When to open windows to cool or warm a building

My uninsulated apartment building went from too cold to too hot in about a week, which is normal in Canberra. People have started to open the windows in the stairwell in addition to their apartment windows. The timing of the opening seems a bit misguided – people open the windows in the morning. During daytime, the air outside is warmer than the air inside the stairwell, but during the night the outside air is colder. To state the obvious: to cool down the building, open the windows for the night and close them for the day. Currently the opposite seems to happen, although I counter this trend by closing the windows in the morning when I notice them open.
In general, if you want the building cooler and the outside air is colder than the inside, then open the windows, but if the outside is warmer, then close them. If you want the building warmer and the outside air is colder than the inside, then close the windows, but if the outside is warmer, then open them. This could easily be automated with temperature sensors outside and inside the building connected to a thermostat and small electric motors opening and closing the windows. Such a system would save some of the heating and cooling costs of the building.
There may be non-temperature reasons to open and close the windows, for example to let smell out of the stairwell or to keep insects from coming in. The second reason is not relevant for my building, because all windows have bugscreens and the exterior doors have a gap an inch wide under them, which the insects can easily use to get in.

2 thoughts on “When to open windows to cool or warm a building

  1. es

    Apart from all possible non-temperature reasons, I feel like this post omits at least one temperature-related one. Preferences for temperature may differ throughout the day. It’s most likely idiosyncratic, but at least I personally prefer my apartment cold in the evening (so it’s easier to go to sleep) and warm in the morning (so it’s easier to get out of bed).

    This argument is most definitely invalid in Australian summers when it’s hot-as-hell any time of the day, it’s more for external validity.

    Also, given all the points about nonexistent insulation in an average Australian home, how reasonable would it be to ignore all temperature-related concerns altogether? That is a weak criticism, more a question of physical curiosity: how long would it take an average home (and all its interiors) at average temperature of Australian summer (25C? 30C?) to cool down to the level of average Australian summer night (20C?) with closed windows?

    1. sanhei Post author

      You are correct that preferences for temperature vary for a given person. To add more reasons why: (1) when a person has a cold, they prefer much warmer temperatures, (2) after exercise, the feeling of warmth persists for a duration that depends on the length of exercising, for example marathon runners have ramped-up metabolism for hours after finishing, (3) food intake – a hungry person feels cold, but overeating causes one to feel too warm, (4) alcohol and various drugs can slow or speed up metabolism, thus cause a perception of heat or cold.
      The question about the average home is perhaps not so relevant as the heat inertia (related to thermal mass) of a specific building. Wooden Queenslander houses heat up and cool down very quickly, but a building with thick stone or brick walls takes days. Before the invention of refrigeration, ice cellars were used to keep food cold throughout the summer. Ice was cut in the winter and the cellar was lined with it, perhaps also insulated with straw. It took all summer for the ice to melt.


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