The black smoke implies some sort of oil/petroleum fire. It's quite possible there was an oil-paint mural on the ceiling of the chapel.

I would assume that the heat of the fire below is pushing air up, while the burning ceiling pushes air back down, and the high pressure is just pushing all the gasses out the nearest exhaust port, in this case the highest window. I would assume the entire outside of the structure is stone, basically turning the entire building into a blast furnace.

But hey, maybe satan was the arsonist.