YEARS ago when most Class C's had no Genset's we installed Gensets and had to build the box and insulate it and make it air tight to the Inside. You build a box in a RV to make it slightly less loud inside and to seal it from the inside. Look at the link I provided. This type material is usually available locally in 4 by 8 foot sheets. It has a foil back and you install with the foil visible. You cut and install on the sides and back and ceiling using liquid nails. If you have no box make one out of 3/8 plywood and then install that insulation. Once the Insulation is glued in you then use rubber silicone to seal all the edges to make it air tight. Most RV Gensets are vented thru the floors now and require no vented door. BUT, I would vent the door if on a 5th wheel or trailer. More air is good. Doug