Eight Terms That Decide
a Shell Jacket Spec

Hydrostatic head

Hydrostatic head is the height of a water column, in millimetres, that a fabric withstands before water penetrates it — the standard laboratory measure of waterproofness in textiles, tested to ISO 811.

A fabric rated 20,000mm resisted a 20-metre column of water under the test conditions. The physics: a microporous membrane resists water because the pores are far smaller than a liquid water droplet, and surface tension prevents the droplet passing through. Raising the pressure eventually overcomes that.

Published benchmarks put alpine-grade laminates above 28,000mm and general performance laminates above 20,000mm (Outside, "The Definitive Guide to GORE-TEX Technologies 2025").

Common misreading: treating a higher number as a better garment. Hydrostatic head measures the fabric, not the garment. An untaped seam leaks at any rating, and a fabric with a high rating and poor breathability will feel wet inside from condensation. Above the level the end use requires, extra millimetres buy nothing the wearer can perceive.

MVTR (moisture vapour transmission rate)

MVTR is the mass of water vapour that passes through a square metre of fabric in 24 hours, expressed in g/m²/24h — the throughput measure of breathability.

Published benchmarks quote above 15,000 for performance-grade membranes and above 25,000 for alpine grades (W.L. Gore & Associates technical documentation).

Common misreading, and it is the important one: MVTR numbers from different test methods are not comparable. JIS L 1099 A-1, JIS L 1099 B-1, ISO 15496 and ASTM E96 upright and inverted all return different values for the same fabric, and B-1 typically returns the largest number — which is why it appears most often in marketing. Comparing two suppliers' MVTR figures without knowing the method is comparing nothing. Always ask for the method alongside the number.

RET (evaporative resistance)

RET is the resistance a fabric offers to evaporative heat transfer, measured on a sweating guarded hotplate to ISO 11092 and expressed in m²·Pa/W — lower means more breathable.

Unlike MVTR it has one dominant standard, so RET values from different suppliers can be compared directly. Published benchmark bands: below 6 for alpine-grade laminates, 6–9 for general performance, and rising from there (Outside, 2025 guide; ISO 11092:2014).

Common misreading: assuming RET and MVTR are two names for the same measurement. They run in opposite directions — high MVTR is good, low RET is good — and they are measured under different conditions. A spec sheet quoting both should be internally consistent; one quoting only a large MVTR with no method is the weaker document.

DWR (durable water repellent)

DWR is a finish applied to the outer face of a shell fabric that lowers its surface energy so rain beads and rolls off — the first of the two water barriers in a shell, and the consumable one.

The membrane underneath is the second barrier. DWR wears out: abrasion, body oils, dirt and UV all degrade it, and when it is depleted the face fabric absorbs water and forms a film that closes off vapour transport through the membrane. The wearer experiences this as clamminess and reports it as leakage.

Fluorine-free (C0, PFAS-free) DWR — now the industry default — repels water comparably to the older fluorinated finishes but has weaker oil resistance and a shorter service interval, so it needs reactivating more often.

Common misreading: treating DWR as a permanent property of the garment, and treating wet-out as membrane failure. Both are wrong, and together they generate most of the false warranty claims on shell jackets.

ePTFE (expanded polytetrafluoroethylene)

ePTFE is a microporous fluoropolymer membrane, industrialised by W.L. Gore in 1969, that made waterproof-breathable garments possible.

Gore's published technical data describes roughly 9 billion pores per square inch at about 0.2μm each, with porosity above 80% — far too small for a liquid water droplet (100–5,000μm) to pass, far larger than a water-vapour molecule (about 0.0004μm). That size gap is the whole mechanism: liquid water is held out by surface tension, while vapour diffuses through, driven by the humidity gradient between the wearer and the outside air.

Common misreading: ePTFE is a fluoropolymer and belongs to the PFAS family — persistent in the environment — which is what drove the industry's move to fluorine-free membranes. Also worth knowing when researching the term: "ePTFE membrane" in search results is dominated by the medical-implant and industrial-filtration industries, which use the same material for entirely different products.

ePE (expanded polyethylene)

ePE is the fluorine-free microporous membrane the outdoor industry adopted to replace ePTFE, keeping the same microporous mechanism without PFAS chemistry.

GORE-TEX began its transition around 2022 and states in its 2023 ePE white paper that hydrostatic head and breathability sit within a small margin of the equivalent ePTFE generation, with a lighter and thinner membrane.

For a buyer, the practical change is not at the membrane. It is at the DWR: a fluorine-free membrane is normally paired with a fluorine-free DWR, and that finish is the one with the shorter maintenance interval.

Common misreading: assuming fluorine-free means reduced waterproofness. Published data does not support that. The real difference sits in oil repellency and DWR service life.

Laminate construction (2L / 2.5L / 3L)

Laminate construction describes how many layers are bonded around the membrane, and it sets a garment's durability, weight, hand feel and cost far more than the membrane rating does.

3L (three-layer) — face fabric, membrane and a knit backer permanently bonded into a single textile. The membrane is protected on both sides; best abrasion resistance and longest life. Benchmark weight band 130–200 gsm for alpine shells. Standard for technical hardshells.

2.5L — face fabric and membrane, with a printed or sprayed protective pattern on the inside instead of a backer. Lightest and most packable; benchmark 80–120 gsm. Abrasion life and next-to-skin feel are traded away for that.

2L (two-layer) — face fabric bonded to the membrane, with a separate liner hanging free inside. Softest hand and quietest, heavier, and the membrane is more exposed to rubbing. Common in urban and 3-in-1 constructions.

Construction descriptions and weight bands: Mountain Equipment, GORE-TEX Product Construction & Performance Technical Manual.

Common misreading: ranking these as good, better, best. They are three different answers to different end uses. 3L on a commuter jacket adds stiffness and noise the buyer will experience as a downgrade.

Seam taping (seam sealing)

Seam taping is the process of bonding a waterproof tape over the inside of a sewn seam under heat and pressure, closing the needle holes that sewing puts through the membrane.

Without it the fabric rating is irrelevant, because water enters at every stitch line. Two levels are standard in the trade. Fully taped (fully seam-sealed) means every seam in the garment is taped; alpine specs additionally use wider tape on primary seams and reinforcement at high-abrasion zones — Mountain Equipment publishes 22mm tape on primary seams with double-taping at shoulder and cuff. Critically taped means only the most exposed seams — typically shoulders, hood and upper chest — are sealed.

Taping is a capability, not a finishing step: it requires the machine, the right temperature and pressure for that specific laminate, an operator who can hold a consistent path through curves and seam intersections, and inspection. Full seam sealing is the accepted dividing line between a waterproof garment and a rain-resistant one.

Common misreading: reading "waterproof" on a product page as implying sealed seams. It does not. Specify the level explicitly in the tech pack, and for critically taped, name the seams.