Just moved into a 20 year old house on the NC shore. It has about 10” of fiberglass batts in the attic floor. There is plywood on top of the insulation over about half of the attic. This is for a walking area, storage and because the forced air furnace and all air ducts are in the attic. As such, adding more insulation to the attic floor would not be practical. Do I just add more insulation to the areas where there is no plywood floor and ignore the rest? That does not seem right. Also, the attic gets quite hot during the long summers, at least six months. This is just solar gain through the roof. There is soffit venting all around the perimeter that I verified is not blocked with insulation and ridge ventilation as well as a powered roof vent and a passive gable vent. It gets to 96 degrees nearly every afternoon and hotter if the powered vent is not on.
Would it be better to insulate the under side of the roof? This would hopefully keep it cooler in the attic where all the air conditioning ducts run.
Which is better?

 

People that put plywood down in their attics don't take into consideration the insulation needs. Current standards are R49 for attics. What I would suggest you do is take up the plywood and if you want to continue having walking space/plywood you should add additional joists to the attic that sit perpendicular to the existing ones to create an elevated platform. Then blow in additional insulation in ALL areas to bring it up to R49 (about 16") and then put your plywood down on the new elevated joists.

Your ducts should all be insulated, I hope. They will be partially buried in your new, thicker insulation.

No, don't insulate your rafters... and don't focus on the temperature in the attic. Attics normally get way hotter than that.

 

The main trunks for the ducting are raised above the attic floor. I could remove the plywood from everywhere except above the uninsulated area above the garage. Then add blown to R49. Is there some sort of chart that shows how much I will save in heating and cooling costs by increasing the insulation? I wonder if I will live here long enough to see a payback other than comfort?

I live just north of Myrtle Beach SC about one mile from the coast.

 

You might message Bud9051... he might have some idea... but there are too many variables to say for sure.