I am doing a home remodeling project. I am finishing some attic space into a small bedroom. The partition walls are 2x4 construction with standard R-13 faced insulation. I am wanting to add some extra insulation between the heated and unheated areas of the attic. I am thinking of adding some rigid foam board insulation. What do you think? Where would it be applied, to the heated or unheated side of the wall?

The heated side would be th best for the foam board then the glass insul with the vapor barrier against the foam board. The foam board is a partial vapor barrier to begin with. Its so tempting to just put the foam board over the exposed glass but that will trap moisture.

Didn't crunch the numbers here and what thickness the foam board you are thinking, but it may be cheaper just to replace the (R 13) glass and go ( R 19 or R 30 ). If the back of the wall cavity is exposed the new fiberglass insul can hang out past the 2/4 wall cavity and its not about looks here if that area is non usable so to speak.

To install the foam board first, removal of the glass insul needs done. But you loose the flap to reuse it. See the problem here.

Vapor barriers need to always be facing the heated side.

Could you recycle the glass there and remove the paper, add that to the attic ceiling unfaced and then go get faced (R 19 ) or better and staple that back into that 2x4 wall cavity with paper facing the heated side. This way you gain more insul in attic ceiling/floor and not waste what's there already.

Your R 13 fits nicely in that 2x4 cavity already and I hope it has a vapor barrier facing the room. I would add the rigid foam to the back side of the studs but its imprtant to seal the seams with foam sealing tape and some expanding foam sealant at the top and bottom edge to stop cold air from creaping in. If you were just starting a new attic room from scratch things might be done differently, here I am trying to work with what you got. Any thoughts on how you will insulate the ceiling?

The ceiling is 2 x 6 framing. I was going to put R19 Kraft faced insulation in the framing and lay additional unfaced bats perpendicular to and on top of the framing.

The ceiling is 2 x 6 framing. I was going to put R19 Kraft faced insulation in the framing and lay additional unfaced bats perpendicular to and on top of the framing.

Perfect... ( faced on the heat side )

What or how is the venting going to change if any, above this area in space?