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In southern climes one can see many such reflective screens on cars in parking lots. Not all drivers use them, but many do.

Is there a meaningful impact on the internal temperature of your car? Does the length of time the car is parked in sunlight matter?

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Cars have matt black dashboards to prevent internal reflection into your line of vision (an image of the top of the dashboard reflected in the windscreen).

Matt black surfaces absorb light and re-emit the energy as heat. Reflective surfaces don't.

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Update:

Undesirable effects that may be alleviated by a sunshade include:

  • Heating of car interior above a comfortable level
  • Heating of car steering wheel (e.g. too hot to hold)
  • Accelerated degradation of plastics - outgassing of plasticizer.
    • consequent deposition on windscreen

Shelby F. Thames, professor of polymer science and president of the University of Southern Mississippi is reported as saying

Car owners can see the results of outgassing on their windshield, particularly during hot weather. Outgassing forms a waxy film on the glass that hampers visibility and is difficult to remove. ... plasticizers make the vinyl dashboard soft and allow manufacturers to shape it. ... It would take years and years to lose the plasticizer at room temperature,But the rays of the sun are magnified as they go through the windshield, and the dashboard can get extremely hot

A review of "Windscreens and safety" at Monash University Accident Research Center says

Almost all cars have a dashboard made from a polymeric plastics material. When the vehicle is left out in the sun the plasticiser is liberated from the dashboard and forms a thin film on the inside of the windscreen.

If the inside of the windscreen is not cleaned regularly this film could build up and cause dispersion of light through the window. This can affect the driver's vision through distortion of the light from the sun or from the headlights of oncoming vehicles.

None of the reports I've read quantify these effects.

The severity of the effect obviously varies enormously depending on geographic location, time of year, weather conditions (cloud cover, smog, ambient temperature), orientation of vehicle, size, slope and composition of windscreen, material and age of dashboard, time left in the sun and so on.

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  • Thank you. I guess what I'd like to see discussed is do we care about the ~15 degree peak difference in that graph? Does that really change anything such as lessen the degradation of dash components or something? That's what I was getting at with the "meaningful impact" phrasing. The goal is not "reduce car temperature" -- that's the method to achieve _____ ? I don't know what goes in that blank. Hope that helps.
    – Drew
    Jan 30, 2013 at 13:51
  • @Andrew, I suggest you consider changing your question from "Does x reduce temperature" to "does reducing temperature reduce harmful effect X" (maybe adding "to what degree" at the start of that).
    – RedGrittyBrick
    Jan 30, 2013 at 14:58
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    I live in Brazil. Pelicules are so popular here and the reflective screens too. In summer, with 40C (104F) temperatures, it's the difference of having to wait a 1 - 5 minutes to car temp gets manageable without refective screen or not wait with them. Reflective pelicules are popular here also because of that effect.
    – Fabricio Araujo
    May 7, 2013 at 20:46

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