This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
summarized from Clive Tully UK Outdoor/Travel Writer
Breathability in waterproof clothing is one of the most misunderstood and
misrepresented technical aspects of outdoors clothing and equipment. It's
all very well listing the technical merits of a particular fabric, coating
or membrane. Too often, the design of the finished garment either makes or
breaks the fabric manufacturer's claim. E.g., a walking jacket with a
permanently vented shoulder flap might as well be made of non-breathable
PU. It can't maintain the partial pressure which makes the fabric work. The
exception is Gore-Tex fabric. Garment manufacturers using their fabrics
have to submit sample products for Gore to check they meet their laid down
standards of manufacture. Not many fabric manufacturers do that, but then,
not many have such a tight grip on their markets.