I'm enlarging an bathroom in a 1940's add on. The exterior wall of the bath is standard 2 x4 with wood siding and fairly recent blown in cellulose insulation. I'm removing an exterior door in that wall and framing it in with 2 x6. I intend to match up the facing of the wall by adding 2 x2's to the old wall then dry walling the whole thing. There will be no plumbing in that wall. I probably will use fiberglass batts in the new section but I was wondering if I should fill the new smaller voids with something, perhaps rigid foam or such. Any thoughts?