How to Wash a Rain Jacket

DWR Coating — How It Works and Why Wetting Out Destroys Breathability

A modern rain jacket has three functional layers working together: the outer shell fabric (typically nylon or polyester), a DWR (durable water repellent) coating applied to the outer face of the shell, and an interior waterproof-breathable membrane. Understanding what DWR is and how it degrades explains why washing requires a different approach than ordinary laundry. DWR is a fluorocarbon-based or silicone-based coating applied to the shell fabric fibre surfaces during manufacturing. Traditional DWR used C8 perfluorocarbon compounds (8-carbon fluorocarbon chains) — highly effective but persistent environmental contaminants. Newer DWR formulations use shorter-chain C6 fluorocarbons, or fluorine-free alternatives based on silicone polymers or wax dispersions. The mechanism is the same regardless of chemistry: the DWR molecules align themselves on the fibre surface with their hydrophobic ends pointing outward. This creates an extremely low surface energy environment on each fibre. Water contacting this surface cannot spread — the surface tension of the water drop exceeds the surface energy of the fibre, so the water forms a bead and rolls off. This is called the Lotus effect in surface chemistry. When the outer shell fabric is properly DWR-treated, the fabric stays dry and light in rain, and the air gap maintained around the fibres allows the waterproof membrane to breathe — moisture vapour from body heat can escape outward. When DWR degrades, two things happen simultaneously that people do not intuitively link. The outer shell fabric absorbs water (wets out) and becomes heavy. AND the jacket loses its breathability. This combination — wet and heavy AND non-breathable — is blamed on the waterproof membrane failing, but the membrane is almost certainly still intact. The breathability failure is caused by the saturated outer shell blocking the vapour transmission path. The membrane is designed to allow water vapour to pass through its microscopic pores, but when the outer shell is saturated, the vapour cannot escape the outer surface fast enough and the system backs up. This is why a wetting-out jacket feels like wearing a bin bag — the membrane is fine; the DWR is not. DWR degrades through two mechanisms. The first is contamination: residue from regular laundry detergent (surfactants), body oils transferred from skin, sunscreen, and general dirt particles coat the hydrophobic DWR chains. The coated chains can no longer present their hydrophobic ends to water — they are covered by the hydrophilic surfactant molecules. Washing with a technical wash (Nikwax Tech Wash, Grangers Performance Wash) removes this contamination without depositing new surfactant residue. The second mechanism is physical abrasion: the fluorocarbon chains are worn off the fibre surface over time by mechanical friction — washing cycles, rucksack strap friction, sleeve rubbing against the cuffs. This is irreversible without reapplication. Heat has a specific, documented effect on DWR: fluorocarbon molecules are flexible and can exist in a relaxed, flat conformation (relatively inactive) or an upright, extended conformation (highly hydrophobic). Heat — from a tumble dryer at low heat, or from a cool iron over a damp cloth — provides the thermal energy to reorient the fluorocarbon chains into their upright, active configuration. This is why tumble drying at low heat for 10–15 minutes after washing can temporarily restore water repellency even when no new DWR has been applied. It is not repairing damaged or contaminated DWR — it is reactivating DWR chains that were present but lying flat. Once the DWR is physically worn down by abrasion, heat alone will not restore it and DWR reapplication (Nikwax TX.Direct, Grangers Performance Repel) is required.