As winter drifts aimlessly into spring and spring metamosphoses
seamlessly into the glorious heat of the classic British summer, you
may notice your pack becoming a tad damp and sweaty.
That's not surprising really as traditional padded back systems
have two major disadvantages, first they prevent air from reaching
your back markedly reducing the area available to radiate excess heat
outwards and heating you up generally.
Second, most foam-backed packs use wicking fabric in an attempt to
move moisture away from your clothing. Unfortunately when faced with
lots of sweat these and the foam underneath them tends to become
unpleasantly saturated like a big sponge. Yuk...
In an attempt to make things more bearable, several pack
manufacturers have produced vented back systems designed to allow air
to flow over you back and increase cooling. Here's what you need to
know if you're after one...
First
Principles
There are two major takes on improving back cooling, but we're not
really going to bother with the first which consists of cunning
channels and funnels in the back pad intended to channel hot and cold
air around.
Why aren't we bothering? Simple, in our experience, they don't
make a blind bit of difference. Which leaves you with systems which
are designed to hold the pack away from your back allowing air to
circulate and cool your body. We'll also take a look at non-absorbent
back systems, which may not improve cooling, but at least avoid wet
sponge syndrome.
Open
Trampolene
It all began with what we're going to call the Open Trampolene.
Early Lowe Alpine and current Berghaus and Deuter packs all use this
system. Essentially 
the
back of the pack is arched away from the back creating an air gap and
a mesh sheet rests against the back.
They're definitely an improvement on conventional back systems and
definitely allow more air to reach the back. One drawback is that as
much as 60 per-cent of a mesh sheet can actually be solid, so the
venting may not be as extensive as it appears. Berghaus has tackled
this on their latest Freeflow IV system by reducing the total area of
mesh using cut-aways.
Lowe Alpine chose to abandon the mesh method altogether. Other
drawbacks are that the arch of the back area significantly reduces
the internal volume of the pack and make it a tad banana shaped. With
heavier loads, the wide air gap can mean that the pack levers away
from the back and feels unstable and, in cold weather, wind blowing
between body and pack can make things feel distinctly chilly... On
the plus side, the plastc mesh fabrics don't absorb moisture.
Reasonable venting, doesn't absorb moisture.
Cold in wintry conditions, heavy loads lever pack away from
back.
A
few companies have taken the 'open trampolene' design and refined it,
notably Osprey with their Atmos and Terra packs, Haglofs and - coming
soon - Gregory.
After
years using the open trampolene-type system, Lowe Alpine has switched
to what they 're calling Pure Air Zone. Rather than using mesh which
can be around 60 per-cent solid, they've designed a simple back
system which holds the pack away from the body with pads at the top
and bottom leaving a total air gap in between. 
A
slightly different approach to the problem is simply to create a back
system that's non-absorbent. Most notable, The North Face uses an EVA
foam-based system called E-VAP on some of their hydration and
multi-sport packs.
Finally,
the increasing light weight and compactness of modern clothing means
y may simply be able to bypass the whole pack question by using a
lumbar pack or 'bum bag' - fanny pack if you're a North American...
A quick look at some ace-looking new packs from the Swedish outdoor brand.
