Because subzero temperatures are expected to affect the vapor resistance of micro porous membranes, the effect of using semipermeable and impermeable rain covers for sleeping bags on moisture accumulation in the bags during 6 days of use at -7°C is investigated. Moisture accumulation is related to the vapor resistance of the materials. The best semipermeable material gives the same moisture build-up as no cover. Semiperme able cover materials are effective at reducing moisture accumulation in sleeping bags at moderate subzero temperatures.