The GO Outdoors Guide To Breathability

When it comes to outdoor wear, from shoes to jackets or mid layers, the best and most useable fabrics are breathable. Arguably more important than being windproof or waterproof, breathable capabilities in a garment mean it allows body moisture and vapours to be released, so you can use it for active wear without overheating or feeling discomfort.

Breathable garments do not ‘stop’ you sweating or producing moisture, which is a natural response, but allow drying to happen quickly. Breathability is particularly important in active wear. The more of your own body heat you will be using and producing, for example on an uphill trek, the more breathability you will need.

The balance with breathability is between letting enough moisture escape, whilst keeping moisture out, whilst at the same time preventing condensation.

Permeability and Breathability

Permeability demonstrates the fabric's ability to transport moisture and vapours, such as sweat through a fabric. Usually this is related to the space in the garment.

High permeability indicates that the garment is highly breathable, as the maximum amount of air can permeate the garment. This is measured by testing.

Testing Breathability to Achieve MVT and RET Rates

There are three main tests that measure breathability used throughout the world, but sadly there is not one standardised way of measuring breathability.

These tests show how many grams of water vapour are able to pass through a fabric in a set time period.

The results of the tests are then expressed as g/m²/24hrs, or as a RET (Resistance of Evaporation of a Textile) or MVT (Moisture Vapour Transmission)

Moisture Vapour Transmission

Moisture vapour transmission relies on the existence of a temperature/pressure gradient between the inside and outside of the breathable garment. In other words, the fabric works by equalising the pressure and heat between the inside and outside of the jacket.

Once body moisture reaches the inner face of the garment, it must pass through the fabric and then evaporate on the surface. If it cannot pass through the fabric, or if the rate of transmission is slower than the arrival of extra vapour, then the vapour condenses on the inside face.

It is for this reason that breathable products work best when the air inside is humid and warm, the air outside cold and dry to evaporate the excess moisture. When the weather produces warm and humid conditions on the outside of the garment, transmission rates will be lower and could allow condensation to build up on the inside face of the fabric.

If the moisture vapour transmission rate is exceeded, for example when walking uphill or carrying a heavy load, it will lead to the possibility of condensation forming.

Breathable garments work best at preventing condensation build up if they are reasonably close fitting. This is because if the mid and outer layers are not in contact then moisture vapour will come into contact with pockets of cooler air between layers, so promoting condensation on the inside of the outer layer.

RET -Resistance of Evaporation of a Textile

Simply put, this shows the levels of resistance the fabric has to allow evaporation.

The lower the resistance, or RET, the better the breathability (the more RETS) or energy it uses, the less able to breathe the garment is.

Basically: A RET of 0 has excellent breathability whilst a RET of 30+ is not very breathable at all.

"The Upright Cup Test"

This measures water vapour transmission through the garment. The fabric is secured onto an upright, sealed cup which is tested over a 24 hour period to see how much moisture is passed through the fabric, into the cup.

The cup is then weighed, and the result of this weight indicates the grams per square metre- written as (gr/24hrs/m²) that the fabric can release as moisture. The higher the amount e.g. 10,000gm , the higher the levels of breathability.

"The Inverted Cup Test"

This measures amount of water absorbed by the garment. The fabric is placed onto a waterproof, moisture vapor permeable membrane and then held in a controlled amount of water.

Then an inverted weighted test cup is sealed and placed on the sample. After a 24hour time frame, the cup is weighed, showing the amount of water picked up and held by the fabric.

This amount shows the MVTR (Moisture Vapour Transmission Rate.)

"The Sweating Hot Plate Test"

This test measures the evaporative resistance of fabrics and the moisture loss of a garment when it is applied to heat, in order to mimic the body heat produced during strenuous activities.

The heat comes from a lab based metal plate which is heated to simulate the temperatures that induce sweat. The garment being tested is added to the plate, as is water, to simulate real life moisture.

The plate is kept at a constant heat, and then measured to show the evaporation process - simply, how much energy is being used to keep the plate cool. This happens in real life when moisture is vented out to the outer layers to keep the body at a standard constant temperature.

The energy that this uses, or the result, is demonstrated as a RET.

Wicking

Wicking is a garment’s ability to progress moisture away from the skin via the fibres. A wicking fabric is designed to move moisture away so it dries quickly, rather than absorbing the wetness.

For this reason, cotton and wool, which are porous, become heavier when wet, and are not wicking fabrics, whereas many fleeces are wicking.

Breathability and Waterproofing

It can be hard to separate the issues of breathability and waterproofing as they are closely linked to one another. This is because waterproofs rely on breathability in order to be wearable.

Water molecules are built from oxygen and hydrogen atoms, and with hydrogen bonding, these fit together in the form of a droplet.

The benefit of water droplets being large is that moisture vapours from sweat are smaller, meaning when the garment is created to be both waterproof and breathable, it can be. Waterproofing and breathability work together. As the factory-applied DWR coating breaks down, the waterproofing technology in a garment has to work harder to keep water out.

This means that the balance between breathability and water resistance alters. The garment has to work even harder to keep water out, therefore it is unable to be as breathable as it once was. This is mainly due to water sitting on the outer surface of the garment, where when the DWR coating was intact, it would have beaded off. This sitting water blocks the pores through which vapours usually pass freely.

The Wearer's Influence On Breathabilty

Oil and dirt from your skin can block the pores of a jacket, lowering it’s breathability, and can eventually lead to a reversal of waterproofing capabilities of the garment, acting as a sponge and drawing water in.

Many manufacturers decrease the likelihood of this by using a PU coating or a wash, which is oleophobic, or oil hating, on the inside of the garment. However, this can decrease the breathability of a garment.

Breathable Fabrics- What To Choose

Just as fabrics can be coated with a membrane to make it waterproof or windproof, breathability can also be aided by the use of coatings.

Coated, Breathable Nylons

These include fabrics such as polyurethane coated ripstop nylons and polystyrene. Hardwearing yet breathable and highly resistant to abrasion, these are used in a variety of products, from tents to groundsheets, hot air balloons, bags and more. Coated fabric construction designed to be breathable is based on the premise that water is directed away using the fibres themselves, rather than using membranes.

A multi-layer, lightweight coating, more suitable for general walking than more demanding outdoor sports. The outer face of the fabric has a standard DWR treatment applied to it to enhance performance.

A multi-layer microporous coating, impregnated under high pressure with millions of ceramic particles, which has the effect of creating microscopic holes around each particle.

A special near-permanent Dry Yarn technology treatment is then applied to the fabric’s outer face to improve performance. Mountain Equipment Drilite is a fabric that exists in both coated and membrane form. In membrane form it is called DLE.

Pertex utilizes a tight weave on the outer yarns of the fabric that allow moisture to pass through with ease. Moisture is absorbed by the inner yarns, creating maximum levels of breathability and complete windproof protection.

Breathable Membranes

These consist of an extremely thin film, containing microscopic pores that are large enough for body moisture to pass through, but small enough to keep water droplets out. Most microporous membranes are laminated to a face fabric and are available in two- and three-layer versions. A waterproof membrane is, by definition, windproof. A membrane jacket will out-perform a coated waterproof by about 25-30% greater efficiency on moisture movement and control.

GORE-TEX is the most established brand of microporous membrane. It consists of a wafer-thin expanded PTFE - Poly-tetra-fluoro-ethylene (Teflon), in which each pore is 20,000 times smaller than a water droplet, yet some 700 times larger than a body moisture molecule. The fabric thus has excellent levels of waterproofing and breathability.

e-Vent fabrics are created from a waterproof membrane which is also breathable. Similar to GORE, e-Vent uses millions of pores in its construction which allow the fabric to release moisture and to breathe. Without a PU layer included, breathability is increased as moisture is moved in one process, known by e-Vent as Direct Venting.

The construction of Paramo is referred to as drop-liner, meaning that the waterproof element is actually inside the lining. There are three constituent components: two layers of fabric and a waterproofing agent. The fabric is impregnated with Nikwax TX Direct treatment that leaves a water-repellent finish on each individual fibre so that the outer layer, as well as being fully windproof, will deflect at least 90% of water that hits it. Paramo is extremely breathable: no matter how hard the wearer works, or how much they sweat, body moisture will be moved away.

Breathability Within Footwear

Typically, GORE-TEX or e-Vent are used as the main breathable membranes in footwear, usually within the inner liner.

These tend to work more effectively within a leather boot than a fabric boot because in the fabric boot it is glued to the cordura and the nature of using an adhesive means breathability is hindered. In a leather boot it is fixed only at the heel and toe and breathability is able to function fully.

The main purpose of the GORE-TEX or e-Vent lining in a leather boot is to aid after-use drying; as a by-product it also makes the boot waterproof. In fabric boots, however, it is put in to make the boot waterproof, with breathability remaining a secondary concern.

Helping Breathability

Jackets, shoes and t-shirts that are designed to be breathable and wicking often utilise other features that can assist them in being able to vent more freely. This speeds up the process of wicking away moisture, as it opens up the garment to allow moisture to be physically released.

Mesh areas – Mesh is a loosely woven fabric that is woven in a way that creates even holes along the surface of the garment. Mesh allows extra venting to occur.

Pit Zips- Found on top layers, pit zips sit under the armpits and allow you to release excess heat without exposing inner layers of your fabric, or your skin to the external elements.

Deep Zips on tops that expose an inner insulating layer, or the face fabric to the elements.

Zip off areas on trousers- These allow you to transform your trousers into shorts or capri lengths, ideal for allowing extra venting to occur. Loosened hems and toggles allow you to expel heat and moisture as required.