Emergency management agencies specifically recommend using aluminum foil-covered cardboard that goes between windows and drapes.
Historic heat waves have rocked parts of the United States this week, leading to record-breaking temperatures over 110 degrees Fahrenheit in cities like Portland, Oregon.
People took to social media to share ways people in homes with no air conditioning or poor air conditioning could fight 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 the heat back outside.
A TikTok video sharing the method got more than 100,000 likes and several tweets sharing the tactic received thousands of likes and retweets. All of the posts tell people to black out windows with aluminum foil.
Does putting aluminum foil over windows help keep homes cool?
Yes. Emergency management agencies specifically recommend using “aluminum foil-covered cardboard” between windows and drapes to reflect heat back outside.
WHAT WE FOUND
The Federal Emergency Management Agency (FEMA) published a June 29 blog post that included the aluminum foil tactic among six ways to combat extreme heat.
“You can keep your house cooler by insulating it and covering your windows with drapes or shades,” FEMA said in the post. “Use window reflectors such as aluminum foil-covered cardboard to reflect heat back outside.”
Local emergency management agencies echo that. Georgia Emergency Management recommends installing “temporary window reflectors” such as aluminum foil-covered cardboard between windows and drapes before extreme heat arrives. Washington Emergency Management tweeted June 22 that residents could prepare for the upcoming heat wave by installing aluminum foil-covered cardboard at their windows. Both say doing so reflects the heat back outside.
A Department of Energy page about radiant barriers, which are often made of aluminum foil applied to cardboard, explains how it works. Heat travels through a combination of conduction, convection and radiation. Radiant heat travels in a straight line from the source of the heat to a surface that will absorb the heat. So when the sun shines through windows, it is heating inside a home through radiation.
Reflective surfaces, like metals including aluminum, act as a kind of heat shield, according to a 2016 blog post from West Texas A&M University physics professor Dr. Christopher Baird. Radiant heat is easily reflected, allowing materials like aluminum foil to prevent most heat from penetrating it.
Aluminum foil won’t block all of the heat; it’s not perfect and heat has two other means of travel even if it was. That’s why emergency management agencies recommend using aluminum foil-covered cardboard alongside other insulation solutions like blinds and drapes.
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