If you live in a hot climate, you probably find your summer economy is worse than your winter economy.
That A/C sure takes a lot of fuel to run.
My van has a lot of roof space, just soaking up the sun. Worse, the cargo section doesn't have a headliner so all that heat is transferred directly to the interior. While the cab has a headliner and is partitioned off, the steel partition isn't insulated so it transfers heat from the cargo area into the cabin in time.
I also have paints and sealants in there that have a longer shelf life the cooler they're kept.
'Before' pic:
Notice the strengthening ribs like a US Boxcar. They're up to 20mm deep, not good for aero.
So time for some chrome wrap.
The air pockets are intentional as this aids insulation, and helps aero. I did the whole thing in one piece, so I was never going to get it perfect anyway. If I was being really pedantic, I would have made little coroplast infills first so as to have something solidish for the wrap to sit on, but the whole thing is being covered in solar panels anyway.
On a cool 25*C day, I found a 22*C delta. That's only going to increase when the sun is really strong, or if you have a darker coloured vehicle. Even compared to white, chrome is still roughly twice as reflective.
With my van being around 7' tall, the wrap is invisible from normal viewing angles, and the paint is now protected from bird droppings (hard to keep something clean if you can't see/ reach it readily.
Unintended consequences
The roof stays so cool now, that a shower in the morning means the roof is still wet in the evening.