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Full Version: Mylar sheets on top of the roof?
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I'm "this close" to purchasing a Dodge Grand Caravan minivan as a newbie, kind of try things out, vehicle to do some boondocking, and maybe a bit of stealth urban camping. I don't want to mess with the interior, at least not at the beginning, in case I want to trade it in later. So, here's an idea, and I don't know if it's stupid or not. I have several of those cheap mylar "emergency blankets," just sheets of mylar, really. Would there be any advantage when parked in hot sunny weather to laying them on the roof of the van, and holding them down with, say, magnets, as way of diminishing the heat coming into the van through the roof?
With air flowing beneath the sheets, however little (1/2"), you would have it made in the shade!
Before buying you can rent one for a couple of days.  Your "conversion" has to be simple put it in, take it out.  

If the vehicle has a roof rack the reflective mylar will be separated from the roof by a space.  If a breeze can get between the two it will act like a heat shield.
Maybe I don't understand how this works yet. Since the mylar would be on top of the van, and there are already 93 million miles of space between the surface of the mylar and the source of the radiation, wouldn't the mylar reflect the radiation, rather than transmitting it to the roof surface, even without an air barrier? Or does there need to be air on both sides of the mylar surface? If there needs to be air against both surfaces of the mylar, I suppose a roll of reflectix with the bubble wrap attached would do the trick. Then the problem of securing it needs to be figured out. But it would all still be better than being able to bake potatoes in the air of my living space.
Imagine the micromillimeter layer of Mylar bonded to your steel shell like paint.

Not optimal is it, still lots of radiant heat gain.

But to the extent there is a gap between the Mylar and your steel, that is reduced.

And if that gap has airflow carrying the residual heat away, even better.

It is also helping remove the heat from your vehicle coming from all the other sources.
simply put, yes you need an air gap. the Mylar will get hot and transfer that heat to the sheet metal via conduction. highdesertranger
The catalytic converter gets really hot.  If you park in long grass it is hot enough to light a fire.  The exhaust doesn't start fires because of the heat shield.  There is a thin piece of steel located a half inch away from the hot parts.  Air on both sides keeps the heat shield cool.  Steel is not a good insulator.  The half inch of air keeps the heat shield from getting a lot of heat input and whatever breeze there is keeps the shield cool.  Good insulation could keep the exhaust as hot as possible but we don't really care about that.  The goal is only to keep the grass cool.  The aluminized mylar space blanket over the roof is like that.  The goal is just to shield the roof to keep the roof cool.
You might want to check this material. Cooltarp
What color is the roof of your van?
  • If roof is shiny white and the mylar sheet is in contact you will see little improvement.  An air gap between roof and mylar will lower the temp of the roof.
  • If roof is a dark color and/or flat (non-reflective) the mylar sheet in contact will give about the same temp reduction as a shiny white roof.  Again an air gap will give more improvement.
Not sure about Reflectix, the stuff I have looked at is not opaque; I can see light through it and it gets worse with handling (reflective coating abrades).  If you decide to use it just pop the bubbles in the places you want to put magnets.  IMO the Reflectix won't work as well as a mylar sheet.

 -- Spiff
I think it would work really well, but I think it will be too fragile. I would use Reflectix myself.
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