[jonheller]

I always assumed running air conditioning with windows open was incredibly wasteful. Is this because Japan's summers are more temperate than where I'm used to (Northeast US)?

I do find myself sometimes running the AC with windows open when it's cooler outside than indoors. For some reason it can be 65 degrees outside but 75 degrees indoors.

[fomine3]

It is said by manufacturer like Daikin that stop&start A/C is inefficient than continuously running. (especially on summer, YMMV)

[fiddlerwoaroof]

Why would it be wasteful? If the house is warmer than the outside air, opening the windows until outside and inside equalize should save a bit of energy. (Assuming that the change to the airflow patterns isn’t relevant)

[smegcicle]

I have no idea to what degree, but I think humid air reduces the efficiency of AC.

[nbutyllithium]

A bit of a complicated subject but the short answer is you're correct humidity does reduce the efficiency of air conditioning.

The reason why(/long answer) is that as it cools the air the humidity in the air (i.e. water vapor) will condense into liquid water. Water requires a fair amount of energy to be removed (known as latent heat) to make this change and none of that energy removed actually changes the temperature of the air (all of it goes into changing the phase of the water). Therefore a high humidity load results in a lot of the cooling potential getting "used" up removing humidity first.