This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Breathable waterproof fabrics operate by one of two ways.They're
microporous, with microscopic pores which permit the passage of water
vapour but not water liquid, or they're hydrophilic, a solid barrier but
capable of absorbing moisture vapour and passing it through its structure.
Either may come as coatings applied directly to a fabric, or membranes
which are glued to the fabric which carries it. Then there are microfibre
fabrics and cotton fabrics.
The top end of the market is dominated by Gore-Tex, and like some of the
other laminates on offer, it comes in a variety of forms. The original, and
still the best for durability, is 3-layer, where the breathable waterproof
membrane is sandwiched between a facing and lining fabric. Garments made of
this tend to be good value, too, because the manufacturing processes aren't
so complex. 2-layer is softer, with the membrane glued to the underside of
the facing fabric, and a loose lining. Not so durable, but usually more
breathable, and more expensive. Other varieties, laminate the membrane to a
lining fabric with loose outer - nice for fashion garments, and sometimes
the waterproof lining has loose outer and lining on either side - again,
more complex constructions generally adding up to more expensive garments.
And the outside pockets will let in water...
A coating is a coating, or is it? Breathable PU nylon doesn't really mean
an awful lot. Individual coatings can have their chemistry tinkered with to
make them more breathable or more waterproof. Cheaper coatings may be
applied in one pass over the fabric, more expensive performance coatings
may be made up of several thinner applications.
You'd expect breathable waterproof fabric to work reasonably well in dry
conditions, provided you're not working so hard as to overload its
capability to transport moisture. The real crunch is when it's raining. How
much does it breathe after 5 hours in pouring rain? Tests showed that all
fabrics lose an element of breathability in wet conditions. The various
configurations of Gore-Tex lost between 34 and 43% of their breathability,
Sympatex 31% on a Z-liner construction, 70% in a double layer. Helly-Tech's
decline was just short of 75%, but perhaps the biggest surprise was Lowe
Alpine's Triple Point Ceramic 1200, losing just 15%.
Whatever the coating or laminate, the facing fabric and its water-repellent
surface treatment is absolutely critical. It's fair to say that the coarse
texturised facing fabrics will fare less well than smooth ones because of a
larger surface area to grab water when the water repellent treatment wears