A waterproof shell with a standard coil zipper is a raincoat that leaks at the centre front. Buyers collapse three products into one word. Water-resistant (water-repellent) zippers shed rain. Airtight (hermetic, watertight) zippers seal gas and water under pressure. They are different chains, different sliders, and different sewing rules. Naming the wrong one is how a hiking jacket gets a dive-suit zipper that a wearer cannot operate with one hand — or how a “waterproof” quote is only a PU film on the tape.
TrimGem University splits the terms because outdoor, workwear and bag programmes all sit on the same Guangzhou sampling desk, and the accessory catalogue will keep adding coated and sealed closures as brands ask for them.
Water-resistant: film on the tape
The apparel default is a coil (sometimes a moulded Vislon / POM chain) whose tape is laminated with polyurethane. YKK’s AquaGuard family is the reference many brands write; other mills sell equivalent PU-coated reverse-coil or face-coil chains. The spiral sits under the film. The slider is a close-fitting, often auto-lock body. The result is a flexible, light zipper that sheds rain, dust and wind well enough for mountaineering shells, military raincoats and technical bags — if the rest of the garment is sealed.
Fabric waterproofness is not garment waterproofness. Needle holes along the zipper tape and along every other seam are leaks until they are seam-taped. Fully taped garments tape every seam; critically taped garments tape only the most exposed lines (shoulders, hood). A PU zipper sewn with extra bartacks through the film is a zipper the mill just perforated.
Coil AquaGuard-type chains stay flexible enough for one-handed use. Moulded VISLON-type water-resistant chains are stiffer, hold shape in a bag opening or a shoe, and are the wrong hand-feel for a running jacket.
Care is chemistry: no solvent on the PU, no spare needle holes, silicone lubricant if the slider drags — not oil.
Storm flaps are a design, not a zipper
Even a good water-resistant zipper is usually backed by a storm flap. Classic military outerwear keeps an external flap (snaps or hook-and-loop) so mud and sand never sit on the chain and so metal does not flash. Lightweight alpine design often drops the external flap for weight and snag risk, and hides a slim internal flap behind the coated zipper. Both are valid. Neither turns a standard #5 coil into a dry zipper.
Airtight: a different product
Hermetic zippers (TIZIP MasterSeal / SuperSeal class, and similar TPU constructions) use a hard coating and pressed sealing lips that lock. They are built for drysuits, survival bivies and some inflatable or chemical-protective goods. They are stiff. They need lubricant. They are not operable at speed with one hand. They do not belong on a land raincoat that must open for a radio or a harness.
If the spec says “airtight” or “watertight to X metres / X kPa,” you are in this family. If the spec says “water-repellent” or “AquaGuard-type,” you are in the PU-tape family. Do not let a quote substitute one for the other on price.
What to write on the PO
State water-resistant (PU-coated tape) or airtight (hermetic). State coil versus moulded. State whether a storm flap is part of the garment (that is a pattern, not a zipper SKU). Forbid extra stitching through the film. Name a lubricant if the chain is hermetic. Put seam-tape on the garment spec in the same sentence as the zipper.
Rain is a system: membrane, seam tape, zipper, flap. The zipper is only one line of that system.