Attic Insulation + Plywood (span chart anyone?)


LB= Load-bearing
First question: Can anyone tell me what weight my attic floor can bear? 2x6's, 16" apart, and the attic is about 28' wide where each joist is about 14' long extending from the exterior LB-walls to where they slightly overlap over the center interior LB-wall (the joist butt-ends do not meet, the joists are 1.5" off-center from each other). The entire attic is about 48' long, just joists & collar-beams with a bunch of flat 2x4's (I think) layed end-to-end on top of the joists from one end of the attic to about the other end.
Anyone know the lb./sq. feet for up there?
Second question: -I currently have faced-fiberglass insulation batts up there between the joists, peeled it up and it says "Johns Manville - Super-thick 6 1/2" R- 22c.24F.19l" on it, why the three numbers? Which is it?
Third question: -I would like to add unfaced insulation to bring it as close to R-48 as possible, should I roll the new stuff out perpendicular to the existing stuff to cover the joists to prevent radiant heat loss?
Fourth question: -For the places where I'd like to put down the plywood, covering about 400 sq. ft. of the total 1400 sq. ft. of my attic circling around the attic stairs opening (all the headers/joists are doubled-up for the stairs btw), should I not bother with adding any extra insulation because it would have to be compressed into 6" of space underneath the plywood, thus negating a lot of the R-value, right? Should I just leave the existing fiberglass there and not add any before putting down the plywood?
Fifth question: Cost: Any ideas for what it will cost for about 1400 sq. feet of R-30 insulation in NY? Home Depot or Lowes the best for large quantities? Somewhere else better?
Thanks for the help, wasn't sure about newsgroup etiquette and whether I should have broken this up to 5 diff. posts for each question?
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Span tables at: http://www.awc.org/pdf/STJR_2005.pdf
You've got 2x6s spanning 14', 16" o.c., so the answer is, "not much weight". For L/240 (about 0.7" deflection) with 10PSF live load, and 5 PSF dead load, you can span 14'1"
That's just about enough (10psf) to support you and a toolbox.

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