Extreme conditions...but instructive about the need to manage moisture accumulation in insulation during below-freezing conditions.
"Modern day adventurers have also been plagued by the problem of moisture accumulation in
sleeping bags. Twenty days out on the polar ice on his way to the North Pole in 1986, Will
Steger noted:
"I hefted my bag and found that it had gained about twenty pounds in
accumulated ice. The inner layers of insulation were still somewhat dry, but
the outer layers were frozen mats. We had been finding that a tremendous
amount of body heat was needed to bring the bags up to a temperature at
which we could sleep with minimal comfort. Some nights we shivered for
three or four hours before we dozed off. (Steger 1988) "
These sleeping bags had been specially made with 5.5 kg of polyester fibrefill and had a total
loft of 36 cm. They were designed with such a great thickness of insulation to compensate for
the expected accumulation of ice.
After 34 days, they tried to dry the bags with stoves in the tent. Steger reported:
"The effort proved futile. The volume of accumulated frost was now so great
-- some bags weighed nearly fifty pounds -- that the minimal heat from the
stoves merely redistributed the moisture rather than driving it from the bags
(Steger 1988)."
The use of vapor barriers is one way to deal with the phenomenon when insensible perspiration passes through your insulation layer only to freeze or condense WITHIN the insulation once it gets cold enough. This thread is probably not the place to go into use of vapor barriers in sleeping or clothing systems, but the physical phenomenon of condensation on the inside of a sufficiently cold vapor-proof (i.e. waterproof) barrier is quite real.
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