Not just maps... there's more to a breathable jacket than the highly techy fabric alone. Pockets for example...
This week's Monday Tip is about breathability. Or maybe 'not breathability' and how there's more to it than just buying a jacket made from the latest whizz-bang SlurpoShell fabric.
Since we've mentioned maps, let's start with them. One of the accepted 'requirements' for a UK mountain jacket is that classic stand-by, the map pocket. The idea is that you must have somewhere to stow your map. Great idea in theory, but in practice, maps simply aren't very breathable.
So you spend a three-figure sum on a jacket made from a fabric designed to allow moisture vapour to move through it as fast as possible then you stick a wad of waterproof laminated paper across an area around a third of the width of your chest.
To make things even less breathable, why not put it in a pocket made with a waterproof fabric pocket bag. Now you have a double layer of fabric there as well as the map. And it's still there even when the map's removed.
And that's just the start of it. Lots of shell jackets designed for mountain use have capacious chest pockets that means a huge area of the front of the jacket is made from a double layer of fabric. Sometimes, if you have hand-warmer and Napoleon-style pockets, there are even three layers of fabric there.
Add in the area covered by your pack and suddenly a big chunk of the surface area of your 'highly breathable' jacket, doesn't look quite so breathable after all.
Pockets And Bags?
So if you want to maximise breathability, look at jacket design and ask yourself some questions about how you operate. Do you really need two massive chest pockets? Some people use them a lot, others, myself included, hardly touch them and prefer to stash stuff in pack lid and stuff pockets. If you don't use them, why have them?
And even if you do, have a look for a jacket which uses mesh pocket linings instead of solid, waterproof ones. You may lose a little ultimate weather protection, though not much if the jacket is well designed, but you'll up the surface area of breathable fabric available and, as a bonus, gain some additional venting if needed.
Gore Active Shell
Interestingly, with its newest and most breathable fabric, Gore-Tex Active Shell, Gore has realised just this, with the result that jackets using Active Shell are only allowed a relatively small area of double or taped fabric. In essence, it means the designer has to avoid double pockets and minimise the number of seams to allow the fabric to work to its full potential.
With the Endo Jacket, for example, Haglöfs has added just a single small chest pocket, and that's made from a non-membrane stretch soft shell bonded to the outside of the jacket just to make the point. Generally any jacket made from the fabric should be specced to allow the material to work at its best.
It's not just Actibve Shell though, the latest lightweight Rab waterproof shells made with Pertex Shield fabric similarly use mesh pocket bags throughout for the same reasons.
So next time you're shopping for a shell, have a think - breathability is about more than just fabrics.