Washington State is home to many rodents that will work night and day to gain access to your warm crawlspace and attic. They chew through plastic, wood and even light gauge screening. Protecting your building from there access is not only recommended but even has building code on how it needs to be done.
Yes there is even building code stating how access points into the Attic and Crawlspace need to be secured off. Exterior openings into the attic space of any building intended for human occupancy shall be protected to prevent the entry of birds, squirrels, rodents, snakes and other similar creatures.
Openings for ventilation having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum shall be permitted. Openings for ventilation having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Where combustion air is obtained from an attic area, it shall be in accordance with Chapter 7 of the International Mechanical Code 701.1 - 708.1.