Originally Posted by SFDavis50
I assume your panels have an inch or more of space between them and the roof so air can circulate and they shade the roof.
51 C (123.8 F) is pretty hot; in Arizona or Texas it would be a lot hotter. If mounted directly to the roof surface they would radiate a lot of heat into the trailer which may be good in winter but bad in the summer.
I don't know if the flexible types generate more or less heat but perhaps you could make window awnings for use in the summer then strap them to the roof in the winter.
I have read on other forums that the cupping of the flexible panels bonded to the top of an RV is caused by the extreme heat and the inability to dissipate that heat. I suspect this is more common when bonded to an "other than aluminum" roof. Lots of people bond them to the top of Airstream trailers but aluminum is a very efficient heat sink and will dissipate the heat very well. Still, the Airstream itself gets pretty hot in direct sunlight and the efficiency of the panels decreases with heat.
Fiberglass is not a good thermal conductor so it won't dissipate the heat like aluminum. Heat from the panels will radiate directly thru the fiberglass into the insulation. Reflectix will radiate some of it back to the panel but it could also get hot enough to damage the insulation. You won't know until after it occurs.
The white gelcoat reflects very well and doesn't get hot in direct sunlight. Touch a white car and then any other colored car on a sunny day and there is a big difference.
Mounting with Velcro is an interesting idea but it would have to be done so that there is no way the panel could melt or soften the plastic hooks on a worst case summer day in Texas or Arizona. Mounting the flex panels onto an aluminum sheet but still having it above the surface of the trailer could be more aerodynamic than the current panels but I wouldn't bond them to the fiberglass surface, although in winter this may be an interesting way to add some heat.
Fiberglass sailplanes are white and the only areas which get painted other colors are non structural areas. The reason is that the fiberglass becomes more flexible as it get hotter. If you paint stripes or numbers on top of the wing the area under will get much hotter, more flexible and will bend more under stress. One wing bending more than the other can flip it.