The pic in post 43 is what I was referencing earlier when I mentioned a "L" bracket to fasten to the top of the existing ceiling rafters. At 3' OC, I'd put metal on the ceiling to support the weight of the insulation and be done with it. The PVC might tend to sag with a bit of insulation on top of it at 3' OC. I would think he could use 2 L brackets as Dad911 mentions with 2x4's basically splitting the ceiling into thirds and then use the PVC since he'd be at 18" OC. I don't think the pvc would sag at that spacing. You'd have to weigh the time and cost for lumber to split the rafters into smaller spacing versus just using metal and keeping the spacing as is.
In my pic below my truss spacing is 4' OC with a metal ceiling. Also shows how I installed my door track support. I said earlier my door is 12' tall but it's 12' wide x 10 feet tall. You'll also see the jackshaft opener I referenced earlier.
