The criterion isn't "where humans would get frostbite". Quite definitely not. Unless you mean "in just a short time".
The main thing to notice about the reprinted articles "open houses" however is that they are DEEP DEEP DEEP. Even the colony-style ones are deeper (measuring back from the open/part-open side) than most peoples' backyard coops; and the long style houses are, what, probably 30+ feet long, with just the one short side open. That means the wind does not penetrate nearly so much. I can tell you from experience with horse run-in sheds that there is a WORLD of difference between a 10' deep shed and a 18' deep shed. The latter gives a really good area of dead air at the back.
So you can't apply features from one size/shape of structure directly to a totally different size/shape of structure. Nor one with *quite* a lot of chickens in it vs one with a lower stocking density.
Some of you recommend open coops, with the openings covered up in winter. That means no ventilation whatsoever in the winter. How is that better than an ordinary coop?
I haven't seen
anyone recommending no ventilation whatsoever.
You cover the big open wall, yes. But the OTHER walls still have their ventilation openings in use, of course!
And in my case if I were building a wire-front coop up here, which I probably *would* if I were building a new coop at all, I'd simply arrange its winter cover to have ventilation openings *in* it
It comes down to what you're comfortable with, risk-wise. Would I leave a backyard size coop totally open front up here? No. Could you? Yes, and if your breeds were chosen intelligently and you didn't have real bad swirling winds at the site you could quite likely get by without serious winter injury most of the time even here in southern Ontario.
Would I build an open-front coop, and leave it open all winter, in Massachusetts? Sure, quite possibly if I had a flock of a hundred kajillion birds and a house the size they show in the old article. But if I just had a dozen birds in a backyard sized coop, no. I might leave it open *some* of the winter but personally I would rather err on the slight side of caution (I would certainly not run a light or heater routinely, but I *would* close the building in so I could control draft and temperature-loss)
JMHO,
Pat