Sunshades Help, but the Real Heat Busters Are Outside Your Windshield

It’s no secret that parking under a blazing sun turns your car into an oven. Sunshades, like window blinds at home, block direct rays. The Romans figured this out centuries ago with wooden shutters. But how well do those foil or cardboard shields actually work when you’re stuck in a parking lot?

A lot better than nothing, as it turns out, but the most effective trick is to put the shade on the outside, not the inside.

Back in 1988, University of Central Florida researchers parked two identical Tercels in the Florida sun and found that a simple cardboard sunshade behind the windshield dropped interior temps by 15 degrees and dashboard temps by a massive 40 degrees. Foil reflectors were even better.

More recently, a 2012 Malaysian study showed a nylon shade cut dashboard heat from a scorching 190°F down by about 33°F. But the German auto club ADAC tested 2020 methods and found that an external car cover beat everything, lowering interior temps from 127°F to 109°F. An exterior reflective windshield shade hit 113°F, far cooler than the 120°F inside with an interior shade.

A white cloth barely helped at 122°F.

Car color matters too: a black car’s interior hit 127°F while a white one stayed at 118°F, saving up to 13% in AC energy according to Lawrence Berkeley National Lab. And never forget safety, even a 61°F day can push a car’s interior to 105°F within an hour, risking deadly heatstroke.

So park in the shade if you can, use an exterior shade or car cover when you can’t, and always check the back seat. Your car, and your wallet, will thank you.