How Poor Insulation Makes Hot Days Even Hotter

Friday - August 4 2023, 13:23 UTC - 1 year ago

Inside even perfectly air conditioned buildings, the mean radiant temperature and operative temperature are often too hot for comfort. Poor modern insulation increases the likelihood of the occupants' heat stroke, and this is especially true for smaller buildings like small houses, mobile homes, tiny homes and more.

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, temperatureand humidity affect comfort, two lesser-known measures help explain how they experience comfort indoors: .

• Mean Radiant Temperature (MRT): How much infrared radiation – heat – is radiating from all the surfaces around us.

• Operative temperature (OPT): This takes into account humidity, air velocity, and MRT, giving a single measure of thermal comfort indoors, regardless of season.

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.

On a hot, sunny day, good insulation and double-pane windows slow heat transfer enough for air conditioning to keep the mean radiant temperature inside the building within a few degrees of the air temperature.

However, in an under-insulated building, such as my house, or in some older public housing projects in Phoenix, the high mean radiant temperature can push the operative temperature over 90 F (32.2 C) – even with the thermostat set to 75 F (23.9 C). When the surface temperature exceeds the temperature of our skin, heat will begin to radiate from the hot surface into the body, making heat stroke more likely.

While the exact threshold where overheating becomes dangerous is debated, most people would agree that 90 F (32.2 C) is far too warm for comfort.

Hot surfaces are why smaller buildings, such as mobile homes, tiny homes, shipping containers and garages turned into apartments, often feel uncomfortable regardless of the thermostat setting. Smaller structures expose occupants to three, four or even six surfaces with the exterior exposed to the sun and hot outside air. More warm surfaces, more discmfort.