We all know what it's like to step out into a raging storm and feel the elements thrash feebly against your seemingly impenetrable outer layer. At times like this you realise that all waterproof breathable jackets are not created equal.

What makes a fabric waterproof and breathable?

All waterproof fabrics have an outer layer, or face fabric. The main function of the face fabric is to provide a flexible and durable outer shell. The inside of the face fabric is either laminated or coated with a protective moisture barrier. This barrier is the part that does all the work of keeping you dry and comfortable in wet conditions. It prevents water ingress from the outside and allows moisture vapour to escape, keeping you comfortable inside.

If it's that simple, don't all fabrics perform pretty much the same?

In a word, no. Firstly face fabrics differ significantly in their abrasion resistance, tear strength, weight and water repellency. It's important that the jacket has the right face fabric for the job. You wouldn't want a nice soft handle, low tear resistance fabric on a full mountain shell, no matter how pretty it looks in the shop. Likewise on the protective moisture barrier, whether a membrane or coating, the range of breathability and durability on offer is vast.

How can one fabric be more breathable than another in the same conditions?

To understand why breathability can differ you have to first understand why and how breathable fabrics work. Regardless of the product: Gore-Tex, Sympatex, Conduit, H2n0, they all work or 'breathe' by temperature differential (Delta T). When the environment on the inside of the jacket is warmer than the environment on the outside a driving force is created. The higher the Delta T, the higher the driving force, and the more the fabric will 'breathe'.

The perspiration you create when active outdoors transforms into water vapour. The structure of the inner protective moisture barrier allows the water vapour to pass through it, but prevents rain from coming in. This is because water vapour molecules are much smaller than liquid molecules. How much the fabric 'breathes' depends upon the inner barrier's 'resistance to evaporative transfer'. This rate of breathability is widely termed RET. The lower the RET rating the higher the breathability, because the resistance to vapour transfer is less. That's why different fabrics can have different levels of breathability (or comfort), even if the Delta T or conditions are identical.

What is DWR?
DWR stands for Durable Water Repellency. DWR is a chemical treatment applied to the outside of the face fabric. This DWR treatment makes water hitting the face fabric surface bead up and run off, like rain on a window. After extended use the DWR treatment on all face fabrics wears out. Water hitting the fabric surface starts to give the impression of soaking into the face, rather than beading up and running off. When this happens it isn't immediately necessary to re-proof the garment (using TX Direct, Grangers Extreme or similar proofing agent).

You can easily restore the DWR water beading qualities of the fabric by applying heat, usually by washing and tumble-drying. The DWR treatment held within the face fabric melts at reasonably low temperatures spreading to the outer face, allowing the water to bead once again. DWR treatments can wear out quickly in certain situations: smoke from campfires, for example, greatly reduces the effectiveness of DWR treatments. Despite claims to the contrary, no manufacturer has developed a permanent DWR finish. All DWR treatments wear off over time and eventually need re-proofing.

What about durability, is it all about the face fabric?
Face fabric is certainly important, but the Number One factor in determining long-term durability is the overall fabric construction. Membranes and coatings are thin and fragile: they need protecting on the outside (the function of the face fabric) and on the inside (the function of the lining).

Serious mountaineering shells all utilise a laminated mesh liner on the inner surface. Under a microscope this laminated lining looks very similar to a fishing net. It protects the membrane or coating, but easily facilitates the passage of water vapour. This construction is described as 'three layer'. This type of construction also yields the lightest weight for strength. Two-layer construction has a loose mesh liner on the inside, rather than a laminated mesh.
  Contrary to popular belief, the primary function of the loose mesh liner isn't to aid breathability. It is to increase durability. It doesn't matter if the fabric is a coating or a membrane; three-layer construction is always far more durable than two-layer construction, in every single case. Any unlined jacket - with the exception of Gore-Tex Paclite (discussed later) - is either not very durable or not very breathable (or both) when compared to three layers.

Now you've blinded me with science, can you summarise?

More demanding users should only consider a three-layer laminate construction for their waterproof shell. In colder weather this means better breathability, and in mild conditions it's more important that you have a highly breathable product (as the Delta T driving forces will be lower).

So what about the Gore-Tex Paclite that you mentioned?

Paclite is sometimes described as a 'two and half layer' construction. Rather than a laminated mesh on the inner of the membrane, Paclite has a pattern of small polymer dots. These dots do the same job as the 3 Layer laminated mesh (they protect the membrane) but with significant weight and pack size savings (hence the name Paclite). Unlike unlined garments, Paclite is still very durable (due to the dots) and very breathable, although still not as breathable or durable as three layer Gore-Tex due to a difference in the membrane technology and construction.

Other considerations before purchase
Jackets and fabrics are a little like football teams. Don't ask a mate who's an avid Liverpool supporter to explain the merits of the Manchester United back four and expect to get a 100 per cent impartial answer.

Don't get too bogged down in fabrics and features but do give some thought to fit. Cheaper jackets tend to use lots of fabric to compensate for poor fit. Go to your nearest specialist outdoor store that is staffed by climbers. It shouldn't be too difficult to find someone who has used many of the jacket and fabric combinations on offer and be familiar with the merits and disadvantages of each one. Explain to them the conditions that you usually find yourself in and what activities the jacket will be used for. Next time you could be the one smiling and it will be everyone else who can't wait to get home and dry.