I have a very mysterious situation that's got me completely baffled. I can't imagine how it's even possible so I'd like some suggestions on what could possibly be going on.
My house is a two-story, wood frame colonial built in 1967. It has a walk up attic that's well insulated. There is a door on the attic stairwell that's kept closed when not in use. The attic has soffit vents and a roof ridge vent. No fans or anything of that nature.
Heat is provided by hot water through baseboard radiators. The furnace is just a few years old and is inspected and cleaned annually. It's a single zone with the thermostat on the first floor.
A couple of years ago we replaced all the windows using double-pane windows with low E-glass and all the usual energy compliance ratings. Being much more energy efficient and tighter sealing than the original windows, we expected at least a slightly warmer house, and that's perhaps true on the first floor, but bizarrely the second floor is now noticeably cooler than the first.
How is this possible? Heat rises and since there's no forced air circulation, I don't think it's possible that heat could be moving downward. We've lived here for 30 years, so there's no question this is a new phenomenon. There are no drafts we can feel, none of the windows leak, and nothing else about the house has changed.
I can't even think of an explanation that fits the laws of physics. Any ideas?
Bonus question: What to do about it?