A guide to the performance
of modern waterproof fabrics
know, once waterproof clothes were just
that. Waterproof! You put them on to
keep out the rain. Unfortunately these
mobile saunas soaked you in sweat as
soon as you attempted any simple task.
Then came W.L.Gore...
Certainly the first
breathable waterproof fabrics were bug-ridden
yet they did start the process that
was to radically change the outdoor
industry, improve the quality of our
life on the hill and bring about some
of the biggest misconceptions of what
a product can do - the biggest of which
being that breathable fabrics stop you
So, what is it all
about? First, let us clear up the sweat
bit. Without sweat you're dead! A person
performing tasks will sweat to cool
the body. The idea of a layering system
is to remove this sweat and keep the
body dry while maintaining the body's
"microclimate" (ie the body's
normal operating temperature and humidity).
All this and keep out the dangerous
Where do breathable
waterproof fabrics fit in? These fabrics
do not breathe. What they do is transfer
body moisture vapour between its surfaces
using the difference between the temperature
and humidity next to the body and that
of the cool outdoors as a driving force.
Wow! Loosely speaking the body sweats.
This sweat is picked up by your layering
system which transports it to the surface
away from your body. Here it evaporates
to form a vapour. Because it is hot
and sweaty inside your jacket ( ie warmer
and more humid) than outside the jacket
there is a difference in pressure across
the fabric. In an attempt to equalise
this difference vapour is driven across
the fabric to the outside.
This is a sensitive
equation failure of which is often incorrectly
put down to base layers not wicking.
You see, as it gets hotter and more
humid outside the jacket the differential
gets less which reduces the force to
drive vapour to the outside. It becomes
more humid inside your jacket and the
sweat is then unable to evaporate away.
It collects in your thermals and you
get wet. Sometimes the system can overload
as you sweat too much and it cannot
be removed fast enough. In this case,
to stabilise the equation (known as
the moisture vapour transfer rate -
MVTR for short), stop and ventilate
to remove excess sweat. If the former
case is a problem then find a good pub
to sit in or learn to live with the