With the sound dampening finished (except for the front cab area), we can now move on to the next layer in the insulation plan: 1/2 inch thick Pink Extruded Polystyrene Foam Board and Pink Fiberglass Batting. We didn’t want to use foam board any thicker than 1/2 inch so that it was easily flexed to the rounding walls and ceiling. If it’s too rigid, it would either break when bent, or overpower the glue used to adhere it (probably). Speaking of adhering it the foam board, we used 3M Hi-Strength 90 spray glue. Hopefully this glue will hold up well, there are many reports both good and bad about using it for this.