I read I should have 1 sq. ft. ventilation per chicken or minimum 1 sq. ft. per 10 sq. ft. of floor area. We only have 2 hens and are building a 4'x8' coop (we will be adding more ventilation that closes down in case we get a couple more). In the meantime that would equal 2 sq. ft. ventilation for the 2 hens or 3 sq. ft. for the floor space- WHICH DO I DO? Keep in mind our very cold winters here in Michigan and w/ only 2 girls they don't have the advantage of a warm chicken on each side of them at night.