I liked at polycarbonate sheets but decided against in the end, partly because I was ending up with 2 interfaces to stick down and the marginal height increase (I’m very close to 2m!).I’ve got a 200W Renogy flexy PV panel that I’ll be fitting after I’ve finished installing a DC-DC charger and battery maintainer, so am interested to see how others are installing them.
My plan is to use a 10mm polycarbonate twinwall sheet between the PV panel and the roof. The PV panel will be attached to the twinwall with Sikaflex 522, and the twinwall will be stuck to the roof with 3M VHB tape. This will provide an air gap to keep the temperature of the PV panel at a reasonable level. As a safety precaution, the PV panel will be attached to the roof bars with two cords fastened to each of the PV panel’s front mounting eyes. According to my measurements, this arrangement should not increase the actual clearance height of my van’s roof by more than 5mm.
I put down a sheet of PPF and the keyed the surfaces with a scotch pad before using Sika 552 to stick the panel down. I applied a 8-10mm triangular bead (instructions on Sika website) to give a 4-5mm deep bead once the panel was applied. I also used a few rubber spacers to ensure the panel didn’t get pushed down too far - I wouldn’t bother next time. The beads are full width at the front then the rest run across the panel but have a 10-15cm gap in the middle to aid airflow and to allow any water to escape. They are placed at about 15cm apart to fall in the gaps between individual cells. The front edge is smoothed so there is no edge to lift in the airflow. It’s survived all weathers and high speeds no problem at all and is in the same state it was when fitted.
