Thoughts on radiant barriers vs. just maxing out the blown-in?

In my 1970s split-level, I spent weeks using Great Stuff Pro to seal every single wire penetration and plumbing stack I could find. Once I finished the air sealing, I acutally decided to pull back on the radiant barrier idea. I found that with 18 inches of Owens Corning Atticat blown-in, the heat transfer from the roof deck became almost irrelevant. The bulk insulation does the heavy lifting for the R-50 target you're aiming for.

My main concern with the foil along the rafters is the potential for condensation during the winter months. If you don't leave a clear 2-inch gap between the barrier and the roof sheathing, you risk trapping moisture that should be escaping thru your ridge vent. In my climate, I saw better results just making sure my baffles were clear and adding an extra bag of loose fill. I’d suggest putting that energy into making sure your soffit vents aren't blocked by the new fiberglass instead of fighting with the foil rolls. It's a lot of extra sweat for somthing that loses its effectiveness once dust settles on it anyway.

I have to disagree with the idea that bulk insulation makes the radiant barrier irrelevant. In my experience with a brick ranch, the radiant barrier is what protects your ductwork. If your A/C unit or any of your flex ducts are sitting in that attic, they are sitting in a 140-degree enviroment without that foil, regardless of how much fiberglass is on the floor. I used the perforated AtticFoil brand and stapled it to the bottom of my rafters, leaving a 6-inch gap at the ridge and at the soffits. Even after blowing in 15 inches of GreenFiber cellulose, the radiant barrier was the only thing that stopped the 'radiant soak' that happens to the ceiling joists themselves. Without it, the wood stays hot long after the sun goes down, wich keeps your house warmer into the night. It’s a lot of crawling around and sweating, but if you're alredy up there with the spray foam, do it now. It's much harder to go back and add it once you have two feet of loose fill making a mess of everthing.

Whatever you decide on the foil, make sure you don't skip the attic hatch. I spent days sealing every wire hole I coud find, but my IR camera still showed a massive heat leak at the fold-down stairs. I ended up building a box out of R-10 rigid foam board and weatherstripping the edges. It made a way bigger difference in my bedroom temp than the last few inches of fiberglass did. Also, if you're still using those individual orange cans, grab a Pro gun. It’s way easier to control the bead and you won’t waste half a can when you stop to move a joist.

Make sure you are actually hitting the longitudinal top plate intersections where the ceiling drywall meets the wall framing, as these linear bypasses typically account for a much higher cumulative CFM leakage than those individual wire penetrations you are currently sealing with the polyurethane. If you do go through with the rafter-mounted foil, you really need to maintain a strictly consistent three-inch offset from the roof deck to allow for the convective cooling required to mitigate shingle temperature elevation; otherwise, the reduction in long-wave infrared emissivity will be completely negated by conductive heat transfer through the inadequately vented air gap.

Quick update—I ended up skipping the radiant barrier and focused on hitting R-60 with blown-in instead. That tip about the top plate intersections was huge; I found way more leaks there than I expected. I also built a foam box for the attic hatch like suggested, and the temprature difference is already noticeable. Thanks for the help everyone!