Brett,
Activity AND environmental factors determine a garment's moisture transport (air permeability) requirements.
Activity is most commonly defined in METS. See http://onlinelibrary.wiley.com/doi/10.1002/clc.4960130809/pdf 1 MET is your energy expended just resting; .8 MET for sleeping; and backpacking is typically the upper limit for sustained activity at 8+ MET. Like a car engine, about 75% of your energy expenditure is wasted heat which needs to be dissipated to stay comfortable. If the heat generated AND the environmental factors raise your core temperature, your body will generate sweat to cool off. As part of the physiological regulation of body temperature, the skin will begin to sweat almost precisely at 98.6F, versus 95F average skin temperature, and the perspiration will increase rapidly with increasing skin temperature. The sweat only cools you off if it can evaporate and the vapor can move through your clothing ensemble. Sweat removes excess body heat through the heat of evaporation. If a clothing ensemble can't pass the vapor through fast enough, the moisture stays on your skin in a liquid state. When the liquid sweat stays on about 20% of your skin surface, you become uncomfortable. Also your body temperature will begin to rise.
As the windshirt's CFM goes up, its ability to pass water vapor increases up to about 35 CFM. After that point, the combined moisture resistances of the base layer, the air gap between the base layer – windshirt, the windshirt, and the windshirt boundary layer prevent further moisture transport increases. Also as the windshirt’s CFM goes up, its wind resistance goes down.
A windshirt not only blocks wind but it also serves as an insulation layer. The air gap between your base layer and a properly sized windshirt provides an incremental .6 clo insulation to your ensemble.
Scenario 1 – If your MET level AND environmental factors result in you being consistently cool, any CFM windshirt (.6 clo), a rain jacket (.6 clo), or insulating layer (? Clo), or combination of the aforementioned, is appropriate.
Scenario 2 – If your MET level AND environmental factors result in you being consistently comfortable, no action is required.
Scenario 3 – If your MET level AND/or environmental factors are varying such that you are sometimes comfortable, sometimes cool, and sometimes warm then a base layer and approximately 35 CFM windshirt is required for optimal comfort. This scenario occurs as you quickly move through varying micro-climates and/or your MET rate varies with the terrain.
Windshirts in the range of 35 CFM provide a hydrostatic head of approximately 400 mm; you need a minimum of 1,500 mm HH to be rain proof. Conversely, rain proof garments, provide a CFM of approximately .5 CFM best case. Contrary to the marketing hype, you will not stay dry on the inside wearing rain gear or a low air permeability windshirt in scenario 3.
© Beartooth Media Group Inc.
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