Keeping dry when its wet - how rain jackets work

SATURDAY SEP 17 / 2016 / by Andrew Talati
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Staying dry at Mt Eliza NRR

There are two main companies supplying this highly specialised fabric, Event and W.L Gore's ‘Gore-Tex’ brand.

These fabrics are a waterproof, breathable three layer membrane with microscopic pores using an oleophobic and hydrophobic (oil and water repelling) chemical application to ‘vent’ the moisture rather than using diffusion (the moisture passing from a concentrated less concentrated area) to transfer the moisture to the outside of the garment.

Traditional fabrics

Traditionally, the waterproof, breathable coatings were applied to jackets that consisted of a monolithic membrane (solid membrane) to transfer moisture from underneath the jacket but the membrane first become wet.

These three layered fabrics are a lot more breathable as it’s an air permeable membrane (allows water vapour/sweat to pass through the membrane), the better the air permeability, the greater you are shielded from the cold air.

Three layer construction

Diagram of 3 layers water proof breathable jacket

A Waterproof Breathable Jacket or 'WPB' jacket typically consists of 2 to 3 'layers'. This refers to the construction of the garment.

Layer 1 - Face fabric 

We start on the outside, first there's the jacket itself, a woven fabric typically a form of nylon or polyester is popular. As mentioned this has a 'DWR' applied to it.

Layer 2 - Direct venting membrane 

This is where the waterproof and breathable layer lives. This layer is either laminated or coated onto the inside of Layer 1. Most microporous membranes are a laminate as they are lighter and commonly only this process will produce the desired effect of air permeability.

Layer 3 - Inner lining

This third layer acts primarily as protection from abrasion on the inside of the jacket but also helps to diffuse any condensed moisture vapour and give the interior of the WPB Jacket a nice feel on the skin as opposed to the 2 or 2.5 layer WPB Jackets. These WPB Jackets either have a basic 3rd layer or none at all. They can feel rather 'plastic' against the skin and do not offer the same abrasion protection as a three layer version. The advantage of the two layer construction is lower cost and in some cases lighter weight.

Micropourous Membranes

These highly sophisticated materials contain billions of holes per square centimetre to allow the water vapours to pass through but require a coating to stop the external contaminants  from compromising the effectiveness of the outer membrane. The integrity of the face fabric (outer membrane) requires the surface tension to be maintained to ensure water or other containments do not pass through. This is referred to as the Durable Water Repellency or DWR where its primary function is being breathable.

Repelling water from the outside

Mt Donna Buang summit snowPhoto courtesy of Brendan Edwards @thedandenongranges

By using a Durable Water Repellency or DWR coating to the outside of the jacket (fluropolymers, silicones or hydrocarbons), the surface tension is increased sufficiently to stop water passing through; the water will simply bead off.

How it works

Two Riders Rain jacketsPhoto courtesy of Brendan Edwards @thedandenongranges

Rather than sweating like a hog and your sweat vapour condensing on the inside of the jacket, the moisture is transported back to the front of the jacket through osmosis, it’s a humidity and moisture deferential that attracts the moisture back to the outside of the jacket again.

Draw backs, it’s a much slower process with your sweat is being transported through the jacket as a sweat vapour but the limit of the technology is on the outside of your jacket, being dry so this is counter intuitive as it’s a waterproof jacket. So a Teflon based water repellant is added to stop the actual material that the jacket is made from, i.e. polyester or nylon forming a solid barrier that your sweat vapour cannot get through, but also stopping the rain particles from the outside from penetrating through the jacket.

However, contaminants and general wear and tear will degrade the effectiveness of the water repellency so the coating must be re-applied depending on the frequency of usage otherwise the fabric will absorb the water. Your sweat will then condense and be transformed from a water vapour back to a solid state or a liquid form and will not pass through the inner layer.

A trade between waterproof & Breathable

Ol Dirty 2015_MuddyPhoto courtesy of Brendan Edwards @thedandenongranges

Like anything there are tradeoffs; to have 100% waterproofing means you compromise breathability so remember when using these jackets to assess the conditions and use a bit of trial and error so you don't end a complete sweat ball and all clammy.

Most importantly - the care instructions

Without following specific laundering procedures, the jacket will quickly fall apart and lose its effectiveness, i.e. not being that breathable anymore.

Normal detergents are for washing out blood, wine and really stubborn stains on normal clothing. It's simply too strong and will try to wash away some or all of the DWR treatment.

1. Get to your local outdoor store to buy a milder detergent such as Tech Wash by NikWax to clean the jacket.
2. Regularly washing your jacket! Funnily enough, as it’s more breathable, it requires washing more often.
3. Re-proofing the DWR - you will need to buy a specific product such as 'TX.Direct' by Nikwax.

A good workaround is getting a 2in1 (wash and proof) by Grangers or using pure soap flakes for washing and the TX.Direct or Grangers alternative to do the re-proofing.

Lastly getting the perfect fit

Lone rider Rain wearing rain jacketPhoto courtesy of Brendan Edwards @thedandenongranges

Finally, fit is king. An ill-fitting WPB jacket on a bike, particularly on a road bike, will flap around like silly and will offer limited movement due to the aggressive position.

Many thanks to Tim Ottaway for contributing the technical know how

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