What is CFM rating for fabrics?

Answer 1

Cubic feet per minute per square meter (CFM) is a measure of the wind resistance or air permeability of a fabric. The higher the CFM, the greater the volume of air passing through. When hard shells dominated the landscape, discussions about CFM didn't come up. Traditional barriers like H2NO, Gore, Triple Point, Entrant, and other respectable waterproof breathable technologies all have a 0 CFM rating. They are absolutely windproof. With the advent of soft shells and more breathable fabrics, the air permeability argument becomes complicated, sometimes heated. Traditional layering has always taught the "vapor barrier warmth" concept. That is, maintain a (windproof) stable dead air space next to skin and you will stay warmer. That's true, if you're watching football game from the stands in November. But what happens when you're pounding uphill to the ridge before someone else sneaks into that untracked line of new powder? You can use a bit of convective heat loss; and you need more breathability to move the extra moisture you create through exertion. And a fabric with 0 CFM doesn't provide it. We've found that fabrics that measure as much as 5 CFM are still functionally windproof: that is, you don't feel the breeze come through. And they afford much greater comfort on the uphill. So we use 1-5 CFM as our standard for weather-protective soft shells (Mixmaster, Dimension, Dragonfly, etc.) Shells for higher exertion activities (e.g. Slingshot, Super Guide Pants, Talus Pants) must be even more breathable. For these products we hold to a comfortably wind-resistant, but not windproof, standard of 10-15 CFM. Beyond this, we don't go. We don't produce shell fabrics with a higher CFM (say, 15-20) because our field test shows that further gains in breathability don't offset the heat loss from wind penetration. (See Schoeller Dryskin on the chart on the next page- offering high breathability, but not enough wind protection) The goal is: both warm and dry. At the other end of the scale, as mentioned, we don't make 0 CFM soft shells. What's the point of a soft shell that doesn't breathe better than a hard shell?