One of the difficulties of discussing breathability is that there are lots of transient factors as conditions change. For this reason it can add some clarity to break breathability into separate phases which may relate to a typical day in the hills:
- Conditions are dry and you are not sweating.
- It's dry and you are working hard.
- Conditions are wet, yet the outer durable water repellency (DWR) is working well.
- It is raining and the outer fabric has become saturated.
- The outer is saturated but the rain has stopped.
1. Conditions are dry and you are not sweating. Even though you have no sensation of dampness, moisture is constantly evaporating from your skin. If you were wearing a non-breathable garment you would notice condensation inside after a time. You may never notice this with a breathable garment, depending on its performance and the ambient conditions (temperature, pressure and humidity).
2. It's dry and you are working hard. The extra moisture you are creating within your garments can overload even the best breathable fabrics. Even if you were wearing nothing at all you may be soaked in sweat because the air temperature, humidity and pressure may not allow it to evaporate it quickly enough.
Sometimes you may be extremely active yet require some protection from wind chill. Windproof garments can offer more breathability than waterproofs. The more wind resistance you get with these fabrics the less breathable they are. FurTech garments are as breathable as the more windproof of the woven wind shells.
Waterproof breathable fabrics can only pass a limited amount of vapour before condensation occurs. Often this is measured in grammes per meter squared of fabric per 24hrs, though the results are dependant on which test is used (please see the Breathability Testing post, below).
The thickness and type of inner layers also has a bearing on how quickly condensation can occur: the more dry air you start with in your clothing the bigger its capacity to absorb moisture and some fabrics absorb moisture (wool, cotton etc.) Unfortunately, both these aspects can also add heat (insulation and heat of absorption). Underlayers can also reduce the effective breathability of the outer fabric which is dependent on the difference in humidity inside and out.
If you are generating much heat, please consider if you need to be wearing your shell. By reducing insulation and increasing venting you can keep cooler and prevent your insulation from becoming damp and ineffective.
3.Conditions are wet, yet the outer durable water repellency (DWR) is working well. In many situations a windpoof with a good DWR is sufficient but the DWR can fail in a number of ways:
- It can be worn off.
- It can become dirty.
- Water can be forced into the fabric.
- Condensation can occur inside the weave or knit, attracting water from the outside (this effect can sometimes be witnessed when wearing just a base layer in the cold - the outer fibres can develop a fine dew).
If conditions worsen, you may wish you had already donned your waterproof.
So long as the DWR is still working a membrane can still breathe. However, rainy conditions are likely to be cooler and more humid, limiting its capability and it will often not work as well as the lab tests indicate.
TGO Gear Editor, Chris Townsend, says "I've found that when the outer fabric wets out the condensation increases in all membrane/coated fabrics (all types of Gore-Tex, eVENT, Sympatex, all the PU coatings)... However even with a good DWR and no wetting out I can still produce much condensation in all these fabrics, especially in prolonged rain. Paramo is far more breathable and my first choice as long as it's cold enough (mostly October to May). " Chris hasn't had the opportunity to test FurTech, which works in the same way as Paramo.
4.It is raining and the outer fabric has become saturated. (This could be called a sentiment change from water hating to water loving.) The membrane becomes chilled to almost the same temperature as the rain its self and breathability all but ceases, resulting in condensation and increasing humidity inside. This prevents your sweat from evaporating so you can't keep cool and the moisture within your insulation means you chill when you stop (the overheat/chill cycle).
FurTech type garments work differently in this phase. The thin saturated outer fabric attracts water from the inner fur lining, sucking liquid water out of the system. Surplus water drains away out of the hem but the inside remains dry.
Normal lab tests do not measure performance in this phase... or the next.
5.The outer is saturated but the rain has stopped. The water in the outer fabric can evaporate if humidity decreases, it heats up or the pressure drops. Typically it's dried by the wind (reduced pressure). However, as it evaporates it has an enormous cooling effect (latent heat of evaporation). This chills the membranes used in conventional systems so they cannot breathe, and you remain uncomfortably cold and damp. These membrane systems don't recover their breathability until the outer fabric has dried.
FurTech type garments continue to breathe because the damp outer fabric pulls condensation off the inner fur lining, allowing it to evaporate off the outside (the outer fabric has become hydrophilic). So they dry very quickly and you don't tend to overheat or chill excessively.
The case study on this link shows some results for how microporous membranes and hydrophilics breathe in the wet and how they recover. FurTech garments weren't tested but I would expect much better results than these! You can feel the difference!