During the winter, the air near your floor is cold, while the air near your ceiling is hot. Similarly, during the summer, the air conditioner keeps your first floor comfortable, while the rooms on the second floor are unbearably hot. What’s going on?
The usual answer is, “Heat rises.” But that explanation isn’t quite accurate. (It’s true that hot air rises by convection. But heat travels in all directions, including sideways and downward, by conduction and radiation.)
The scenarios I’ve described are variations on the theme of temperature stratification — problems caused when indoor air temperatures are layered horizontally, with cold air down low and hot air up high, like layers in a parfait.
Let’s start with a simple example: a single-story slab-on-grade house with temperature stratification problems. The thermostat is 48 inches above the floor, and it’s set at 72°F. On a winter day, when you are sitting at your desk, the air near your feet is at 65°F. Up near the ceiling, the air is at 79°F. Why?
There are two main causes of this phenomenon.
Air leaks coupled with the stack effect. The most important factor in this type of stratification is air leakage. Holes near a home’s ceiling allow warm interior air to escape into the attic. As this air escapes, it pulls in cold exterior air through cracks near the floor — for example, cracks near the threshold of an exterior door. The engine for this type of air movement is the stack effect. Since the warm interior air near the ceiling is at a positive pressure with respect to the outdoors, it wants to escape. Since the cool interior air near the floor is at a negative pressure…