FurTech Science

Can waterproofs breathe in 100% humidity?

100% humidity occurs in wet and misty weather. It means that the air is saturated with water and it can't absorb any more. Is it possible for our sweat to evaporate through our clothing in such conditions? Two factors come into play:

1. The heat of your body warms the surrounding air, which can then absorb more moisture. But as ambient conditions reach skin temperature, breathability ceases because there is no temperature differential to drive the process. (This can also be explained in terms of a differential in saturated vapour pressure.)
2. Wind has an effect: The air that has been warmed by your body and which contains the extra moisture from your clothing, is removed by the wind. You then warm the new air and impart moisture to it, thus improving breathability.

Wind can reduce the pressure on parts of your clothing allowing air at less than 100% humidity to absorb more moisture (it increases the pressure on other parts, reducing local breathability). When pressure is reduced relatively slowly (as is the case with wind) it is an isothermal process, however, when pressure is reduced rapidly it almost becomes an adiabatic process which can cause condensation as can be seen above the aircraft wing in this picture.

Thanks to www.flyart.biz

Durable Water Repellancy

Abrasion of DWR treatments can remove water repellency fairly rapidly. (See this link to Trail magazine.) The outer fabric wets out, which causes plummeting breathability in conventional membrane hardshells and softshells... you then get wet from condensation.

The DWR applied to the FurTech lining is protected from abrasion by the outer fabric. Even when the outer saturates, the lining remains active. In fact the wet outer fabric draws moisture away from the body and out of the fur. We call this Wet Weather Breathability. It also allows our fabrics to dry rapidly (unlike membrane garments).

Pressure Test of Breathability

This link shows tests carried out on breathable fabrics. It demonstrates that Paramo (Fur type Technology) is more breathable than eVent which is more breathable than Gore-Tex!

This experiment shows what force is required for air molecules to pass through the fabrics, which is a good indication of conventional breathability. However, it does not take into account any of the thermodynamic effects and real world problems that lead to condensation inside waterproofs.

The report also suggests that feather and fur type garments are too warm. We would recommend wearing less beneath them and utilising the integral vents. This gives them a far greater temperature flexability than conventional layering systems with less time consuming layer changes! Wear them just like a softshell, which happens to keep you dry. Here's some recommendations for different activities.

Aerodynamics and Breathability

Imagine that your body is made up of cylinders representing the torso, legs and arms. As the wind blows around you it effects the breathability of your clothing. Where it increases pressure, into wind, breathability is reduced. Where the pressure is reduced, breathability is increased.

This image shows stable air flow around a cylinder (white). The red and yellow region represents increased pressure and the blue area represents reduced pressure. As there is more blue than red, this indicates that, on average, the breathability is improved by the action of wind. (Reynolds number = 10. Click here for other Re.)

Note that this does not take account of the fabric flapping, which should improve breathability.

Wind and Breathability

The wind changes the breathability of a garment in two ways:

High pressure on a fabric reduces breathability and low pressure increases breathability.

It makes the fabric flap and pump air around inside, improving moisture transport.

Wind blown fabric can also shake water droplets off the fabric. I sometimes see large drips shooting away down wind of my hood.

Rain Room Testing

Waterproof testing can be carried out as a field test, pressure test or rain room test.

Some waterproof membrane fabrics claim high resistance to pressure, in the order of 20m of hydrostatic head (until they get punctured by a thorn or crack after being stuffed in your sack!) This is irrelevant because rain exerts little pressure.

FurTech jackets have just passed Leeds University's Rain Room Test. This simulates rain at 5 times typical heavy British rain and is the same test used by Paramo.

According to Leeds University staff, not all Gore-Tex garments pass this test because of their design, despite the fabrics having high hydrostatic pressure results!

Layering Hardshell over Softshell

Adding a waterproof jacket or trousers to softshell or conventional layers causes a problem. Often you are already damp by the time you get them on. This can overload the breathability because:

1. The inside of the waterproof gets wet, retarding breathability by cooling the membrane (latent heat of evaporation is absorbed, perhaps chilling the membrane below the dew point).
2. There is more humidity in your clothing system, requiring more breathability to allow it to escape.
3. Water must evaporate to pass through the membrane.

In my experience these problems are particularly prevalent when adding overtrousers to wet leggings. I've also noticed the problem when adding Gore-Tex to Buffalo, though it does create a warm damp atmosphere inside.

Fur type garments can be worn across a wide range of conditions in the same way as other soft shells, but when it starts to rain you don't need to stop and add waterproofs... commonly known as faffing!... And the water stays on the outside!

Insulation and Thickness

It is intuitive to think that thickness equals insulation, but that isn't quite the whole story. Lets look at the four types of heat loss:

1. Different materials have different rates of conduction even if their thickness is the same. For example air may conduct 1/10th as well as nylon (materials' conductivities).
2. Convection currents effect insulation. Take for example double glazing. Once the air gap becomes too big convection currents inside make it less effective. You may also have experienced the lack of insulation that a thick conventional fleece offers in windy conditions.
3. Heat radiation is all about the surface emissivity of a material and isn't related to thickness but instead is based on its chemistry.
4. Limiting evaporation (with low breathability) slows the loss of latent heat.

In conventional garments the materials are the same or very similar, so conduction and radiation provide little differentiation between types (unless we are talking about Blizzard Survival Equipment, foam or phase change fabrics).

However, convection and evaporation are greatly effected by a fabrics design and garment features. For example:

• Thick fabrics with little wind resistance can be much cooler than thinner windproof fabrics.
• Thin fabrics with poor breathability may cause more overheating than thicker fabrics with better breathability.
• Vents can increase breathability and convection inside a garment, reducing overheating and the effects of thick, windproof insulation.
• Loose fitting garments (or sleeping bags) may allow more convection inside and feel cooler.

Some of these factors may be why I often feel more comfortable in FurTech garments than conventional waterproofs, even in the summer months, when intuition would claim the opposite. It would be interesting to get some experimental data on this phenomenon to quantify my experience.

Layers and Breathability

Lack of breathability can be considered as resistance to water vapour transmission. Very air permeable fabrics have excellent breathability and very windproof/waterproof fabrics have low breathability. This link demonstrates the difference by forcing air through eVent, Gore-Tex and Nikwax Analogy.

So what about layering a waterproof over a windproof?

If the windproof is 90% breathable and the waterproof is 80% breathable, the combination is 90% times 80% or 72% breathable. And every other layer further reduces moisture vapour transmission.

Please note: this only relates to conventional breathability and not wet weather breathability.

Phases of Breathability

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:

1. Conditions are dry and you are not sweating.
2. It's dry and you are working hard.
3. Conditions are wet, yet the outer durable water repellency (DWR) is working well.
4. It is raining and the outer fabric has become saturated.
5. 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!