from some blogging from a patagonia insider....
It turns out that obtaining an accurate MVTR is incredibly difficult. When Patagonia first decided to test the breathability of our fabrics, we chose six different fabric labs around the world and sent off a piece of fabric cut from the same bolt to each lab. When the results came back it was both disappointing and enlightening. All six results were substantially different, so much so that you’d have sworn they tested different fabrics. The Quality Team then went one step further; they took the exact same piece of fabric and sent it back to the exact same lab to be retested. Wouldn’t you know it, the second laboratory tests did not reliably match the first set. Obviously, if independent laboratories couldn't provide similar results from the exact same fabric, comparisons between different fabrics (and brands) were impossible. This discrepancy is the reason we don't publish hard and fast numbers; any comparison with other fabrics would be meaningless. Anyone picking a product based on one published number is putting a lot of faith in a marketing team and an unknown lab.
then some details of how they do it 'in house'
An MVTR test uses a humidity difference to create a vapor pressure between the inside and outside of a fabric. We run ours so the inside of the fabric is at or as close as possible to 100% RH and the outside is maintained at a very even (computer controlled) 50% RH. These settings may not sound like they are "real world" but the test must be run with no temperature gradient between the inside and outside, and it is run at our standard controlled lab temperature of 73F. What these operating conditions nicely create however is a small vapor pressure similar to what exists on a very warm and humid day. It's a worse case scenario so we make sure that materials will perform to our standards even when it's hot and humid. Of course the cooler and dryer it is outside the better the breathability performance.
