There are essentially 3 options that can be used to make a waterproof cycling jacket:

Polyvinyl chloride or PVC – a plastic polymer, which is great for keeping the rain off, but offers zero breathability, so you will soon start to rain from the inside as you begin to sweat.

Membranes made from polytetrafluoroethylene (PTFE) – this is the original Goretex fabric and there is a myth that the way it works is that it has microscopic pores that are big enough to allow water vapor to squeeze out, but small enough to not allow water droplets in. The truth is that the pores are big enough to allow water droplets through, but the surface is hydrophobic, so water droplets are repelled, whilst water vapor can pass through. One downside of this membrane is that if it gets contaminated with grease, oil or even detergent, it can lose some of its hydrophobic properties, so with time it will tend to degrade.

Membranes made from polyurethane (PU) – absorb moisture from inside the jacket because of its hyrdrophilic properties and it passes through diffusion to the outer surface, driven by the high concentration of moisture on the inner surface of the membrane next to our bodies and the low concentration of water on the outer surface where there is fresh air and then the water can then be released as water vapor. This is a process known as wicking, which also occurs with good quality custom cycling jersey fabrics. The secret to making PU membranes work well is to make them as thin as possible, to allow the water to diffuse through the membrane as fast as possible.

We have four different kind of fabric options for our jackets and gilets and they all use cutting edge PU membranes, to give the best possible breathability. There is no point making promises that we cannot keep however, because if you start to get hot and sweaty, no matter how good the membrane, it is not going to be able to wick away all the moisture and you will start to get clammy. So with our new biofit range, we have engineered in breathable underarm panels to improve the breathability of the jacket, whilst maintaining the back, chest and topside of the arms completely waterproof (with the exception of the training jacket). We believe that this gives an optimal solution, combining the very best PU breathable membranes with some clever engineering.The other benefit is that for the side panels we use lycra so it holds the jacket in place and avoids any flapping in the wind and maintains the aero silhouette of the rider.

Here is some information about how our Biofit jackets work (the part about the jackets starts at 1.03):

Our 4 custom cycling jacket/gilet options are:
Training jacket – windproof/waterproof/breathable chest protector, brushed microfibre back and arms and brushed lycra side panels
Minerva jacket – medium insulated windproof/waterproof/breathable fabric with brushed lycra side panels
Rain jacket – non-insulated windproof/waterproof/breathable fabric with lycra side panels
Drystorm jacket – thick insulated windproof/waterproof/breathable, with brushed lycra side panels