How Breathable Raincoats Work

Plastic raincoats are waterproof but because they trap virtually all vapour inside, the wearer can get saturated with moisture and sweat, particularly if they exert a lot of energy e.g. heavy manual tasks or hiking. That is why you don’t see serious hikers or skiers wearing cheap plastic coats.

A person recently asked why they got wet wearing an expensive breathable raincoat.

How are Breathable Waterproof Clothing and Coats Made

The cheapest and most common method for making “breathable” material is to spray a coating of polyurethane or ePTFE (expanded Polytetrafluoroethylene) on the surface that keeps large droplets of water out but allow smaller moisture droplets to pass through. This process is referred to as Moisture Vapor Transmission (MVT). These products are typically water-resistant, but not waterproof.

Images showing how the Gore-Tex layer allows moisture to pass through, but not water droplets

The more expensive brands, like GORE-TEX®, laminate a permanent breathable coating to the base material. Laminated coatings are more durable as the sprayed on coatings more easily wear out and loose flexibility and can peel off.

Returning to the person who got wet although they were wearing a breathable raincoat.

The breathable coatings allow water vapor to pass from the hot humid side (interior), out to the cooler (exterior side). If you wear breathable coats in environments that are hotter and more humid on the outside than the inside, the moisture can pass the other way and the wearer may get so wet they think the raincoat is leaking.

However, under those circumstances, you would be sweating so much it would not make sense to wear the raincoat at all.

Examples of breathable suits for emergency personnel which are also rated for chemical protection

Note: Most breathable products are not intended for chemical protection, but some are, so make sure you check with the manufacturer.