I'd insulate, cost so ittle. I've used a aircon product "Duct board" its a foil covered f/g;ass insulatioon board, just cut it with a razor knife. Great for sound as well , used it on my engine rooms over the years. You can just tape the seam after.
Master Flow 48 in. x 120 in. Duct Board - R-6-DBOARD1 - The Home Depot