Breathability is often mentioned as an important feature in connection with shell or outdoor clothing and sportswear. But what does breathability really mean, and does it matter? In this article, we’ll answer these questions.

Breathability in a nutshell

In general, breathability is defined as the fabric's ability to let water vapour pass through. A loosely woven cotton or linen fabric is more breathable than a tightly woven nylon fabric. Breathability is an important feature both in hot weather clothing and in the layers that protect you from the elements.

That said, typically breathability is only measured for water- and windproof fabrics, and only water- and windproof products include information regarding their breathability. To some extent, a membrane always decreases the fabric’s breathability, but the breathability of the membrane has a significant effect on whether you feel like you’re wearing a plastic bag or a high-quality outdoor garment.

Read more about waterproof and water-repellent clothing

How can a waterproof fabric breathe?

Waterproof membranes are full of microscopically small holes, designed to be precisely the size that liquid water cannot pass through them, but water vapour can.

Breathability is based on a difference in the pressure inside and outside the clothing: the higher heat and humidity inside the clothing attempts to level out with the outside air. As a result, when you move and sweat, not all the moisture accumulates inside your jacket as some of it is transferred outside through the membrane. Heat also evaporates with perspiration, making you feel more comfortable.

The greater the differential pressure is, the more efficiently sweat will be transferred outside. That is why breathability works best in cool and dry weather. If the outside air is also hot and humid, a sufficient pressure difference doesn’t occur.

How is breathability measured?

Breathability can be tested in various ways. Our breathability ratings are based on the JIS method in which a higher rating always means better breathability.

The fabric to be tested is placed over a cup, and the cup is then placed upside down in water. The moisture-absorbing substance inside the cup tries to pull moisture through the fabric into the cup. After a while, the fabric is taken out of the water and weighed to see how much liquid has passed through the fabric into the cup.

A vapour permeability value can be calculated based on the result. The value can be, for instance, 5,000 grams per square metre per day, i.e. 5,000 g/m2/24 h.

This value is called a MVP (moisture vapour permeability) or MVTR value (moisture vapour transmission rate) – they both refer to the same thing.

What is a good breathability rating?

The higher the MVP value, the more breathable a fabric is. 5,000 g/m2/24 h is sufficient for relaxed activities. For more energetic activities, we recommend products with a breathability rating of at least 10,000. A rating of 20,000 is a top-of-the-line breathability rating.

Factors that decrease breathability

Breathability decreases if the membrane is blocked from the inside or outside, either partially or completely. That’s why, when carrying a backpack, your back gets wet quickly. Water can also temporarily block the membrane: if the surface of the fabric absorbs a lot of water, water vapour isn’t able to evaporate. Therefore, waterproof clothing is almost always impregnated or treated with a water-repellent finish.

The inside of the fabric is often ignored, but dirt and grease that accumulate on the fabric when the garment is used can also block the tiny holes in the membrane. To maintain breathability at the original level, wash and maintain your clothes regularly according to their care instructions.

Are ventilation openings necessary?

During sports or other vigorous activities, even the most high-quality breathable fabric might not be enough, and ventilation openings can bring added comfort. On the other hand, a clothing made of a less breathable fabric can still be suitable for a wide range of uses if the additional ventilation has been designed properly.

The following two methods are the most common methods of ventilation, and they both have their own strengths:

• A back vent: Warm air will escape naturally through the opening on the upper back. This ventilation method is easy and reliable but not very well suited for outdoor clothing as the opening is typically covered by a backpack. Wind can also blow in from under the flap.

• Armpit zips: Armpit zips are common, especially in outdoor, hiking and skiing clothing. Through the openings, moisture can escape straight from the areas where perspiration is the highest. Armpit zips also work well with a backpack, and you can close them completely if necessary.

The suitable ventilation method really depends on the intended use of the clothing. It’s also worth remembering that having separate ventilation openings isn't the only ventilation method. Loosening your collar or opening a zip can already make you feel much more comfortable. You should also check the pockets: if the insides are made of a mesh fabric or otherwise let air pass through, the pockets will also act as vents.