A recent issue of Backyard Poultry had articles on winterizing. The consensus there was that chickens should not be shut in without outside ventilation in winter. The closed in coops trap moisture, which leads to molds and respiratory infections. Mine are all in buildings with an open (wired) side or in individual coops with wire fronts. Everyone does fine with the exception of an occasional frostbitten comb when I forget the vaseline. I do make sure that there are at least two birds to a coop so that they can snuggle.