PU Membrane Explained

In brief: A PU membrane is a waterproof-breathable barrier made from polyurethane, applied as a coating or a laminated film to the inside of a jacket's fabric. PU membranes come in two breathing styles, microporous (tiny holes) and hydrophilic (a solid film that passes moisture by diffusion). They are cheaper than PTFE membranes and power a large share of mid-priced waterproofs.

What a PU membrane is

PU stands for polyurethane, a versatile polymer used everywhere from foam to coatings. As a waterproofing technology, a PU membrane is a thin polyurethane barrier bonded to the inside of a garment's face fabric to block liquid water while allowing some moisture vapour to escape. Because polyurethane is inexpensive and adaptable, PU membranes underpin a huge proportion of the world's mid-priced waterproof jackets, including many brands' own-name technologies.

Microporous versus hydrophilic

There are two ways a PU membrane can breathe, and the difference matters. A microporous PU membrane is riddled with tiny holes too small for liquid water but large enough for vapour, so it breathes by letting vapour pass through the pores, similar in principle to ePTFE. A hydrophilic PU membrane has no holes at all; it is a solid film that attracts water molecules on the warm, humid inside, passes them through the film by diffusion, and releases them on the cooler, drier outside. Hydrophilic films are very durable and easy to keep clean because there are no pores to clog, but they rely on a humidity and temperature difference to drive breathing, so they can feel less immediate than a porous membrane during stop-start effort.

Coating versus laminate

PU can be applied two ways. A PU coating is painted or rolled directly onto the back of the face fabric as a liquid that then cures, which is the cheapest method and common on entry-level waterproofs. A PU laminate is a pre-made film bonded to the fabric, which tends to be more durable and consistent. Both can be genuinely waterproof; the laminate generally feels and lasts better, which is part of why prices vary.

How waterproof PU membranes are

Quality PU membranes typically deliver hydrostatic-head figures in the region of ten to twenty thousand millimetres, comfortably past the roughly 1,500mm the trade treats as the waterproof threshold and plenty for everyday rain and most outdoor use. They are generally regarded as less breathable and sometimes less durable than the best PTFE laminates, which is the trade-off for their much lower cost. For a commuter, dog-walker or casual hiker, a good PU membrane is often all the jacket they need.

Caring for a PU membrane

PU membranes, especially coatings, benefit from gentle care. Keep the outer DWR finish working so the face does not wet out, wash according to the care label, and avoid harsh detergents and very high heat that can stress the polyurethane over time. Hydrophilic films are easy to keep clean since they have no pores; microporous ones, like all porous membranes, breathe best when not clogged with dirt and oils.

PU membranes at OD's Designer Clothing

Many capable, sensibly priced waterproofs use a PU membrane, and for a lot of wearers that is the smart buy. If you are weighing a PU-membrane jacket against a pricier PTFE laminate, the team can help you judge whether you will ever use the extra performance, or whether a good PU shell is the better-value choice.