ActiveOutdoorsOutdoor TechWhat’s breathability: Understanding breathability ratings of fabricsManufacturers throw around all kinds of breathability numbers, figures and facts, but what do they all mean?When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works.
ActiveOutdoorsOutdoor TechWhat’s breathability: Understanding breathability ratings of fabricsManufacturers throw around all kinds of breathability numbers, figures and facts, but what do they all mean?When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works.
Manufacturers throw around all kinds of breathability numbers, figures and facts, but what do they all mean?
When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works.
(Image credit: Pat Kinsella)
(Image credit: Pat Kinsella)
When you invest your hard-earned cash in expensive outdoor gear, such as thebest waterproof jackets, thebest hiking bootsor even thebest walking trousers, one of the first things you will most likely look at is how waterproof the product claims to be. However, a good waterproof rating means nothing if the fabric isn’t breathable. What’s breathability, and what’s a good breathability rating? Read on to find out.
The Hydrostatic Head (HH) rating (more on HH here:How to stay dry outdoors) of the materials used won’t tell you everything you need to know about how waterproof a garment genuinely is (it’s just as important to look at how well the seams have been sealed and how good the zips and other design elements are), but a crucial factor to weigh up alongside waterproofing credentials is how breathable the boots or jacket is.
There is no point keeping all the exterior moisture out, only to end up soaked from condensation and sweat that has built up on the inside of your clothes because you’re wearing something essentially made of non-permeable plastic. This is where breathability comes into play.
(Image credit: Getty Images)
(Image credit: Getty Images)
What’s breathability: Understanding breathability
Various things play into determining how breathable a material is, including its thickness and composition, but in technical apparel and footwear, the most important factor is the performance of the technical membrane – for example, Gore-Tex – incorporated in the design of the product.
This is an extremely important factor when you’re weighing up waterproof jackets for high-intensity outdoor activities, including hiking, biking, running, climbing and cross-country skiing. When it comes to mountaineering, the degree of breathability in something like a jacket can be absolutely crucial because if the moisture can’t escape, it will cool your body down dangerously fast and can lead to life-threatening conditions such as hypothermia.
But if you think waterproof rating methods are complicated and opaque (and they are), then just wait until you try and get your head around how the breathability of outdoor products is worked out and presented. Luckily, we’re here to help untangle it a bit.
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Breathability is presented in several different ways. Two of the most commonly seen are the Moisture Vapour Transmission Rate (MVTR) scale and the Resistance to Evaporative Heat Transfer (RET) scale.
(Image credit: Pat Kinsella)
(Image credit: Pat Kinsella)
Moisture Vapour Transmission Rate
Let’s take MVRT first. This rating is calculated during laboratory tests, in which the quantity of water vapour that passes through the fabric under scrutiny during a 24-hour period is recorded.
On a product, this is usually shown, for example, as ’15,000 g/m²/24h’ as in theFinisterre Stormbird jacket– which means 15,000 grams of water passed through one square metre of the material over the course of a full day during test conditions. So, with an MVRT rating, the bigger the figure, the more breathable the product is.
However, these are just a series of numbers and letters to most people, so here is what that means in reality:
(Image credit: Montane)
(Image credit: Montane)
Resistance to Evaporative Heat Transfer
If the RET rating is 0–6, the garment will have superb breathability and be suitable for use during highly energetic activities; a RET rating of 6–13 is still very good, and will work for most outdoor pursuits; 13–20 is fine for gentle walking; anything with a RET rating of 20+ will barely breath at all.
Other considerations
But, just as a HH rating doesn’t tell the whole story about how waterproof a garment is, so an RET or MVRT rating only give you an indication of how truly breathable a piece of clothing is. You also need to take into consideration things like design and vents.
And one last tip: garments that are capable of breathing will operate at their best level to prevent the build up of condensation if they’re quite close fitting, because moisture will build up within air pockets if the inner and outer layers are not touching.
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