Breathability of outdoor fabrics can refer to two different things:

  1. Air permeability, or the rate at which air flows through a fabric
  2. Water vapor permeability, or the rate at which water vapor flows through a fabric, also known as the moisture vapor transmission rate or MVTR

Water vapor permeability is typically discussed in the context of waterproof-breathable (WPB) rain gear whereas air permeability is applied to most other types of gear, particularly wind shirts.

Air permeability is measured as the volume of air that can pass through a fabric and a common choice of units is cubic feet per minute or CFM. A recent post in the Ultralight subreddit summarizes CFM measurements for many popular wind shirts.

Lacking access to industrial scientific equipment, the average Joe can use a method of approximating CFM known as the Darth Vader test. This involves breathing through your chosen outdoor fabric and then comparing the effort required to breathe through one or more single layer coffee filters whose CFMs roughly translate as follows:

  1. One coffee filter: ~75 CFM
  2. Two coffee filters: ~35 CFM

Using this method you can quickly determine which breathability bucket the fabric falls into (under 35 CFM, around 35 CFM, between 35 and 75 CFM, around 75 CFM, and more than 75 CFM).

For wind shirts 35 CFM is considered the optimal level of breathability for high-output activity (in comparison, my Massdrop Veil is rated at 11 CFM which makes it more suitable as an insulation piece for more moderate activity or just standing around). Compare this to the maximum of 0.2 CFM for eVent, widely considered one of the most breathable families of waterproof-breathable membranes. No wonder rain gear is so miserable to hike in, it’s 55 times less breathable than a fabric that’s intended to be windproof.

In the absence of coffee filters you can use the Darth Vader test to compare the relative breathability of any two fabrics you have on hand. I used this method to determine that my new Armachillo button-down shirt was significantly less breathable than my Montbell Wickron button-down shirt which it was intended to replace. In addition to having button-up sleeves (which the Montbell lacks) the Armachillo was lighter and I had interpreted this as necessarily being more breathable. However, a passage in the socks chapter of Fixing Your Feet makes me wonder whether my thinking is backwards:

“A loose weave feels coarse and provides less insulation. A dense weave feels softer and generally provides more insulation and cushioning.”

The Armachillo fabric is definitely silkier than the Montbell, so perhaps feel can be another indication of relative breathability. In any case, a subsequent breath test confirmed that the Montbell is in turn less breathable than my previous hiking shirt, the Outdoor Research Echo.

My recommendation is that you always compare the breathability of your current hiking gear with any gear you might be replacing it with. This may include bringing your current gear to outdoor stores and breathing through clothing you find on the rack.