The effect of geometric factors on insulation was calculated mathematically for standing humans. It was found that internal radiative heat transfer in an ensemble was significant for insulation, that intrinsic clothing insulation is a useful concept only for indoor climates, and that shape plays a minor role. The literature agrees closely on insulation and clothing surface area figures, and the latter are compatible with model predictions. Finally, it was shown that wind, body motion, the effects of posture, and the fit of garments are predictable. Sitting provides more insulation than standing for light clothing, but the reverse is true for heavy clothing. Insulation is decreased by about 20% by cycling and by about 40% by walking, and a reasonable estimate can be made of the effect of wind and wind and motion together. The effect of air motion on vapor permeability is stronger than the effect on heat transfer.