The existing joists seem to be at least 2 X 6 lumber. You could sister 2 X 12 lumber next to each joist for the entire span of the garage. You can then use the R-38C EcoTouch® PINK® Fiberglas™ - from Owens Corning. The thinkness is 10.25 inches which would make it flush with the height of the 2 X 12 joist.

Are you planning to add ventilation to the roof? If you don't I would be concerned about moisture accumulating in the garage attic.

Thank you Mike. Yes, I’m using baffles at both ends of the trusses to keep airflow up through the soffit. Trusses are 2x4s, just double checked.

I hadn’t seen the R-38C, looks like a great option. I’m okay to just screw the 2x12s to one side of the 2x4s in that middle section only? Understood theyre not structrual, and only to support the light load above. No concerns about sagging with the weight of the 2x12s?

2x12’s are heavy but what about attaching 2x4’s to the angled struts at the 12 inch height spanning the middle section to support your plywood?

Sounds even easier. Then I can adjust the height based on insulation height. Wont need to hold much weight, just me (200 lbs) and empty boxes

That should work also. You can limit it to the area where you want the storage. Are you thinking of installing a pull down stairway?

Perfect, thank you!

I considered it. But just going to have a pop out section and ill climb up with a ladder

Please do not sister 2x12 against the bottom truss chord. First, that's crazy expensive just to give you height for a platform only in the middle of the truss. Have you tried getting 24' long members, which weigh around 100lbs, and negotiating them into the rafter heel space? Do you realize the dead weight you are adding to the truss bottom chord?

Also, please do not attach anything to the (angled) truss web chords. They certainly cannot have a load on them that will want to rotate them.

Be realistic. Your bottom truss chord cannot take a lot of weight (typical 10psf, but you will have a ceiling + insulation). Your garage is very air leaky because of the overhead garage doors. So, do you think you will notice a comfort difference if a say, 4x8 area above your ceiling does not have R38? What if the entire ceiling is R38, but that small area is R15? And do you have a plan to cover the top of the bottom chord with insulation anyway, if you are just setting batts between the trusses?

If you want to put a platform and have a little more insulation below it, run a 2x perpendicular on top of the truss bottom chords. Space 3 of them at 24"o.c. and use no thicker than 19/32" OSB (is rated to span 40") for the platform. The bit of deflection you feel when standing on it will ensure you do not store anything heavy up there.

Empty boxes won't compress the insulation enough to make a difference. Lay a 4x8 1" foamboard across the insulation. You can collapse the boxes also.