Jerry’s review of the thermal conduction (thermal diffusion) profile though our sleeping kit, from body temperature to surrounding ambient temperature is very well done.
Also, it behooves to observe that the freezing point temperature of 32F (0C) is NOT an impenetrable steel wall forbidding additional removal of water vapor from our kit by mass diffusion — any more than 0C might be (is NOT) an impenetrable barrier for additional loss (diffusion) of thermal energy.
WHAT????
Reflect that the vapor pressure of water-solid-ice at its freezing point is MORE than zero.
– Ice sublimes directly to water vapor, without melting first.
– All of us who hung laundry on clothes lines in subfreezing weather before owning clothes driers well remember stiffly frozen towels and underwear drying during one sunny daytime.
– And, the energy required for sublimation of solid directly to vapor is only about 10% more for evaporation of liquid at the same temperature
In fact, the vapor pressure of water-solid-ice at its freezing point is EXACTLY the same as water-liquid at this freezing point.
This means that water-ice frozen at 0C has exactly the same driving force for mass diffusion outwards into surrounding ambient air as does water-liquid condensed at 0C just before it froze to ice, because the driving force for said mass diffusion is vapor pressure difference.
Let’s look at vapor pressure values for water, which are the upper limits for diffusive vapor permeation driving force at a given temperature.
98F 0.066 atm
80F 0.031 atm
50F 0.012 atm, about 5x reduced from our skin temperature.
32F 0.006 atm, SAME for ice or liquid, and 10x reduced from skin temperature.
14F 0.003 atm, ice of course, and a further reduction of 2x compared to when ice first forms.
Reflect that warm humid air at 80F and 80% Relative Humidity (RH) can accept twice as much more additional water vapor partial pressure: (1-0.8)* 0.031 = .006 atm; than can cold, totally dry air at 14F and 0% RH: (1-0)*0.003 = 0.003atm.
The loss of loft problem is truly rooted in the declining vapor pressure of water, declining exponentially with local and eventually surface temperature.
The accumulation of water in our sleeping kit during prolonged “icy cold” temperatures can be usefully understood as an extension of the same problem encountered with “breathable” rain gear when ambient temperature cools.
When actively exerting (hiking hard) our body produces a lot of perspiration to provide cooling. As ambient temperature cools and/or ambient RH increases, this amount of perspiration can overwhelm the transport capacity of any breathable membrane.
Accordingly, we must change something: increase the amount of vapor venting (zips, etc.), remove clothing layers to improve body cooling without as much perspiration, and/or reduce our hiking intensity to avoid soaking ourselves with condensed sweat that cannot diffuse (permeate) sufficiently rapidly through an outer, chilled membrane fabric into a cooler and/or more nearly saturated outer ambient.
Fortunately, when resting in our sleeping kit, our body produces a VERY much-reduced amount of at-rest insensible perspiration. This enables us to delay vapor condensation and liquid freezing inside our sleeping insulation to temperatures usefully colder than for condensation woes with rain jackets.
Generally speaking, and as observed by diverse experiences rather than calculations, the vapor pressures of cooler water-liquid and of frozen water-ice are typically sufficient to support diffusive removal of our at-rest insensible perspiration through the relatively large outer surface area of our sleeping kit for ambient temperatures down into and below the teens F.
Of course, this balancing also depends on how wet our clothes are inside the sleeping kit (how much water vapor is being produced near our warm skin surface), the temperature of the surrounding ambient air, and the “dryness” of the surrounding ambient humidity (which sets a limit of the driving force based on water vapor pressure in the outside air).
In fact, our down sleeping insulation can begin to wet with condensed vapor even when temperatures are a little above freezing if, for extended times, we keep adding wet clothes or are camped inside a fog-cloud or inside a tent that is condensing.
Or, if the surrounding ambient air is low-humidity, crystal-clear air, we can defer condensation and subsequent icing down to single digit F range, variously depending on details.
TLDR, the root cause for moisture balance problems becoming more difficult at increasingly cooler temperatures is the exponential decline for the vapor pressure of water with temperature declining and not simply the initial formation of ice crystals per se.
Staying dry when cool or cold is a balancing continuum: Exertion level. Ambient temperature. Ambient humidity. Accumulated liquid on skin, on clothing surface, and absorbed inside fibers, down plumes, etc. Venting. Duration of the events. All are factors for the moisture condition of our clothing and other insulating layers.
When sufficiently cold that diffusive permeation of our bodily-produced water vapor cannot avoid condensation and freezing, then a VBL usefully keeps water from our bodies from entering insulation. Turning the VBL inside out the next morning, the water can freeze and then be shaken away as ice crystals.
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Returning to the original post:
Yes, a synthetic over-quilt can extend the useful comfort temperature range without having to purchase a new primary insulation.
Yes, an over-quilt can be easier to dry in cold conditions, even when iced, since it is thinner than a sufficient single layer of insulation and because it can be more conveniently laid open on both sides. (Permeation drying time varies about with total thickness-squared (1-sided) plus 2-sided provides about another factor of 4.)
Yes, an over-quilt can accumulate less condensed and frozen moisture in the same bodily and ambient conditions owing to the gaps and convective air flows at the warmer interface between the inner and outer. This is akin to opening pit zips on a rain jacket; but it is hard to control while asleep tossing and turning and, like pit zips, it comes with a convective heat loss, which is less desirable when at rest.
There are no perfect rules for condensation and icing control because ambient conditions are so variable: temperatures, humidities, winds, durations; and because each event comprises a different human body starting with different accumulated moisture and having different sleeping patterns.
The situations/ranges cited by others for when a VBL becomes useful and indeed necessary are valid experientially.
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