Hi. Recently I had two double hung windows replaced with a shallow box out picture window with a 14″ deep x 18″ tall seat under. The way they framed the seat (odd to me) was to attach a 2×10 box 76″x 14″ (w 16″ o/c joists) and then frame another 2×6 box strapped on top of this. On top of the box sides they build standard 2×4  wall framing  so R15 could be installed there (wall height is about 64″). The boxed walls were strapped to roof rafters so the top of the box is/was open to the attic. The box width was built to be the exact picture window width plus 2×4 side walls and the front was clad/sheathed with 3/4″ OSB (most of which, save the 18″x76″ below the window was cut out.) Sorry I know I’m not describing this well. The point is after day one of their framing, I caulked every seam and joint I could find. Next day they insulated the 14″ side stud walls, put R38 in the seat (like you would expect in a floor) and, upon my request, (re) insulted the (now cut down) 18″ knee wall/cripple stud cavities below the seat. Okay so then I had a framed box seat window, insulted (in a way typical of contractors). Next the they built a cabinet-grade box for the finish detail. It looks great. However, I am concerned about the  1/2″ gap between the insulation on the top and bottom and the finish plywood and insulation. On the sides there is no way the insulation is up against the inner side of the plywood, but I cannot access it.  The very, very long winded explanation finally leads to the question: Should I try to treat this like a giant extension jam and get window expansion foam into these gaps down the sides and across the top or am I confused about how all this works? I don’t have trim on yet and want to address all I can before that happens (oh and, if it helps, when I gutted the house in 2017 I air sealed everything. Since then I’ve been well below the energy usage of the most efficient homes. Not claiming it’s all perfect, but providing this info since it’s relevant, IMO, to understanding everything I’ve done to air seal and insulate up to this point.) Appreciate thoughts and guidance from experts on this site.