I'm planning on reinsulating my garage which shares a common wall with my kitchen. The garage is mostly unfinished with insulation only on the one common wall. The wall currently has some sort of MDF as the "wall" with R-11 fiberglass underneath. Living in the northeast (Vermont) this seems insufficient and I can definitely feel the lack of insulation in the part of the kitchen with the R11.

Here's the question: The common wall is constructed of 2x4's. I'd like to remove the R11, add Great Stuff around all the cracks/joints and then replace with R15 AND a layer of 1 or 2" rigid extruded/expanded polystyrene to bump the R value to 20-25. Is this doable? Can I put the rigid insulation board over fiberglass?

Any thoughts from insulating experts would be greatly appreciated!