The fitting of the wall installation and the Headliner were done in parallel. I used white 3/4″ foam board insulation which came in 4’x 8′ sheets. Insulation was measured and cut for each void in the side walls. The foil layer faces inward to contain the radiant heat inside the trailer. Supposedly you need a bit of air space for this to fully work correctly. The insulation was not permanently glued as it will need to come back out for the wiring later on.