Breathability in Waterproofs Explained
Breathability in waterproofs explained.
What is breathability?
When we talk about gear being ‘breathable’, we are referring to the extent to which it allows moisture to escape. Even during light activity, the body responds by sweating. In a ‘non-breathable’ garment there is no way for this moisture to escape. It collects on your skin and on the fabric as condensation.
Trapped inside your jacket, this moisture acts as a thermal conductor. It transfers your body heat nearly 25 times faster than dry clothing. This feels very uncomfortable, chilling, and in extreme environments, it can lead to hypothermia. A breathable waterproof garment will allow moisture to escape through the membrane. It reduces peaks and troughs in body temperature and keeps you comfortable for longer.
How can a membrane be waterproof and breathable?
A microporous membrane has millions of tiny holes that are large enough to allow water vapour to pass through, but too small for water droplets to get in.
What are they made from?
In waterproof garments, these membranes can be made from a number of substances such as ePE, ePTFE or PU, in a network of tiny, interconnected pores.
How do they work?
Individual water molecules (around 700 times smaller than the pores) in sweat vapour can pass through the membrane.
Groups of water molecules (around 20,000 times larger than the pores) in rain droplets are too big to pass through the membrane.
A hydrophilic (water-loving) membrane attracts moisture to its inner surface which it then moves to its outer surface.
What are they made from?
Often made from PU or polyester, this is a solid-state membrane without any microporous holes.
How do they work?
You might have heard of osmosis from your days in biology class. That’s exactly how a hydrophilic membrane works. Moisture from your sweat is absorbed into the membrane and then diffused into the atmosphere on the other side.
As you get warmer and produce more sweat, the diffusion gradient increases. The membrane responds by increasing the rate of moisture transmission, working to equal the balance of water vapour on the inside and outside of your gear.
How breathable is my waterproof?
Here’s where things get technical. You can measure the breathability of fabrics in a variety of ways in a lab.
You might have seen the MVTR rating quoted before – this refers to the Moisture Vapour Transfer Rate and is an industry standard test. We also use RET, another industry standard test that simulates sweat transfer through fabrics.
MVTR (Moisture Vapour Transfer Rate) records the amount of evaporation through a square metre of fabric over a 24-hour period.
There are two kinds of MTVR test, inverted cup and upright cup, but the premise is the same. They calculate the change in weight created by the movement of moisture from one side of the fabric to the other.
This weight difference is calculated after the test to give a moisture vapour transfer reading of grams per square metre of fabric over 24 hours. This describes how much sweat will pass through the fabric.
What should I look for?
Figures can range from less than 10,000 MVTR to up to 40,000 MVTR.
High MVTR = High breathability.
Low MVTR = Low breathability.
RET shows the fabric’s level of resistance to evaporation.
In this test, otherwise known as the Sweating Guarded Hot Plate test, fabric is placed above a porous metal plate. The plate is heated to a constant temperature between 33°C and 36°C and water is channelled into the metal plate to simulate sweat.
As water vapour passes through the fabric it causes evaporative heat loss. The amount of energy that is needed to keep the plate at a constant temperature is used to extrapolate an RET figure.
What should I look for?
The better the breathability, the lower the RET value.
High RET = Low breathability.
Low RET = High breathability.
Does breathability mean comfort?
Using the same test method, we may be able to confirm that one product is more breathable than another in the lab, however this doesn’t take into consideration the other properties that impact overall comfort. Comfort can come from many elements, including the breathability, weight, and stretch of the fabric. It may have specific features designed to improve ventilation, like pit zips to dump heat or a soft, fast-wicking lining.
We use these laboratory tests as a guide for understanding how breathable a fabric is. However, we know that comfort is personal and depends on the design of the garment as a whole.