"My only question, for now, is: can I just add the blue polystyrene on top of the existing fiberglass insulation?" ----- Yes.
I would air seal the attic first
as you said: http://www.rd.com/how-to-seal-attic-...icle18158.html
This will stop the stack effect of heated air escaping there: http://www.wag-aic.org/1999/WAG_99_baker.pdf
The attic is like a wall on its side, attic side equals outside:
“Foam board insulation is commonly placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulk. However, this practice may worry some builders in cold climates since the foam board may act as a second vapor diffusion retarder. Studies have shown, however, that condensation rarely occurs
in these areas unless something else is seriously wrong with the wall assembly (i.e., massive uncontrolled air leakage
into the walls from the house). If the assembly is constructed correctly, the inside surface of the foam board stays warm enough
to keep water vapor in its gaseous state long enough for it to escape.” Bolding is mine, from: http://www.energysavers.gov/your_hom.../mytopic=11620
Board over the loose fill is similar to this:
“One of the final experiments included the additional layer of a 1-in. unfaced fiber-glass insulation 'blanket' (density of 1 lb/ft3) over the loose-fill insulation. Thermal resistance increased 12 to 16%, with a heat flow of 10-13% lower than the results with the pillow in place. The blanket was actually or slightly more effective than the pillow. This is believed to be due to the blanket having a slightly larger thermal resistance and/or air flow resistance due to its higher density.
(see Figure 5
.)” Bolding is mine, from: http://www.homeenergy.org/archive/he...92/920510.html
R-38 required in attic for NY. http://www.ornl.gov/cgi-bin/cgiwrap?...le/ins_fact.pl
The problem is installing batts perfectly. Keep them level with the ceiling framing to install the next rows perpendicular to the first. No air tunnels above or next to the tops of the joists to negate the batts from wind-washing: “One of the heat exchange mechanisms that cavity insulations are intended to control is convection. When convective looping is able to occur in or around the insulation, its function is subverted. Also, air moving through insulation because of wind or other pressures can also subvert its performance. Therefore, low density fibrous insulations should be covered by air barriers in areas subject to air movement.
Rolled Batt Insulation: Batt insulation can be a very cost effective option where the framing cavities are of uniform size and where the width and depth matches that of the rolled batt insulation. With rolled batt insulation care must be taken to cut the insulation to fit snugly in cavities and around all obstructions without compression. When batts are cut too short, too narrow, or too wide around obstruction, gaps and the associated problems result. Also when batts have been cut too big for the cavity or have not been cut to adequately accommodate obstructions there will tend to be compression and gaps of incomplete thickness (“rolled shoulders” at framing or wrinkles in the field of the batt). Compressing batt insulation also reduces its thermal resistance (effectively, its R-value). Batts should also be fluffed to full thickness so that they will be in contact with the cavity enclosure on all six sides and not leave gaps for convective looping. Higher density rolled batts are more effective at inhibiting convective looping within the insulation.” From: http://www.buildingscience.com/docum...archterm=insul
Use foam boards at exterior wall line: http://www.homeenergy.org/archive/he...96/961110.html
Be safe, Gary