They’ll be fine as long as they’re dry and there’s no wind/drafts. Make sure they have ventilation. Mine handled a week of highs in the single digits and lows in the -10s with no problem in that cold stretch around New Years 2018. Even if it gets a little below zero in the morning I let them out into the run. They’re birds…they can adapt to relatively slow changes in temp. You have orpingtons and mine handle it better than most of my other birds. Like I said though…the big thing is ventilation as it can lead to respiratory problems over time. The front of my coop is 8ft and I leave the eaves wide open so their heat and moisture can escape up and out. I don’t do the deep litter method in the coop…they’re in the run all day and I just pick up the poop in there daily. They roost in the coop over a poop board which catches their poop like a litter box. So you definitely want to have a place for the moisture to escape aloft if you’re doing DLM. Like you said, the decomposition will provide some heat and convection for the warmth to rise.
I do have a barred rock hen with a large comb that has occasionally gotten frostbite on it, but she’s the exception. All of my orpingtons tend to have smaller single combs. Also keep in mind that people keep chickens in the upper midwest and Alaska where it gets -30° to -40° or lower. Keep’em dry and with no air movement and they’ll be fine.