This paper reports on an experimental investigation of the effects of garment fit on clothing thermal insulation and moisture vapor resistance, both of which increase with the thickness of the air gap between the garment and the body when the air gap is small. The rate of increase gradually decreases as the air gap becomes thicker, and is much less than the theoretically ideal still air due to natural and forced convection. When the air gap exceeds a certain value, thermal insulation and vapor resistance may decrease with increases in the air gap. Thermal insulation and moisture vapor resistance reach a maximum at a certain air gap thickness depending on fabric properties, wind conditions, and garment fit. Tighter fitting garments are preferable to keep the body warm in windy conditions.