I have many similar jobs with substantial floor decking, upon strong supports, found by picking blog Label: Strong Attic Floors , here, or at the right. The unique importance of this job is use of attic floor supports to keep attic weight from loading very-long 2x4 roof joists. Second floor walls do not directly support the roof, here, and fairly often. Add label: Roof Strength for this post. Try that to find more-complete discussion.
This photo is from a 1:120 scale drawing, structures in overlay upon a satellite photo, drawn in Adobe FrameMaker.
I think that bonding the composite beam top elements to the roof joists, reduces propensity for bowing of roof joists. Bowing may happen when the cross members can shrink in twist action.
With very-extensive support framing, it is easy to place flooring that comes closer than ever before, to a complete hard covering of insulation. The motives of strength and the protection of insulation integrity and value, are of equal importance.
Plywood is 1/2" CDX supported 16" on-center. With composite beam construction, it is readily set flat, with no panel edges to catch and trip upon. Plywood is screwed to upright 2x4 beam top elements called nailers. Nailers are carefully aligned and straightened using a good six-foot level. Bottom elements are often wildly twisted, and a surprisingly large range of revealed nailer is seen above the consistently-ripped plywood webs. Web rips here are 9" wide. Decking is also 1/2" CDX, pieces reassembled why-not, mid-ripped 24" x 96". I use lots of plywood, in creative ways.
Besides being a foundation for protective covering, the beams serve as thermal breaks, even where batts are not crossed. Conduction of energy is with high resistance transfer from 2x framing, to the thin plywood webs. Gaps behind the webs, between lower and upper 2x elements, are stuffed with insulation. Here insulation is in three layers of unfaced batts: upper and lower R19, and wider R11 batts filling 2x spaces, in between.
The total of insulation everywhere is R49, in a depth 9.25" where compressed under decking. I think the air barrier and the added boundaries of conductive layers is good compensation for the small batt compression. This attic is improved from R4, to better than R38. Always add as much insulation as space allows.
Batts wider than spaces in deck framing are thinned by stored energy along edges. In-between there is billowing that will be leveled, pressurized and stilled of air circulation, by decking plywood. Wall insulation done right, is "bottled" this way. Why wouldn't you do the same in an attic floor?
None of this is for storage, though the space is now usable.
I would not go further in this home, to apply hard covering along the difficult edges. I would like to cover the periphery instead, with heavy cotton throws. Those throws would collect dust carried up into the attic through the heat engine of roof/ ceiling slopes. The throws could be laundered in periodic attic maintenance that includes vacuuming of roof vent screens, reducing need of a respirator in-attic. Where I will comment on thought of hard-covering skylight shaft insulation, I will point out that air-barrier fabric will be a preferred solution most of the time in existing-home retrofits. What will it be? It must not decay. It must be washable.