Hello, I am designing an interior of a RV to be built inside of a Promaster cargo van. The living space will be 12' long x 6' wide x 6' tall. In winter the van will be heated with a heater that uses gasoline for energy source. In summer, a 24VDC air conditioner will use limited battery power when 120V AC is not available. Not planning to do much winter camping and then there is climate change, hence the objective to optimize the insulation design for minimum cooling load, even if it results in sub-optimal heating load. The company that will build out the interior prefers to use sheep wool for insulation without a vapor barrier. I am focusing only on the ceiling area for a better approach that will minimize the cooling load on hot sunny days. Please refer to the rendering of the metal ceiling in the attached image. The metal ceiling has stiffener ribs. 1x2 wood furring strips can be screwed to both sides of these ribs to create a framing structure and also a 1-1/2" deep insulation space between the ribs. I was thinking of stapling a reflective radiant barrier to the framing such that an uniform 1/2" air gap would be created between the metal ceiling and the radiant barrier. The rest of the insulation space below the radiant barrier would be filled with sheep wool or batt insulation and a laminate surface will form a finished ceiling. The radiant barrier could be thin foil or foil faced insulation whichever is easier to install & be more effective. The ceiling is not flat and the 1/2" air gap will be 3/4" for about half the area. In the image, the grey color is for metal roof & ribs, yellow for 1x2 furring, light purple is 1/2"x1/2" thin plastic corner protector stapled to the furring, the magenta is the radiant barrier and the green is final finished sheathing. The radiant barrier can also span the two furring strips on either side of the metal rib or that space can be filled with insulation. Please let me have your advice on whether a radiant barrier will make a big enough difference to make the extra effort worthwhile as opposed to filling the entire 1-1/2" deep area with insulation. Thank you very much for your time.