Should I use the blue or silver side of my car sun-shade?

May 27, 2008 9:54 PM

Car Sun Shade Question: I have a car sun-shade that is blue on one side and silverish on the other side. Which side should I face out to keep my car cooler?

I know this is a silly question, but I think about it almost every time I use my sun-shade. I have been putting the silvery side out. Both sides have sort of a cloth texture, so it is not one of those fancy mirror sun-shades.

But also, just for my curiosity's sake, are the mirror sun-shades the best type to use? Wouldn't these reflect the light away from the car? Does this even matter where temperature is concerned? Just curious.

posted by boots77 to science & nature (12 comments total)

The silver side. Yes, it will make a difference.
posted by kindall at 10:05 PM on May 27, 2008

Visible sunlight is a mix of colors. Remember that the color you see when you look at something is the color that's not reflected. For example, when you look at the leaf of a plant, it is absorbing green and reflecting all other colors — that's why it looks green.

So a sunshade with a lighter hue (probably the silvery side) will reflect more of the spectrum of sunlight than the darker color and will help reflect more thermal energy away.

If it helps, think about the difference in wearing a black or a white t-shirt on a hot summer day.

A mirrored sunshade will be more efficient at reflecting UV, visible and thermal energy, which means more of that energy is kept outside of your car and off your dashboard plastics.
posted by Blazecock Pileon at 10:07 PM on May 27, 2008

Whichever's lightest.
posted by pompomtom at 10:13 PM on May 27, 2008

@Blazecock Pileon: Are you sure about this? What if the leaf reflects green but absorbs all other colors? Seems that would appear green, too.
posted by mr. creosote at 10:15 PM on May 27, 2008

For example, when you look at the leaf of a plant, it is absorbing green and reflecting all other colors — that's why it looks green.

Blazecock, you have that backwards, the leaf absorbs other frequencies and mostly reflects back green light.

Anyway, you should place the shiny side out. The blue side would absorb more radiation and thus heat the car. That might be useful in the winter...
posted by wfrgms at 10:18 PM on May 27, 2008

Blazecock got it backwards. Creosote's right. A leaf looks green because it reflects (technically, scatters) green light.
posted by phunniemee at 10:19 PM on May 27, 2008

The silver side. Don't most sunscreens say CALL POLICE on the other side, anyway?

There's a tiny bit of thermal heating associated with the reflected light hitting your windows, but it's much less than the heating of the direct sunlight that gets through your glass on the cloth surface itself. On the other hand, it won't be any worse than the thermal heating of the sunlight on your interior car surfaces, from the dashboard to the seats. This is the other reason for a sunscreen: it protects your interior from sun damage. The sun will make the colors fade, and make materials like vinyl more brittle and easily creased and torn.

If you WANT it to heat up, say on a winter day, then use the blue side. If you want to minimize interior heat, use the shiny side.
posted by dhartung at 10:48 PM on May 27, 2008

Definitely the silver side out. The silver will reflect all frequencies of radiation.

The blue side will also play a role in keeping the car cool, however. Its blueness means it will absorb the red and infrared wavelengths, which are the wavelengths ordinary hot and warm objects emit here on the surface of the earth. So if the interior of the car is warm the blue side will absorb some of the heat, causing the reflector to heat up, and much of that heat will be emitted out of the windshield. The blue side causes the reflector to perform something like a wick to get rid of some of the heat of the car.
posted by jamjam at 10:57 PM on May 27, 2008

I hate to add a new part to this question so late, but what does all of this mean about window tinting? Tinting is traditionally dark, so doesnt' that draw heat to the car?
posted by boots77 at 6:19 AM on May 28, 2008

I'm not sure of the precise physics behind it, but tinting keeps your car cooler, and I think it's due to the reason mentioned by dhartung. Less sunlight gets into the car to heat up the interior surfaces. Tinted windows are a measure to use on buildings that will have a lot of solar heat gain as well, so there's some background to it.

So if the interior of the car is warm the blue side will absorb some of the heat, causing the reflector to heat up, and much of that heat will be emitted out of the windshield. The blue side causes the reflector to perform something like a wick to get rid of some of the heat of the car.

I don't think it works like this. One, there's a reason that they make greenhouses out of glass--glass traps infrared radiation. Other types of light energy come in through the windows and heat up the surfaces inside the car, but the glass prevents the heat produced by that from escaping. The silver layer isn't going to emit anything, it just reflects, and would actually probably just reflect any heat picked up by the blue side back into the car. The sunscreen's primary effectiveness comes from keeping the sunlight from entering through a fairly horizontal surface.
posted by LionIndex at 7:18 AM on May 28, 2008

boots77: tinting heats up the glass (ie the exterior of the car) instead of the interior. Since the exterior (the glass) is air-cooled by the outside breeze, air currents, and convection, this means the heat goes away, whereas heat inside the car is trapped and accumulates, raising the temperature higher and higher.
posted by -harlequin- at 9:30 AM on May 28, 2008

Another thought on tinting - if you used a reflective, silvery material instead of dark, that would be unsafe and make other drivers insane. When you use a sunshade, by defintion you're parked.
posted by desjardins at 9:01 AM on May 29, 2008