How can I insulate this junction box?

Question

I have a junction box in my unfinished attic that supports a light fixture in the room below. It doesn't fit exactly to the lath and plaster and I'd like to insulate for sound intrusion (the attic is well insulated so I'm not worried about temperature).

You can buy recessed lighting insulators like the one in the picture, but they're $15 and way too big. These are 2x4 joists and I'd like to put some plywood down so there's not a lot of headroom to work with.

I can spray foam and caulk all of the holes but that seems like a hassle and an opportunity to make a giant mess. Is there a better way?

Answer 1

Fiberglass insulation works well for sound buffering as well as controlling heat loss. It sounds as if you're planning to lay a plywood sub-floor in which case you'll want to insulate between all of the joists for sound mitigation. Plaster and lathe isn't reliable for sound suppression. Better to do it now rather than regret it after you lay the plywood.