How are materials made waterproof and windproof?

Question from Louise Peacock

Weatherproof garments are made by sandwiching together different layers of materials. The outer layer is normally a tightly woven fabric, such as nylon or polyester, combined with a membrane. A membrane is a barrier that allows certain things to pass through but not others. In a waterproof jacket this layer is covered in tiny holes called pores that are small enough to prevent water droplets getting through but large enough for sweat vapour to escape. For clothing to be fully waterproof the seams must be sealed with heat-activated tape. A different kind of membrane is used for windproofing. Like the waterproof membrane, this is often made from breathable polymers (a type of plastic) full of micro pores that allow perspiration to pass through but not the elements.

Answered by Jodie Tyley for Brain Dump in How It Works issue 113.

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