I'm doing a similar project. As others have noted, you may only have 4 or 6" of insulation under your plywood. That's in the R14-R20 range if fiberglass. Not good. My house has mostly blown-in cellulose that's 6-8" deep. It's crap, especially in the summer. And I've found several large uninsulated areas and gaps allowing attic air into the space between floor joists (2-story house).
I'm putting in radiant barrier stapled to the bottom side of the 2x6 rafters leaving an open channel between rafters from perforated soffit to ridge vent. I'm using a micro-perforated double-sided product from
here. This stuff is very durable and unlike the cheap foil from the big box stores. It's a heavy plastic core with metalized film on both sides (and small perforations to allow moisture to move). You cannot tear it.
Last spring I did about 500 sq ft in a south-facing attic off of my mancave (FROG) and over our dining/kitchen area. On frosty mornings the difference is obvious looking at the shingles. Summer attic temps in that area are about 10F lower than before. I've got an indoor/outdoor thermometer setup in that attic to monitor.
I'm also rolling out unfaced R30 over the existing blown-in stuff to give me something in the R40-R50 range. As someone else mentioned you need to put a barrier at the bottom of your trusses where they meet the top of the wall so that insulation doesn't block your soffit to ridge flow. It's a job, but will pay back every month if done well. Good luck!