Waterproof and Breathable Membranes—Answers to Frequently Asked Questions

Gore-Tex, eVent, Pertex Shield, Futurelight? The range of membranes and waterproof systems is wide, and the differences are not always clear. Here, we will clarify which information will help you make the right choice according to your activity.

Waterproof and breathable membranes are essential for mountain activities. Photo Rab

What is a membrane?

A membrane is a thin waterproof and breathable material placed underneath the outer fabric of a garment.

Unlike fabrics that are purely waterproof, such as those used for plastic raincoats or tents, fabrics with a membrane have the capacity to wick sweat away from your skin to the outer layer.

How do membranes work?

Currently, all membranes on the market use the same structure: the material consists of millions of microscopic pores that stop water molecules getting through, but allow water vapor caused by perspiration through. This means, the entry of water is prevented but the evacuation of sweat is facilitated.

Being a somewhat fragile material, membranes are protected internally and externally by other fabrics that also help reinforce the properties of the membrane.

What are membranes made from?

The most common material is ePTFE (expanded polytetrafluoroethylene), which is better known by its commercial name, Teflon. Among its many properties,Teflon has some very interesting features for use in mountain environments: it is very flexible and resistant and when it expands it forms the micro-pores that we have mentioned above.

The nanospinning used by Futurelight is another waterproofing system that uses an injected polyurethane solution to form a layer with properties similar to those of membranes.

Microscope comparison between FutureLight (left) and Gore-Tex (right).

Do all membranes have the same properties?

There are several waterproof membranes or technologies (Gore-Tex, eVent, Futurelight, Pertex Shield...) and among them, there are many variations depending on the construction process or properties. All are waterproof and breathable, although to different degrees depending on the activity they are designed for. The way they are attached to the outer fabric, the lining or the construction method used can also vary.

Depending on the type of membrane, the wind permeability performance will also vary. Some are totally windproof, while others allow a certain degree of air exchange and it is this second type that is said to have high breathability.

How is the waterproofing capacity of a membrane measured?

The usual system is the water column or HH (Hydrostatic Head, hydrostatic load) measured in millimeters. This test consists of placing a bottomless tube on the fabric and gradually filling it with water. When the fabric begins to lose impermeability, the height of the water in the tube is measured and this measurement is the HH.

In clothing, 10,000 millimeters is considered adequate for sporadic rain and moderate intensity, anything over 20,000 millimeters is considered excellent waterproofing capacity and the most waterproof membranes have 28,000 to 30,000 millimeters.

In tents, where the fabric is always taut and not in contact with the body, the water column requirements are much lower.

A graphical representation between different water columns. Graffics by Rab.

How is the breathability of a membrane measured?

There are two methods; MVTR and RET. These concepts are explained in more detail in this article on technical terms for mountain equipment, but here is a brief summary:

  • MVTR (Moisture Vapor Transmission Rate) also known as MVP (Moisture Vapor Permeability) specifies how much water vapor can pass through a fabric. This is measured in grams per square meter in 24 hours. Anything above 10000 g/m²/24h is suitable for mountain activities, while ratings over 20000 g/m²/24h are recommended for intense aerobic activities.
  • The RET measures the resistance of the fabric to moisture transfer and, therefore, the lower the RET, the more breathable the fabric. RET (Resistance to Evaporative Heat Transfer) is measured in Pa.m²/W and ratings below 14 are suitable for mountain activities and below 7 are suitable for aerobic activities.
  • What are the layers of a membrane?

    A two-layer membrane, a three-layer membrane or even a two-and-a-half-layer membrane are terms used to refer to the adhesion system that attaches the outer fabric, the membrane and the lining of the garment. This should not be confused with the layering system of mountain clothing.

    • Two-layer membranes laminate the membrane to the outer fabric, but the lining, which is usually mesh, remains unattached, which results in slightly reduced packability and breathability.
    • Two-and-a-half-layer membranes laminate the membrane to the outer fabric, but in this case, there is no lining. Instead, a coating is applied to the inner face of the membrane (printed or sprayed) to protect the membrane from abrasion. This method is lighter and more compressible than two-layer membranes, but not as durable.
    • Three-layer membranes laminate the membrane to both the outer fabric and the lining. This results in a compressible, highly breathable and very durable garment.

    Do membranes provide warmth?

    Although any fabric covering your body will provide a certain amount of warmth, it is not the function of a membrane to maintain the body temperature. This is achieved by using other garments such as a fleece, insulated jackets or thermal undergarments.

    Any warmth provided by a jacket or pant with a membrane has little to do with the membrane itself, but depends on the other materials the garment is made from. To stay warm in a mountain environment, the membrane (in the shell layer) should be accompanied by other garments with insulating properties (mid layer and, more importantly, a base layer).

    Does a membrane need special care?

    As the membrane is placed on the inner face of the garment, it does not damage easily in normal circumstances and its materials are especially durable, much more so than normal linings and outer fabrics. However, as it wicks body moisture away from the body, it can accumulate traces of salts and dirt that can clog the pores. The outer waterproofing treatment also loses its properties with time, use and friction that occurs during activity and with the use of backpacks.

    For this reason, it is a good idea to wash the garment periodically according to the manufacturer's instructions, which can include ironing or tumble drying to reactivate the waterproofing treatment of the outer fabric.

    For more information on this subject:

Leave a comment

Be the first to comment on this article.