I would like to stabilize the temperature in our nursery. Currently we need to overheat/cool the rest of the house (using central forced-air heating and cooling) to maintain our desired temperature in a single room. The room is on a southwest-facing corner on the top floor and has two large windows on the exterior walls. We are located in a very temporate region, where the highs in the summer are rarely above 85f and the overnight lows in the winter almost never get below 40, and we do not have much humidity in the summer (Zone 3, near the coast).
Currently the windows are aluminum double pane, but the seals on the panes have "blown out" and air can move between them. Otherwise they are in good shape and there are no obvious cracks or drafts. There is no insulation in the walls, and there is 4" of paper-backed fiberglass batt with some gaps in the attic above. The building is modern construction (1994) but has very little insulation due to our locally moderate climate.
My first step will be to rearrange the attic insulation to eliminate the gaps and caulk/foam any gaps around the ceiling fixture box.
My question is, after that, where will I get the most bang for my buck? Should I install new windows, blow cellulose insulation into the exterior walls, or add another layer of batting in the attic? Will any one of these help without doing the others? Am I wasting time trying to improve one room without looking at the rest of the house?
I also have access to a thermal imaging camera if that will help make this decision.
I found a few related questions, but none of them directly address my situation (single room, willing to do moderate improvement projects.)