The outbuilding we will be using for a coop is about 15'x 15'. We have 6 pullets that will be moving in there soon. We live in a true 4 seasons climate with some hot and humid days w temps in the low-mid 90's and cold snowy winters with lows around 0 with a wind chill up to -15 on the coldest of days. How much ventilation should we add? Is there a formula to figure out how much we should have? There are some holes at the top on all 4 sides, but from what I've read that's probably too little. Anyone have any tips? Thanks!