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.

The Mystery

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?

  • Is it a single zone system or is the second floor a separate zone?
    – mikes
    Nov 26, 2019 at 22:31
  • @mikes Single zone. I added that to the question. Nov 26, 2019 at 22:39
  • The low e glass is preventing the heat gain inside the house that old windows allowed. It’s cooler in the summer and cooler in the winter.
    – Kris
    Nov 26, 2019 at 23:00
  • Where is the thermostat for the single zone located?
    – Ecnerwal
    Nov 26, 2019 at 23:13
  • @Ecnerwal First floor. Added to the question. Nov 26, 2019 at 23:34

2 Answers 2


That's easy. Thermostats don't work by duty cycle. They work by sensing the temperature in the room. The upstairs is cooler because the upstairs is now (let's say) 10% better insulated, but the furnace is running 20% less often.

Why would it do that? Where's the thermostat?... Downstairs. Downstairs is in fact 20% better insulated, so the furnace gets downstairs to a happy temperature 20% sooner, so the thermostat shuts off the furnace.

The house is now out of balance. It was balanced, but it isn't now.


OK Let me try. Originally you had your single zone heat on. My experience is first floors are usually warmer than second floors. Hot air wants to rise but it can't unless there's some cooler air replacing it. That's where the drafts come in from the older windows on the first and second floor. So heat rises to the second floor in addition to the second floor heating and all is well.

Then you change the windows. No more drafts from outside. Now the hot air on the first floor can't rise like before because there's no air coming in because no drafts. The second floor now has to rely on only the second floor heating which is usually lower then the first floor heating, single zone.

End result, a warmer first floor and a cooler second floor. Try to regulate the first floor heating, close the valves a bit so hotter water get to the radiators on the second floor. Good luck

  • Hot air rising does not require "drafts". The cooler air replacing it can be found upstairs, and those two masses switching place is the motion.
    – Sneftel
    Nov 27, 2019 at 9:52
  • @Sneftel Yes, but the intake drafts will accelerate it.
    – JACK
    Nov 27, 2019 at 12:33
  • How would a draft make the first-floor hot air want to rise more? There's already cool air upstairs, otherwise the OP wouldn't have the problem in the first place.
    – Sneftel
    Nov 27, 2019 at 13:11
  • @Sneftel Some convection could occur in the stairway connecting the floors. More convection would occur if the warm air on the first floor were displaced by cold air entering through drafty first floor windows, and only needed to rise through the stairway (instead of traveling both directions) and escape through drafty second floor windows. Nov 27, 2019 at 18:44

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