Aaron Fude wrote:

My external 2x4 walls (Philly, PA) are currently open (from the
inside) and I am about to put R13 insulation in them. Are there some
additiona quick-an-easy things that I can do at this point to improve
insulation. I'm thinking some reflective surfaces or something like that.

That would be hard to do without increasing the wall thickness.

An airtight taped plastic film vapor barrier would help.

Nick