@EatenByLimestone nailed this one.
As
@Circus said, improvements to existing insulation can have lousy payback. If you are losing a lot of heat to an apparently insulated attic, its not 'convection' in the FG (which is overblown as a problem) its good old fashioned conditioned air leaking into the attic through the insulation.
In my case, I had an older house with ~14" total of FG (old, blown between joists) AND crosswise batts on top. I thought I was covered. And then I noticed that my attic was always as warm as the house, and the snow melted off the roof darn fast. When I airsealed my attic floor (over a few weekends, over a couple years) I dropped my heating bill 30%, without adding ANY new insulation!
1. Airsealing between the house and attic is the priority. You need an air (not vapor) barrier which will be your plaster...this is a good air barrier except where it meets the framing, and small penetration like light fixtures....
You need to take up the T&G decking (hard) and dig under the FG (easy) in locations around the perimeter of the house and over interior walls. You want to see if there is a wide open path (or a long narrow crack, or holes) between the framing below and the attic air space. If there is, you need to seal all that....
Open stud bays, you can stuff 16" hunks of (new) FG batts into kitchen garbage bags, fold them double and stuff them in. The FG holds it in, the bag blocks air motion. If the house is framed balloon style, the whole perimeter might need this treatment.
Interior walls often have a top plate, and the plaster might not be done up to the top plate, creating an air gap the length of the wall, often on both sides. You can seal this with caulk or foam. If you need to tackle a 100' of such a crack or more, buy a powered caulking gun and the big pro tubes, or a foam gun.
You might have a plumbing chase, a couple square feet in cross section tying your house into the attic, with a little FG batt or sheet good sitting on top. That you just cut some rigid foam sheet like XPS or polyiso to size, and caulk/foam in place.
And so on. Google is your friend.
A company would do this for you for a few thousand $, less if you removed the decking first. Payback would be good, and they would estimate it for you.
2. When the above is done, and you want to add insulation, you will have to reduce the area that is decked out (which I assume is for storage). In the decked smaller area, I shoveled the loose FG into the larger area and spread it out, then stuffed new FG batts into the joist space. I bought thicker batts and compressed them slightly into the space. They cost the same per sq ft, will have slightly higher R-value, and will take longer to collapse vertically due to age. And there is no convection in dense batts. You can use a different insulation here, but loose cellulose would be a poor choice, since it will settle under the boards.
In the larger, undecked area, I had a company just blow cellulose over the top to a total depth of 14" over the joist tops. They made a little foamboard dam around my decked area, and blew it in in no time. This would DIYable if you like...cellulose blow is easy.
Now all of my attic floor is airsealed and most of it is >R-50, and a small decked portion is ~R-20.
So, if you are uncertain, you can
1. look at snow on your roof
2. put a wireless thermometer in your attic, and measure overnight/predawn temps veruss outside temps.
3. buy a cheap IR thermometer, and scan it around your plank attic floor and look for hotspots.
If the snow melts fast, you have giant icicles, your attic runs 10-20°F warmer than outdoors, you can see warmspots over interior or perimeter walls, then the above plans will save you a bundle on your heating bill and will greatly increase house comfort year round and reduce draftiness.
