Tim, your problem might not be how cold you're getting, but rather a moisture problem. We typically get down to the minus 20 range in the winter. The only time I've had any kind of real problem with frost bite is if their coop gets damp. That happens when those silly women sit out in a full on blizzard and then cart all of that snow back into their coop! The Dingbats!But, if I can keep them from doing that, and keep the coop dry, the petroleum jelly applied to rooster combs at bedtime seems to do the trick.
I believe you're right. The only rooster who didn't get frost bit was the BR in thing hoop coop. I have humidity gauges in both coops. I don't know how to fix the ventilation problem in the wooden coop.
Too, I think the ceiling of the coop is to high. If you notice most of the old coops have a slanted roof and are not high at all. Not like some of the new ones I'm seeing made.