what would be advice for soundproofing material for my mechanical room where a new hrv hepa is being installed? the hrv is a lifebreath 300dcs, 220 cfm at 0.5 wg. the hepa is a lifebreath tfp3000hepa, 150 cfm.

this room will be just off my home theatre and i don't want to hear any mechanical noise. i'm sealing all holes, installing a good weatherproofed door and pretty much paying attention to good soundproofing practice. we didn't have the room for a staggered stud wall. the mechanical room has 3.5" wood stud walls with 0.5" sheetrock on each side. my thought is to line the wall cavity with owens corning selectsound acoustic blankets (black rockwool). but this stuff is pricey. a case of eight 4x8' x 1" sheets is $400 ! any thoughts on something more reasonable?