Solar heating panels

[matt4x4]

If you're going to build racks for your panels, I presume you will use wood - possibly PT. In this case, I don't think any underlay is needed, it will certainly NOT help with heat gain, the only possible extra heat gain I could see is if you were to use something reflective like aluminum foil or the silver foil covered foam boards used in house construction. Reason being, the black underlay would actually absorb heat that's allready in your panels, where the reflective would bounce the heat trying to push through the panels right back at them.

The only reasons I used my black winter cover as underlay on the ground was to keep the weeds down and that I had it kicking around and it saved me from buying tons of landscape fabric which is also black, however, for a raised install I would not use it.