hikers in a snowy mountain landscape

Not all waterproof-breathable (WPB) jackets are fully waterproof – by design. Let’s explore why, and how you can deal with that in extremely wet conditions.

Marketing and test standards lead us to think that a “rain jacket” should be waterproof under all conditions. However, even high Hydrostatic Head (HH) scores don’t guarantee that you will stay 100% dry. Instead, there is a spectrum of water resistance, from windshirts with DWR to highly breathable electrospuns, to a variety of other membrane types.

Some of the issues:

Mismatched Expectations, a.k.a. “Leaks”: Backpackers commonly choose not-entirely-waterproof WPBs in search of improved breathability. It works, but people complain of wet out, leaking, and other water ingress in some of their more breathable layers. I originally thought they must be wrong — that users were mis-reporting condensation.

Condensation: Some of it really is condensation. Most DWR fails in half an hour of hard rain (even when new). A wet out face fabric prevents vapor transmission, is cold, and rainy air may be humid. The combination encourages condensation inside the jacket. But there is more to it than that…

Hydrostatic Head: This standard test works well for fully waterproof shelter fabrics. It involves measuring a column of water suspended by the fabric. A taller column applies more pressure to the fabric, simulating the impact pressure of a raindrop, or the pressure of a shoulder strap on a shoulder, or a knee against a tent floor.

Wicking: However, WPB membranes may have other characteristics that the HH test does not measure. It turns out that some membranes are designed to pass moisture as liquid. In the past, this was advertised as a feature: the membrane can pass sweat even when the face fabric is wet out. But the fine print is that the membrane may also pass a small amount of water from the outside to the inside of the membrane. It is a slow process, over time, so the wearer continues to benefit from partial resistance to rain. This ingress can happen with negligible hydrostatic head pressure.

(Note that keeping the jacket zipped up may cause more vapor pressure inside the jacket than outside, which would encourage moisture movement toward the outside. However, wearing a zipped up jacket in humid weather can be unpleasant.)

Stretching, Wear and Tear: Well-worn membranes are especially vulnerable, which can lead to pinholes and/or stretching of the membrane, which increases some of the gaps that allow air to pass. If those gaps become large enough, then liquid water can pass through the membrane. Of course, a new membrane with effective DWR and intact membrane will allow less penetration.

Field Reports of Wetness: People report this phenomenon in many popular rain jackets such as the Outdoor Research Helium (and other jackets made with the Pertex Shield fabric), Polartec NeoShell and PowerShield Pro, TNF FutureLight, Outdoor Research AscentShell, ZPacks Vertice, and other electrospun membranes. I do NOT see those reports for Gore-Tex and other, heavier, less breathable, membranes.

For example, Columbia Outdry (heavier membrane-on-the-outside) has a modest but useful amount of MVTR, while being apparently 100% waterproof in my field testing. It is not as pleasant to wear while active as my Outdoor Research Motive AscentShell (electrospun), which has a very high MVTR. However, the AscentShell leaves me slightly damp in extended or heavy rain. I feel safe in Outdry in a cold storm, but I would be concerned about the AscentShell in heavy and prolonged cold rain.

Summary: So don’t dismiss reports of “leaky” WPBs. Instead, recognize that they may be working exactly as intended. The disconnect is that the marketers never fully explained the intent, which was to improve breathability in light or intermittent weather conditions. That is exactly when a WPB jacket is most helpful.

A practical strategy is to pair a more-breathable-less-waterproof WPB jacket with a fully waterproof layer as backup in case of heavy or prolonged cold rain. Some UL examples might be:

ZPacks Vertice, Outdoor Research Helium, Rab Phantom, or even some windshirts for light weather.

Frogg Toggs Emergency Poncho (3.5 oz), Coghlan’s or Ozark Trails Emergency Poncho (1.5 oz), Timmermade Megazip, RockFront Rain Hoody, LightheartGear or Anti-Gravity Gear or other sil jacket as backup.

Above: High-exertion trekking in raingear during a Backpacking Light guided expedition in the Absaroka-Beartooth Wilderness, Montana (Ryan Jordan photo).