Aluminum foil over windows can keep homes cool in heat wave

Emergency management agencies strongly recommend the use of aluminum foil-covered cardboard inserted between windows and curtains.

Historic heat waves shook parts of the United States in June 2021, resulting in record-breaking temperatures topping 110 degrees Fahrenheit in cities like Portland, Oregon. A heat wave the following June brought various heat warnings to almost a third of the US population.

People took to social media to share how people in homes without air conditioning or with poor air conditioning can beat the heat and stay cool. One method in particular stood out because users claimed it was both cheap and effective: covering windows with aluminum foil to reflect sunlight and heat out.

A TikTok video sharing the method received more than 100,000 likes and multiple tweets sharing tactics received thousands of likes and retweets. All posts urge people to black out windows with aluminum foil.

THE QUESTION

Does installing aluminum foil over windows help keep the home cool?

THE SOURCES

THE ANSWER

This is true.

Yes. Civil protection agencies specifically recommend the use of “aluminum foil-covered cardboard” between windows and curtains to reflect heat outward.

WHAT WE FOUND

The Federal Emergency Management Agency (FEMA) published a June 29 blog post that listed the aluminum foil tactic among six ways to combat extreme heat.

“You can keep your home cooler by insulating it and covering your windows with curtains or blinds,” FEMA said in the post. “Use window reflectors, like cardboard covered in aluminum foil, to reflect heat out.”

Local emergency management authorities agree. The Georgia Emergency Management recommends installing “temporary window reflectors,” such as cardboard covered with aluminum foil, between windows and curtains before extreme heat arrives. Washington Emergency Management tweeted on June 22 that residents could prepare for the upcoming heatwave by putting aluminum foil-covered cardboard on their windows. Both say this reflects heat outward.

dr Mahabir Bhandari, a member of Oak Ridge National Laboratory’s Building Envelope and Urban Systems research group, agreed that aluminum foil and cardboard are useful for cooling the home in emergency situations. But if it’s an option, he felt it would be more effective to cover the windows from the outside rather than the inside.

“Now, [when the aluminum is covering the window from the outside, the sunlight] doesn’t hit the window directly, it hits the aluminum foil,” Bhandari said. “It will absorb some and some will come in, but then most of it will be reflected.”

dr Bhandari said the aluminum foil could transfer some heat through the window as the aluminum itself gets hotter, and through a process called conduction, the glass could then be heated. But this is where the box from the FEMA recommendation comes into play.

The cardboard serves as insulation and absorbs the heat from the aluminum foil. That means the window doesn’t get as hot, and therefore the space behind the window doesn’t get as hot either, Dr. Bhandari.

Heat travels by a combination of radiation, conduction, and convection. According to a Department of Energy page about a cooling solution called this radiation barrier, radiation travels in a straight line from the heat source to a surface that absorbs the heat. Conduction, explains the University Corporation for Atmospheric Research, is when heat is transferred directly between two things that are in contact.

“Basically, when the sun hits your window, some heat can be reflected out and some heat can be absorbed,” said Dr. Bhandari. “And then some heat can go straight in, we call that direct radiation, and then also because your window is hot, it can also emit heat later on too.”

“So when the heat comes in, it gets locked in,” he continued. “It can’t go out the window the way it came in.”

Because of this, explained Dr. Bhandari, many windows are already built with a certain degree of reflection. Heat will not pass through a reflective surface, most of it will just bounce off. And if something prevents the window surface from getting hot, like a shutter or a piece of cardboard, then the window can’t emit heat into the room air.

If aluminum foil is not available, Dr. Bhandari that the next best strategy is to find anything at all to cover the windows to keep sunlight and the heat that comes with it from getting inside. Cardboard works well here too.

According to Dr. Bhandari energy efficient windows that reflect more sunlight, window films that can be attached to windows, and shutters that can be opened and closed as needed to prevent sunlight from reaching the window in the first place.

More from VERIFY: Yes, heat is historically the number one weather-related killer in the US

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Alley Einstein

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