About That Waterproof Jacket…

If you have participated in a professionally organised trail running event with a compulsory kit list, then you will have been faced with the “waterproof jacket” requirement and the resulting debate on which one deserves your hard-earned cash. They aren’t cheap, but for good reason.

Waterproof fabrics are fabrics that have been treated to become resistant to penetration by water. The term "waterproof" refers to conformance to a governing specification and specific conditions of a laboratory test method. A truly waterproof rain jacket, the kind that will save your life when put to the test, will have features such as tape-sealed seams and an integrated hood.

We interviewed tech fabric guru, Morne Strydom, for an in depth look at what goes into, and comes out of, a waterproof jacket. Morne was part of the original Cape Storm team in 1998; during the company’s infancy. At the time, they were market leaders when it came to local research and the development of specialist, technical outdoor garments, and in particular waterproof breathables. Morne joined First Ascent in 2007, and in 2014 moved on to the Adventure Inc team. In short, there isn’t much that he does not know about the development or application of outdoor gear.

“The first question you might have is, what is waterproof? In its most basic definition, a black bag qualifies, as does a cheap poncho. However, there are complex levels of waterproofing, as outlined in the label of a waterproof garment”, says Morne.

“The Hydrostatic Head is the globally recognised definition in mm. Some garments refer to the schmerber rating, which is the same thing. The Hydrostatic Head is determined through intense fabric testing, where a column of water puts pressure on fabric. A tent, for example, might have a Hydrostatic Head of 2000mm. This means that the tent fabric could withstand the pressure exerted by a column of water that is 2000mm tall.”

But when it comes to waterproof jackets, particularly those used for a sport like trail running, the Hydrostatic Head is just one element of the overall product concept.

“Design is another major influencer when it comes to the garment’s performance,” emphasises Morne, “and you pay more for a superior design. An integrated and functionally designed hood, for example, improves the functional waterproofing of the garment substantially. Fabric flaps over the top of front-facing pockets will also reduce the chance of water coming through the garment, as will aqua guard zippers. A Raglan sleeve design takes the seam away from the top of your shoulder, which is the area that takes the most water pressure in a downpour.”

The actual manufacturing process and quality control is the next major influencer. Each time a needle hole is put into the fabric the garment’s performance is compromised and therefore needs to be sealed to maintain waterproofness.

“A tapesealing machine utilises heat and pressure to fuse the tape into the coating or membrane on the inside of the fabric and ultimately ensures complete waterproofing” says Morne.

Tape Sealed Seams as seen from inside a waterproof jacket

A DWR finish is added to the outside of the face or carrier fabric to assist breathability by shedding the water it encounters from the outside and thereby creating the opportunity for the moisture from inside to pass through the fabric. This is why the maintenance of this DWR finish is so important and why proper care and washing of a waterproof breathable is essential.

The actual waterproofing is a result of the membrane or coating which is applied to the inside of the face or carrier fabric.

Waterproofing of a technical garment is one thing, and achieved through a combination of the above, but breathability in conjunction with waterproofing is the key balance. A plastic bag does not breathe. This breathability refers to the fabric’s ability to allow body moisture to pass through as vapour.

The Moisture Vapour Transmission Rate, or MVTR, of a fabric is indicated in mm per 24 hours per square metre of fabric. An MVTR of 5000, for example, allows 5000ml of body moisture through per 24 hours, per square metre.

“You can continually improve the waterproofing of fabrics, but your breathability will be reduced. Modern fabrics seek to find the happy medium between the MVTR and the Hydrostatic Head”, explains Morne. “To further understand the performance of your garment, consider that the harder or faster you run, the more moisture is produced. The ideal conditions for a breathable fabric to perform is a big differential, with heat and relative humidity being the key factors to be considered. Exercising at altitude would represent the ideal environment for optimal breathability as there is likely to be a significant heat differential and very little moisture in the air as it will be frozen solid. On the other side of the spectrum running in a waterproof garment in Durban or similarly humid climate reduces the breathability of a garment, as there is limited differential to drive moisture out. Premium technical garment brands offer the best combination of features to release internal moisture, and keep external moisture out.”

It is critical to remember that this optimal balance will be best maintained as you keep moving to create driving pressure. If you are injured or unable to run, a waterproof breathable garment will allow the moisture inside to dissipate as your core body temperature starts to drop thereby reducing the ‘cold and clammy’ feeling while cutting the wind-chill which is a major cause of hyperthermia. The use of the correct under layers become even more critical in this situation and this is exactly why compulsory kit lists specify (non cotton) base layers, micro fleece tops and space blankets.

“Once you have invested in a quality waterproof jacket, basic care of the garment is critical. Check the label for specific instructions, but in general do not use normal washing powder, as it will strip the DWR. Fabric softener is about the worst thing that you could do to your jacket! Specialist outdoor stores have appropriate cleaning products as well as solutions that can assist in replenishing the garments DWR if it has been compromised”, concludes Morne.

Our recommendations when you are out shopping for the most important garment in your mountain running bag: 

  1. A Hydrostatic rating of 10 000mm to 15 000mm is suitable for just about all weather conditions but equally at home in lesser ones
  2. A MTVR or 'Breathability' ratio of 15 000mm to 20 000mm / 15 000g to 20 000g would be considered optimal for a quality waterproof running jacket

The following Waterproof Running Jackets meet the above listed criteria, in particular, the optimal Hydrostatic Head vs Breathability ratio.