Hello, we are about ready to start insulating our bus as the rest of the furring strips will go up today or tomorrow. We have decided to use insulation board but have a few questions. Does the board need to be flush with all surfaces? Or can boards be stacked to the needed height? For example our ceiling gap is 2 inches but our floor will only be 1.5 and the walls are closer to 1. So if we were to get 1 inch boards for the walls and 1.5 inch boards for the floor and then could we either use the 1.5 for the ceiling and there would be a small air pocket or could we stack two 1 inch boards to completely fill the space? And in any spaces where two boards meet should tape that or fill that gap somehow as well? We are also planning on gluing the boards in and wondering if there is a particular glue that would be best to use? Any insight into this or any friendly suggestions regarding insulation are much appreciated.
There's nothing wrong with a gap except that it's a space you could be filling with insulation. 1.5" XPS has an R-value of 7.5 while 2" XPS (or two layers of 1" XPS) will have an R-value of 10 - a 33% improvement.