Breather membranes are an important component of building envelopes, as they allow for the escape of moisture vapor from the interior of a building to the outside. Two common water-resistant standards for breather membranes are W1 and W2, as defined by EN 13859-2.

Membranes that achieve W1 are used in open applications where the breather membrane is directly exposed to the elements. Membranes that achieve W2 are used in protected applications, such as behind cladding or a rainscreen system, where the breather membrane is not directly exposed to the elements.

The W1 standard requires that a membrane be tested for a minimum of two hours at a pressure of 2 kPa, which simulates driving rain in severe weather conditions.

The W2 standard, as defined by EN 13111, requires that less than 100ml of water penetrates the membrane when a volume of water is poured down an inclined plane onto the membrane. The W2 standard does not require testing for water penetration resistance under dynamic wind loads, meaning that breather membranes that achieve W1 are more resistant to water penetration than W2 membranes.