Agreed that the standard is a minimum of 4 s.f. in coop and 10 s.f. in run per bird. While it is possible to cram more birds into a tighter space, such crowding can result in increased stress, (resulting in increased disease and parasite loads) behavioral issues including aggression, feather picking, blood shed and cannibalism. Most folks keep back yard flocks so they can provide humane care which moves away from the factory farm mentality, where a chicken has approximately 1 s.f. of space in which to live.
OP lives in PNW, where increased humidity is going to be problematic. Cold weather is not going to be your enemy as much as moisture will be. In such a tiny coop (18 s.f.) with that many birds, they will put out more moisture in a single night than any amount of ventilation in such a small coop can handle. The general recommendation for venting is 1 s.f./bird or 10% of floor space.
OP lives in PNW, where increased humidity is going to be problematic. Cold weather is not going to be your enemy as much as moisture will be. In such a tiny coop (18 s.f.) with that many birds, they will put out more moisture in a single night than any amount of ventilation in such a small coop can handle. The general recommendation for venting is 1 s.f./bird or 10% of floor space.