Ben said in part: “I understand that the Patagonia Houdini is even more breathable than the Marmot Ion. Those who own a Houdini and perhaps something esle present or past -- pray tell -- does Houdini's superb breathability come at the expense of wind blockage? If so, then I wonder at what point chasing after maximum breathability would result in a windshirt that no longer blocks the wind optimally???”
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.
Traditional barriers like H2NO, Gore-Tex, eVENT, Triple Point, Entrant, and other respectable waterproof breathable technologies all have a 0 CFM rating. If you are just sitting still or doing camp chores a 0 CFM garment will block all wind and keep you the warmest. Both eVENT and Gore-Tex Direct Venting achieve this with a pore size of about .2 microns.
With the advent of soft shells and more breathable fabrics, the air permeability argument becomes complicated, sometimes heated. If you are backpacking, you can afford a bit of convective heat loss. You need more breathability to move the extra moisture you create through exertion. And a fabric with 0 CFM doesn't provide it. 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 with the high exertion rates during backpacking. Popular wind shirts, like the Patagonia Houdini, fall into this class as do the best soft shells. The average pore size for this class of fabrics is about 20 microns or 100x times larger than eVENT. Any larger pore size will result in more than 5 CFM. After 5 CFM you can start to feel the wind through the fabric.
To put the Houdini pore size into perspective: it is about ¼ the diameter of a human hair; it is about 10x smaller than the finest water mist; and it is about 100,000 times larger that a water vapor molecule.
In my opinion, the Houdini’s encapsulated silicone micro-fibers make it the best the best wind shirt on the market for thru-hikes and expeditions because there is no need to periodically renew the DWR for water resistance. For conventional trips of a week or less there are many excellent alternatives such as the Montane LiteSpeed, etc.