In his building-science presentations, John Straube often talks about how skyscrapers co-evolved with revolving doors. This is because of the stack effect: Warm air rising in a tall building creates a pressure difference that makes opening a ground-level swinging door very difficult. The taller the building, the greater the pressure difference and, as a result, the harder it is to open the door. A similar relationship exists with temperature: The greater the temperature difference, the faster the heat flows to the cooler space.
This explains why in hot, Southern climates, the International Residential Code (IRC) requires less insulation. Even on the hottest of days, there is only a 40 to 50 degree Fahrenheit temperature difference between indoors and outdoors. This contrasts to the far North, where there may be a 70 to 80 degree Fahrenheit temperature difference between indoors and outdoors on the coldest day.
With a larger temperature difference, more insulation is needed to effectively slow the movement of energy. This concept also informs how and where we insulate. For example, it makes more sense to insulate an attic floor than it does to insulate a basement ceiling. The attic floor separates the living space from the unconditioned attic, while the basement temperature is much closer to the temperature of the living space.
It makes more sense to add insulation where the temperature difference is greatest. Vented attics over living spaces experience the greatest temperature differentials, so it often makes the most sense to boost insulation levels there. This is one reason that fully inspecting your home for leaks is important. Preventing the flow of heat and air into areas that are unnecessary is wasteful and not energy efficient. Taking advantage of building sciences is an efficient way to productively protect your home and your wallet.
For more information on building sciences, visit FineHomebuilding.com/extras