The level of breathability is determined by how easily air can pass through the fabric. Fabrics that naturally breathe well are assigned a rating of 3 or better. The breathability of a fabric should always be considered before selecting a fabric for a specific application. If an air tight enclosure is made of fabric that does not breathe well (waterproof fabric), condensation and mildew will become a problem. Waterproof fabric is an excellent choice for bimini, awning, and dodger tops, but enclosed covers may require ventilation with screens or vents.