Picture two homes on the same street: one constructed in the 1950s and the other in the 1990s. There are no trees or other shade. The air conditioning units are identical, recently replaced, and operating perfectly. Identical thermostats are set at 82 degrees Fahrenheit (27.8 Celsius).

When it’s 110 F (43.3 C) outside, the 1950s house will likely feel at least 10 F (5.6 C) warmer inside, even with the same air temperature.

Why?

The answer has to do with radiant heat. Radiant heat is what keeps you toasty warm at a campfire on a cold winter night. The fire doesn’t warm the air much; rather, like the Sun, most of the fire’s heat moves through invisible waves directly from the campfire to your body.

In the radiant heat of the Arizona sun, the surface temperature of the uninsulated post-and-beam ceilings in my house, one of 41,000 built in Tucson during the post-World War II era, can reach over 100 F (37.8 C). The single-glazed steel windows register 122 F (50 C), and the uninsulated concrete block walls aren’t much cooler.
Inside my house on triple-digit days, it can feel like I’m standing near a campfire, even with the air conditioner roaring to maintain 75 F (23.9 C). And when the system breaks – as it did during the long-running 2023 heat wave, when Phoenix hit 110 F (43.3 C) every day for weeks – temperatures rise dangerously fast. Without the AC, the hot surfaces plus the swirl of air from the ceiling fan makes the house feel like an air fryer.

Air temperature: An incomplete indicator of comfort
While people are used to thinking about how clothing, air movement, temperature and humidity affect comfort, two lesser-known measures help explain how they experience comfort indoors:

Mean radiant temperature. This is the average temperature of all the surfaces that surround us: ceiling, windows, walls, floor. For radiant heat to move between an object and the human body, it needs an uninterrupted line of sight, so ceilings and unobstructed windows have an outsized influence on the radiant temperature experienced in a specific place in a house.

Operative temperature. This can be approximated by averaging the mean radiant temperature and the average air temperature in a room. Other calculations of operative temperature take into account effects of air movement, humidity and additional variables. Roughly half of how you experience comfort is determined by the radiant environment.

Unfortunately, as the building scientist Robert Bean (no relation) says, “an entire industry of manufacturers, suppliers, builders and tradespeople incorrectly equate thermal comfort with air temperatures.” The result is that most people are completely oblivious to what actually makes a space feel comfortable — or uncomfortably hot.

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