I just got a used spa and I wanted to make it quieter and retain heat more efficiently. Can anyone recommend soundproofing material and insulation that I should use? Thanks!

I have not done this yet but I have been told that spray foam on the shell (not the plumbing) works good for sound deadening and insulation. Foil sided bubble-rap placed like a blanket over the shell and plumbing helps to insulate and reflect some of the heat back to the tub. Lastly you can place foam insulation board on the inside of the spa panels and under the spa too. Layering of the insulation is going to give the best results, kind of like layers of clothing.

Jeff