Technical outerwear is one of the few clothing categories with published performance numbers, and the numbers mean specific testable things.
The two measurements
Waterproofness is measured as hydrostatic head — the height of a water column the fabric resists before leaking, in millimetres.
Breathability is measured as moisture vapour transmission rate — how much water vapour passes through in a given time.
These are in tension. Anything that stops water coming in tends to stop vapour going out.
Every membrane technology is an attempt to manage that tension.
How membranes work
Microporous membranes have pores large enough for water vapour molecules and too small for liquid water droplets.
Which works because a water droplet is orders of magnitude larger than a vapour molecule.
Monolithic membranes have no pores and transmit moisture by absorbing it on one side and releasing it on the other, driven by the humidity difference.
Combinations of both exist, and the practical difference is that microporous membranes can be contaminated by oils and dirt while monolithic ones cannot.
Why jackets stop working
Usually not the membrane. Usually the face fabric.
The outer fabric carries a durable water repellent finish that makes water bead and roll off.
When that finish wears, the face fabric absorbs water and saturates, which is called wetting out.
A saturated face fabric blocks vapour transmission even though the membrane is intact, so the jacket feels like it is leaking when it is actually failing to breathe.
Which is why the fix is reproofing rather than replacement, and why washing a technical jacket properly and reapplying the treatment restores most of the performance.
Washing technical fabric
Counter-intuitive and worth stating.
Dirt and body oils degrade the repellent finish and block membranes, so technical outerwear needs washing more often than people think.
Standard detergents leave residues that attract water, which is why dedicated technical washes exist.
Tumble drying on low heat frequently reactivates the repellent finish, which is one of the few cases where heat helps.
Seams
A waterproof fabric with unsealed seams is not a waterproof garment, because every stitch is a hole.
Taped seams have a heat-sealed strip applied over the stitching on the inside.
Fully taped means every seam. Critically taped means only the most exposed ones, which is a cost saving and is disclosed.
Tape delaminates with age, and it is the most common permanent failure in older technical jackets.
Layer counts
Two-layer construction bonds the membrane to the face fabric with a separate hanging liner, which is cheaper and bulkier.
Three-layer bonds face fabric, membrane and a backing textile into a single laminate, which is lighter, more durable and more expensive.
Two-and-a-half layer prints a protective coating onto the membrane instead of a backing, which saves weight and is less comfortable against skin.
The chemistry question
Durable water repellent treatments historically used long-chain fluorinated compounds, which perform exceptionally and persist in the environment.
Regulatory pressure and voluntary commitments have moved the industry toward shorter-chain and non-fluorinated alternatives.
The alternatives perform less well against oils and require more frequent reapplication, which is a genuine functional cost that manufacturers have been reluctant to state plainly.
Choosing sensibly
The highest specifications are designed for sustained exposure in serious conditions, and for a commute they are considerably more than is needed.
Ventilation features — pit zips, mesh-backed pockets — frequently do more for comfort than membrane performance does, because they address the actual problem, which is usually sweat rather than rain.
And a well-designed hood that turns with your head is worth more in practice than a thousand millimetres of hydrostatic head.
Insulation
Separate from the shell and worth understanding on its own terms.
Down is measured by fill power, which describes loft per unit weight rather than total warmth.
Higher fill power means more warmth for less weight, and total warmth depends on fill weight, which is a different number and less often advertised.
Down collapses when wet and stops insulating, which is why treated down and synthetic alternatives exist.
Synthetic insulation retains loft when damp and is heavier and bulkier for equivalent warmth, and it degrades faster with compression cycles.
Fit and layering
A shell sized to be worn over insulation must be larger, and one sized to be worn alone should not be.
Which means trying a shell on over what you will actually wear underneath, rather than over a shirt in a warm shop.
Sleeve length with arms raised, and whether the hem rides up, are the two tests that reveal a poor fit and are never done in shops.
Hood design
The difference between a functional hood and a decorative one is whether it turns with the head.
Which requires adjustment at the back and at the face, and a stiffened peak that holds shape in wind.
A hood that does not adjust will blow off or block vision, and this is checkable in thirty seconds.