How do breathable fabrics like Gore-tex and DWR (Durable Water Repellent) work?

Question

Gore-tex, and other waterproof breathable fabrics are often coated with a durable water repellent (DWR) finish that helps prevent the fabric from wetting out. It is not uncommon to hear people complain that their rain jacket has wet out and is now leaking. While obviously if the fabric is so worn it is compromised, or if the seams are not properly sealed, a rain jacket will leak, but what if the jacket is in relatively good shape by the DWR is compromised.

In the absence of a DWR and the presence of dirt, oil and other contaminants on the outer layer, does a Gore-tex jacket become more permeable to outside water seeping in or does it become more like a non-breathable waterproof fabric? In other words, in the absence of a DWR finish, if I prevent overheating and sweating and ventilate the jacket to mitigate the effects of perspiration, will it still keep me dry?

Answer 1

In the absence of the DWR, it would still be waterproof, but not breathable.

The DWR coating is to keep the rain off of the layer of Gore-tex but does not provide the waterproofing. Once the DWR coating wears off, the Gore-tex is no longer able to work and then the person's sweat starts building up inside and you will get wet from that.

Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane (Teflon) coating that is bonded to a fabric. This membrane had about 9 billion pores per square inch (around 1.4 billion pores per square centimeter). Each pore is approximately 1/20,000 the size of a water droplet, making it impenetrable to liquid water while still allowing the more volatile water vapour molecules to pass through.

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The DWR prevents the main outer layer from becoming wet, which would reduce the breathability of the whole fabric. However, the DWR is not responsible for the jacket being waterproof - this is a common misconception. Without the DWR, the outer layer would become soaked, there would be no breathability, and the wearer's sweat being produced on the inside would fail to evaporate, leading to dampness there. This might give the appearance that the fabric is leaking when in fact it is not.

Wikipedia