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Why Arizona homes often feel warmer than thermostats suggest – and what to do about it

Imagine two houses on the same street. One was built in the 1950s and the other in the 1990s. No trees or other shade. The air conditioning unit is the same, recently replaced and working perfectly. The same thermostat is set to 82 degrees Fahrenheit (27.8 degrees Celsius).

If the outside temperature is 110°F (43.3°C), your 1950s house will feel at least 10°C (5.6°C) warmer inside at the same temperature.


The answer has to do with radiant heat. Radiant heat keeps you warm by the campfire on cold winter nights. Fire does not heat the air very well. Rather, like the sun, most of the fire’s heat is transferred from the campfire directly to your body through invisible waves.

In the radiant heat of the Arizona sun, the surface temperature of my home’s uninsulated post-and-beam ceiling is 41,000 built
In post-World War II Tucson, temperatures can exceed 100°F (37.8°C). The single-pane steel windows register 122°F (50°C), and the uninsulated concrete block walls aren’t all that cool.

Inside the house on a triple-digit day, it can sometimes feel like you’re standing near a campfire, even though the air conditioner is blaring to keep it at 75°F (23.9°C).And when the system breaks – like in an accident Prolonged heat wave in 2023, temperatures soared dangerously quickly as Phoenix reached 110°F (43.3°C) daily for several weeks. Without air conditioning, hot surfaces and swirls of air from ceiling fans heat up your home. Feels like an air fryer.

Temperature: an imperfect indicator of comfort

People are accustomed to thinking about clothes, air movement, temperature and temperature, humidity Two little-known metrics that influence comfort help explain how we feel comfortable indoors.

  1. average radiant temperature. This is the average temperature of all surfaces that surround us: ceilings, windows, walls, floors. An unobstructed line of sight is required for radiant heat to travel between objects and the human body, so ceilings and unobstructed windows have a large effect on the radiant temperature experienced in a particular part of the house.

  2. operating temperature. This can be approximated by averaging the average radiant temperature and average air temperature in the room. Other calculations of operating temperature take into account the effects of air movement, humidity and other variables.almost Half way to experience comfort is determined by shining environment.

Unfortunately, as an architectural scientist Robert Bean
(Irrelevant) says, “An entire industry of manufacturers, suppliers, builders and traders incorrectly equates thermal comfort with air temperature.” As a result, most people are completely unaware of what actually makes a space feel comfortable, or uncomfortably hot.

On sunny and hot days, the good insulation and double glazing slow the heat transfer of the air conditioning, keeping the average radiant temperature inside the building within a few degrees above the air temperature.

However, in poorly insulated buildings like mine and some older public housing projects in Phoenix, even with the thermostat set to 75°F, the high average radiant temperature can cause operating temperatures of 90°F. °F (32.2 °C) may be exceeded. (23.9°C). When the surface temperature exceeds the skin temperature, the hot surface begins to radiate heat into the body, Heat stroke is more likely to occur.

There is some debate about the exact threshold at which overheating is dangerous, but most people would agree that 90°F (32.2°C) is too warm for comfort.

Small buildings such as mobile homes and small houses have hot surfaces, shipping container
Garages and apartments have often been uncomfortable, regardless of the thermostat setting. In smaller structures, the occupant is exposed to his 3rd, 4th and even 6th sides, the outside being exposed to the sun and the hot outside air. The warmer the surface, the greater the discomfort.

Cooler surface for more comfort

If you live in a poorly insulated building and don’t mind using more power, you can set your thermostat lower. But with a high average radiant temperature, a 2°F (1.1°C) drop in air temperature only feels like 1°F (0.6°C). Still, the hot surface makes it uncomfortable.

Add insulation to the roof and replace the single pane windows with double glazing units. Low emissivity (Low-E) glass Helps reduce average radiant temperature and utility bills.They are expensive improvements, but the new federal tax credit and Upcoming rebatesmanaged by individual states.

Trees, shades, and outdoor shades can also reduce average radiant temperature by blocking direct sunlight. but, Glass is a poor insulatorSo in very hot climates even veneer windows with full sun protection can get uncomfortably hot.

Adding a curtain to the inside and keeping it closed can reduce the average radiant temperature because the curtain is closer to the air temperature than the glass.

What happens to the old building renters?

Tenants in older, poorly insulated buildings often cannot afford large utility bills, and landlords may be unable or unwilling to undertake expensive renovations. What makes the situation even worse is the Older air conditioning systems consume 2-3 times more energy Provides the same amount of cooling as a new unit.

Because thermostats need to be set lower to create comfortable operating temperatures, HVAC systems in poorly insulated buildings have to run harder for longer periods, using more energy and costing money. will rise further. And the price of discomfort isn’t just financial. in hot buildings Adverse health effects and productivity.

Millions of Americans now live in places like: Cooling is the only way to prevent mass casualty incidents. in Phoenix, City code requires an air-conditioned rental unit
Maintain temperature below 82 F (27.8 C) measured 3 feet above the floor in the center of the room. Unfortunately the code doesn’t specify whether 82 F is the operating temperature or the air temperature.

That one word can change the world.

In an old, poorly insulated building like my house, or in the worst-case scenario, a sunny southwest facing unit on the top floor of an uninsulated concrete skyscraper, seemingly safe temperatures are 82 degrees. F can easily mask dangerous operating temperatures above 96 F (35.6 C).

The key to better design

as a professor of architecture and architectural scienceI believe that today’s Byzantine building codes and rental rules can greatly improve comfort by: Adjustment of average radiant temperature
not the temperature. Vast sections of code could potentially be abandoned by requiring internal surfaces that can be easily measured with inexpensive tools. infrared thermometerwithin a comfortable range of 60 F (15.6 C) above and 85 F (29.4 C) below.

Architects and engineers can apply for more comfortable buildings simple and well-established principlesSuch as natural ventilation, shading, appropriate insulation and windows depending on the climate. Insulating heat in the first place means you don’t have to spend a lot of energy on cooling. Studies have shown that these measures provide the following safety benefits: keep buildings cooler longer during summer power outages.

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

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