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We don't use the second story of our home. Is it better to shut off the air conditioning completely upstairs to save money, or keep it set but at a higher temperature, such as 78-80 °F?

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Almost certainly you should shut off the upstairs AC completely. Ideally you would also seal off the second floor as much as by shutting a door at the top or bottom of the stairs (or installing a curtain across it or something).

The only reasons I can think of to leave it on would be:

  • If you have any valuable artwork or antique / fragile wood furniture upstairs. Excess heat and humidity could be a problem, especially if the temperatures get really high.

  • If you don't have a good thermal separation between the upstairs and downstairs it's possible that by leaving the upstairs unconditioned you'll create uncomfortable hot areas downstairs. E.g. if you have a flight of stairs between the floors, the bottom of the stairs might get warm.

  • If you have separate AC units for upstairs and downstairs and the downstairs one is very inefficient, you'll be making it do more work.

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I don't know about closing off the second floor. Depending on your climate it could possibly get to over 100 degrees if it is totally closed off - and possibly well above that. It could also get really humid.

And I doubt you have insulation between the two floors. So not only with the temperature changes are you rolling the dice with drywall cracking, door/trim issues, hardwood floor issues if you have them, and issues with tile... but you could also have issues on the first floor in the ceiling area.

It really depends on the climate and what you have upstairs if you are willing to chance a couple of cracks in your drywall. I would leave it around 80.

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I guess you're right that if you expect the temps to be over 100 for long periods of time that might be bad. But I seriously doubt that drywall and floors would be bothered by temps in the 90s. I'm from New England where AC is nice but not ubiquitous, and lots of upper floors get quite warm in the summer. – Henry Jackson May 16 '13 at 13:02
I was guessing he was in a very hot climate based on the comment about setting the AC to 80... I guess the answer is dependent on region/climate. I have bought foreclosed houses in my area and have spent lots of time doing a lot of repair because of this. – DMoore May 17 '13 at 5:43

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