I like the comparison between 3 years of different conditions with the same flock, it's evidence based , not just a one-off observation. I'm fortunate in being in the Uk, so -11 is normally the worst cold; wetness is the biggest problem!
I don't know where to put the following ideas, so hope its ok to put it here!
I give my chickens (a standard Orp, & silver-laced Wyandotte, & 6 silver Sebrights) a conservatory, made from a 4' x 2' x 3' high wooden dog kennel, with a very thick piece of perspex for a roof. This ensures a warmer, dry place at cooler times, & a great 'chicken-spa' for socializing.
Inside it is on earth, with a large, high-side cat litter tray, filled with a 2"-3" -ish mixture of absorbent clay cat litter, dry compost & approx two cupfuls of diatomous earth. The weight of the cat litter keeps the stuff in the tray when they fling it about, the compost attracts them in the first place & the DE kills mites in their feathers. (I also put this in their bedding , sprinkle on housing & under roofing, & on the hens, as mites get everywhere. I've not had a problem with it over 20 years.) They also use the outdoor dog kennel & run when it's wet (the dogs aren't often in it!), which I designed to have an upstairs & downstairs, as dogs like to roost too! My chickens are free range & have lots of shrubbery & different garden areas to tootle around, & also the patio & 20' corridor when it's very windy & wet. Sorry I have no photo of the conservatory-it's old & not photogenic!