Making sense of air permeability ratings

Spotted Montane using these on their website now

8 messages
02/03/2013 at 15:24

While prerusing Montane's latest offerings I noticed that they are now giving air permeability ratings for some of their products, which I assume is a measure of wind resistance or is it just for breathability?

For example, the new Mountain Star jacket has "Air permeability 4.5cc max (BS EN ISO 9237)", whereas the old faithful Litespeed jacket has a rating of 1cc. So I assumed that the lower the rating the more wind resistance you get, but what confused me was that the Mountain Star is not billed as particularly wind resistant yet has a higher rating than some of their softshells like the new Alpine Stretch jacket (13cc) and the Dyno jacket (8cc). So is this new Mountain Star jacket actually more weatherproof than those softshells?

02/03/2013 at 16:54

These are all properly breathable fabrics so air permeability IS breathability This only breaks down with membrane waterproofs which don't let any air through and even then Event lets a tiny little bit through and Noeshell a little bit more.

Onto the other question, yes the mountain star is quite a bit more wind resistant. Their quantum GL based stuff is even more so Rain resistance basically DWR based so pretty constant with maybe an edge to the thicker fabrics. Or does the weave tightness affect this too? I dunno.

As for why you'd want more air permeable stuff like the dyno or even the alpine stretch? Well they're even more comfortable to wear than 'fully' windproof windshirts due to the air transfer. When the wind gets too strong/cold you can always shove your (totally windproof) waterproof shell on top.

Seems very sensible to me, although not everyone quite likes it. What is key is having useful information as it varies all over the place between fabrics (even ones with the same name!) and is one of the most important factors in what a soft shell is for.

The 'standard' measure is actually CFM. On that scale up to 5 is 'essentially' windproof up to maybe 15 is fairly air resistant but can feel a bit more through it and higher is getting quite air permeable. Search backpacking's forums for more detail than you want.....

As far as I can tell from there there's roughly a 1 - 2 cc - cfm conversion ratio but not sure! So the trail shirt stuff won't feel totally wind proof (which is the point vs the Pertex things! For running etc.), the dyno even less windproof and the alpine stretch is quite close to the level of hardfaced wind resistant fleece.

(Since its made of polyester I wouldn't be surprised if it basically was something like hardfaced windpro.).

Whatever its very positive to see Montane (and RAB) putting this sort of information up publically. Its often rather hard to find and rather important with it.

Just think how tricky it'd be sorting out the sabertooth/dyno/Alpine stretch without it

Edited: 02/03/2013 at 16:57
02/03/2013 at 17:46

These are all properly breathable fabrics so air permeability IS breathability

Quite so, but breathability and perceived breathability are not always the same.

For example, Rohan took some of their Airlight polycotton and had it treated with the Epic silicone encapsualtion DWR.  Measured air pereability was around the same, so it is therefore as breathable, so everyone's happy, right?  Wrong...  Epic is a very effective DWR so condensed sweat, rather than soaking in to the polycotton and subsequently evaporating from the outside, would stay in liquid drops inside just as rain would on the outside, with the result that wearers found the garments clammier than the (no more breathable) "normal" version, and associating clamminess with lower breathability complained that the new garments were not as breathable.

So while I applaud extra information from vendors it remains the case that the Real World tends to be more complicated than simple numbers suggest will be the case.  You can still easily get wet inside a very breathable garment if it has a very effective DWR coating.  Of course, if it doesn't have  aDWR coating you can get even wetter, if it happens to be raining...

In short, breathability measures have no direct relationship to liquid water transmission.


03/03/2013 at 13:19

I do not think that "air permeability IS breathability" statement is correct. "Breathability" as used by the manufacturers, usually refers to the capacity of the fabric to transfer water (sweat) from the inside to the outside (so the sweat is eliminated as fast as possible). Air permeability refers to wind resistance.

"When the wind gets too strong/cold you can always shove your (totally windproof) waterproof shell on top."

If it is not raining, I prefer to use a non-membrane wind shirt instead of a waterproof jacket (with Goretex or similar membrane). You could use a waterproof jacket as a wind shirt, but you will sweat a lot more.

03/03/2013 at 13:51

You missed Martin's caveat of "properly breathable", i.e., allow air flow.  Gore etc. prefer their measure because Goretex comes out kinda crap if you do it with air permeability!  But note it's not water it meaures transfer of, it's water vapour.  Big difference.

But for a non-membrane soft shell the permeability is probably the best measure of basic breathability. 


Edited: 03/03/2013 at 13:53
03/03/2013 at 15:46

Thanks for the replies, chaps. It does confirm what I thought (that the rating is related to wind resistance). It's very curious how Montane advertises that Mountain Star as a 'trail shirt' (they need to check the definition of shirt) and doesn't mention in the copy how windproof it is, when it's actually more windprood that Pertex Equilibrium jackets that are advertised as windproof (such as some from Rab are).

03/03/2013 at 16:49

"windproof" has always been a relative term.  For example, Buffalo advertise their Pertex tops as "windproof to 50 mph", which is one of those statements that seems really clear until you start to think about exactly what it means.  Does anything get through at 49 mph?  How much gets through at 51 mph?  And so on...

As for the definition of a shirt, that's not quite as obvious as you think either!


03/03/2013 at 19:31

Well Montane have a tricky thing to explain here as they look very like their pertex wind shirts. Ok so they'll function like that too

Still the air permeability does I think maybe put them into a gently different category/intended user group. Guess they've been fairly rational overall. It isn't easy when you've got people sticking waterproof membranes in things and calling them soft shells too......

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