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It’s 40°C at dusk during a summer heat wave. My electric utility is threatening blackouts and begging people to conserve.

I have a couple unused rooms in a detached suburban house that don’t need air conditioning. If I close the air vents and the doors to these rooms, will my air conditioning usage (and my electric bill) decrease, increase, or stay about the same?

My first thought was that less conditioned air volume absolutely means less cooling. Then I started thinking about the lack of insulation in the interior walls and about air leakage in ducts that now carry greater static pressures. Is there any guidance that doesn’t require a full building energy audit?

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  • Despite the things you mention (lack of insulation etc - though an unused room with no ventilation also by itself acts as a bit of extra insulation from the outside), the sheer decrease in volume and wall area wich is in contact with the heat source (i.e. the outside) directly should still make this worth it and result in decrease in energy usage. I can't pinpoint the exact physics laws behind it though, would guess 2nd law of thermodynamics.
    – stijn
    Commented Aug 19, 2020 at 7:44

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No, closing the air vents will increase your energy bill. The added pressure from closing a vent can cause air leaks in your system, causing long-term and unnecessary energy waste. The heater or air conditioner will produce the same amount of air whether or not you have them closed. Not recommended.

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    Hi wilkvolk welcome to the site and thanks for your answer :) I do have one question - how is an air leak any different or worse than cooling an entire room that is not being used?
    – Robotnik ♦
    Commented Aug 30, 2020 at 23:21

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