal conductivities

Wool (kw) = 0.02 Btu/(h ft °F) and air (ka) = 0.014 Btu/(h ft °F)

ASSUMPTIONS

Heat transfer can be approximated as one dimensional, steady stateconduction