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.

1. 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 outsize influence on the radiant temperature experienced in a specific place in a house.
2. 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.

Adding insulation to your roof and replacing single-pane windows with double-pane units with low-emissivity (low-E) glass can help reduce the mean radiant temperature and your energy bills. They’re expensive improvements, but new federal tax credits and forthcoming rebates, to be administered by individual states, can help.

Trees, awnings, and exterior shades can also reduce mean radiant temperatures by blocking direct sunlight. However, glass is a lousy insulator, so in very hot climates, single-pane windows completely protected from the sun can still become uncomfortably warm.

Adding a curtain inside—and keeping it closed—can help decrease mean radiant temperature because the curtain will be closer to the air temperature than the glass.

What about renters in old buildings?

Renters in older, under-insulated buildings are often less able to afford large energy bills, and landlords may be unable or unwilling to make expensive improvements. Making matters worse, older air-conditioning systems use two to three times as much energy as newer units to deliver the same amount of cooling.

Since creating a comfortable operative temperature requires setting the thermostat lower, an HVAC system in an under-insulated building must work longer and harder, using more energy and further raising the cost. And the costs of discomfort are not only financial: Hot buildings also have adverse impacts on health and productivity.

Millions of Americans now live in places where cooling is the only thing preventing a mass casualty event. In Phoenix, city code requires rental units cooled by air conditioning to maintain a temperature of no more than 82 F, measured 3 feet above the floor in the center of the room. Unfortunately, the code does not specify whether 82 degrees is the operative temperature or the air temperature.

That one word makes a world of difference.

In an older, under-insulated building similar to my house—or, in what might be the worst-case scenario, a sun-fried Southwest unit of the top floor of an uninsulated concrete high-rise—a seemingly safe air temperature of 82 F could easily mask dangerous operative temperatures of 96 F or higher.

The key to better design

As a professor of architecture and building science, I believe today’s byzantine building codes and rental rules could be greatly improved for comfort by regulating mean radiant temperature rather than air temperature. Vast sections of code could be jettisoned by requiring that interior surfaces, which are easy to measure with an inexpensive infrared thermometer, be kept within a comfort range above 60 degrees and below 85.

For more comfortable buildings, architects and engineers can apply simple, established principles, such as natural ventilation, shading, and the right insulation and windows for the climate. Keeping heat out in the first place means we don’t have to spend so much on energy for cooling. Research shows that these measures can also make us safer by keeping buildings cooler for longer in summer power outages.

The happy result: homes and other buildings that are not only comfortable but also safer and more affordable to operate.

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Jonathan Bean is an associate professor of architecture, sustainable built environments, and marketing at the University of Arizona.

This article is republished from The Conversation under a Creative Commons license. Read the original article.