The back staircase has always been a little cold. For 4+ years, we just assumed it was due to missing insultation that was also absent everywhere else in that end of the house.
But Jeff found a hole in the coat closet where the insulation was dried up and pitiful.
This shot is looking down at the floor of the closet on the left and the stairs on the right. Notice the gap. So typical of this house.
This explains the cool breeze in the stairwell. I wish I'd kept a running tally of Great Stuff cans used this winter.