Where Should AC Be Installed?

Automatic climate control (HVAC) is a beautiful invention. But, for some reason, it seems to be the solution in search of a problem. People install AC all over their homes, not realizing that air conditioning can make you feel hotter than no AC at all! It’s pretty counter-intuitive but true.
So before you go out and buy that $1000 unit, read on.

It All Starts with Heat Flow

Understanding the principles behind heat flow is key to understanding why too much air conditioning will leave you feeling hot and miserable during the summer months. (1) As shown in the graphic below, warm air naturally rises while cool air descends. An AC unit will cycle cool outdoor air through your house regardless of where you install it. And if that air is more relaxed than the warm air inside your home, you’re going to feel chilly—and the air outside will be wasted as it heats up in your AC unit and blows out hot!

Measuring Heat Flow

7 Btu/hr-ft² of heat flow coming through a single layer of 1″ drywall.
Heat Flow on the Vertical Plane

The energy required to cool your house is directly related to the difference in temperature between the inside and outside airflows. If that difference is most significant at the ceiling, then you’re going to overcome that area, wasting energy without keeping you comfortable. (2) To demonstrate this concept graphically, these are plots of heat flow plotted against height above grade for both hot & cold climates. Note how much more heat comes into the building where it meets the ground—almost 3x as much!

This plot shows how hot & cold airflow interacts with a wall… notice what happens around 2′ off the ground!

A similar principle applies to walls. Air that meets a wall is cooled and rises vertically until it hits the ceiling. (3) As shown in this illustration, cold outdoor air will first cool the top of your house (red), and as it moves down and meets warm inside air (blue), it will chill you out (purple).

This graphic shows how heat flows up & over a structure if there’s no insulation. Here, we can see where cold spots might be found on an uninsulated roof.

The same principle also applies to roofs (and any other surface, for that matter). The roof deck acts as a giant radiator, cooling hot attic air as it hits. If that cold airflow can’t quickly go back outside, it will chill the roofing materials and whatever is below.

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