A cold snap in early January caught plenty of humidity inside the house and, in fact, the attic temperature dropped so abruptly that the nails protruding through the roof iced over:

You’d think they were coated with plastic:

The incandescent bulb lighting the attic highlights the crystal planes:

As nearly as I can tell, the ice sublimes or melts-and-evaporates, because the plywood floor under the nails doesn’t have the drip marks you’d expect.