MLucas wrote:Why not just trim it a bit and put it on the inside of the car? That material is flexible enough to fit inside the plastic fascias and dashboard, ......
She could, but INSIDE the window it is not as efficient as keeping the car cool as outside.
Reasons(with heatshield inside): First, once that sun hits the glass itself (which has a slight tint and does absorb some energy) it warms the glass itself. Once the glass is hot it is transmitting heat into the car by convention, conduction, and radiation. While most of the heat it reradiates toward the inside of the car is then reflected back out by the heatshield, the hot air and adjacent metal and plastic heated by the hot glass does get inside the cabin. In short, with it INside a nice layer of hot air between the shield and the window is created and that does leak, significantly I'm sure if you're parked for hours in direct sun, into the cabin.
With the shield OUTSIDE the light/radiant energy is never allowed to even hit the glass, let alone go past it. Cooler windshield, cooler air layer behind it, etc.
With it outside it is also blocking/insulating the surface of the windshield from conductive heating by the hot air itself (consider a 100 degree day).
In terms of the difference between having it inside vs outside if just parking in the sun for an hour I don't it's significant. Bet it is if parked for many hours...particularly if the air temperature itself, not just the sunlight, is high.
Since in terms of easy of use with that material of inside vs outside being about equal, I'd say her method is better.
That said, ANY windsheild sunshade is going to be radically better at keeping the inside from becoming suitable for baking bread than having none. I like the idea (see earlier in this thread) of a total cabin cover, and if in a really hot place and lifestyle had us parking in the sun would go for that. But not worth the bother as ours is parked in garage most of the time and our town has a half decent scattering of shady parking spots.