Vapour transfer in two-layer clothing due to diffusion and ventilation

An experiment was carried out to measure the vapour resistance of two-layer clothing ensembles as a function of air permeability of the outer layer, open or closed apertures, wind, and walking, both for the total ensemble and for the outer garment alone. Six subjects walked on a treadmill (0.0, 2.5, and 5 km.h-1) which was placed in a wind tunnel (0.2, 0.7, and 3.0 m.s-1). They wore long underwear and an outer garment made of impermeable (imp), microporous (mpo), low air permeable (loa), or high air permeable (hia) fabric. Vapour resistances were determined by a trace gas method, calibrated against water vapour resistance. The vapour resistances of the underclothing and the outer garment were calculated from the measured data, as was the ventilation through the apertures. The vapour resistance of the underclothing was almost constant at 5 mm air equivalent. The ventilation was strongly dependent on wind and motion but still so low (54 l.min-1) that only the impermeable garment could benefit from it noticeably. The vapour resistance of the garments also varied strongly (imp 55-200 mm, mpo 12-20 mm, loa and hia 1-14 mm air equivalent). For the imp garment, this is due to leakage of air, whereas the vapour permeable garments were dominated by the diffusion and air penetration through the fabric. It is concluded that ventilation with vents cannot match the effect of vapour permeability, and that real low vapour resistances are only possible with air permeable fabrics.