A waterproof jacket has to let moisture out, and that is the harder half

Two flows in opposite directions through one wall

A rain shell has to stop liquid water travelling inwards under the pressure of wind and rain, while allowing water vapour produced by the wearer to travel outwards. A sheet of plastic does the first job perfectly and the second not at all, which is why a garment made of one leaves you as wet as the weather would have.

The trick that makes both possible is the size difference between a liquid droplet and a single water molecule in vapour form. Liquid water in a raindrop is held together by surface tension and behaves as a bulk fluid, whereas vapour moves as individual molecules. A barrier with openings between those two scales lets one through and blocks the other.

Everything else in a waterproof breathable garment is arranged around that principle. The membrane is a fragile film, so it’s laminated to a face fabric for abrasion resistance and usually to a backing for protection from the inside as well.

Microporous and hydrophilic membranes work differently

A microporous membrane is a physically perforated film, stretched or cast so that it contains an enormous number of extremely small passages. Vapour diffuses through the holes; liquid water cannot, because surface tension won’t let a droplet enter an opening that small at ordinary pressures.

A hydrophilic membrane has no holes at all. It is a solid polymer film containing chemical groups with an affinity for water, and moisture is absorbed on the warm, humid inner face, moves through the polymer, and evaporates from the cooler, drier outer face. It is a chemical transport process rather than a physical one.

Each has characteristic behaviour. Microporous films tend to transport vapour faster but can be blocked by contamination from body oils and detergent residue, which is why washing such a garment restores performance. Hydrophilic films are harder to contaminate and depend more heavily on a difference in humidity across the barrier to work at all.

Breathability is driven by a difference, not by a property

The engine for outward vapour transport is the gradient between the warm, humid air next to the skin and the cooler, drier air outside. When that difference is large, a membrane moves a great deal of moisture. When the outside air is warm and already saturated, the gradient is small and the same garment barely breathes.

This is why a jacket performs beautifully on a cold, dry, windy day and feels like a bag on a mild, humid one. Nothing has changed about the garment. The physics that drives it has simply stopped supplying much of a difference to work with.

It is also why breathability figures should be read as laboratory comparisons rather than promises. Test methods differ, conditions differ from any real day out, and two garments quoted under different methods cannot be compared at all. The ranking is more useful than the number.

The rate you produce moisture usually exceeds the rate it can leave

A person working hard produces a great deal of water vapour, and no membrane in ordinary clothing keeps up with a high output for long. The excess condenses on the coolest surface available, which is the inside of the shell, and the wearer concludes reasonably enough that the jacket is leaking.

It isn’t. Condensation is water that came from you, and telling the two apart is the first diagnostic step: leaks appear at seams, zips and abrasion points and follow the pattern of the rain, while condensation appears evenly across the cold inside face and is worst when you are working hardest.

Which is why vents exist. Underarm zips and mesh-backed pocket openings dump humid air by bulk airflow, and bulk airflow moves orders of magnitude more moisture than diffusion through a film. Using them is generally the difference between a comfortable day and a damp one.

What to expect from a shell, honestly

A waterproof breathable garment is a compromise device. It is far better than a plastic sheet, considerably worse than no jacket in dry conditions, and it works best when the weather is cold and hostile enough to justify wearing it in the first place.

Layering underneath makes a measurable difference, because a next-to-skin layer that moves moisture as liquid to a place where it can spread and evaporate helps the shell do its job. Cotton against the skin does the opposite, holding water where it makes you cold.

And durability is worth as much as performance. The membrane is the fragile part of the garment, and delamination, abrasion at pack points and contamination all end its useful life well before the outer cloth wears out. Washing it properly, rather than avoiding washing it, is the single most useful habit.