Gosh, I couldn't say...but, if you're asking because you're trying to choose which one to use I have to say DONT use polystyrene..not in the loft anyway! In the loft, the beads will attach themselves to your wiring, over time, and eat their way through them. And are a fire hazard. BUT, if it's for in your walls, I believe polybeads have a higher u-value thus insulate better. Rockwool is a very good product though, been around for decades, is inert and also used in hospitals (internal walls) to stop the spread of fire.