Hi all,
I've been slowly insulating my new house over the past 6-7 weeks, deciding to go the whole hog and fill every wall cavity, internal and external. The main reason for doing the internal walls was the cut down on noise transmission between rooms, and to give the house a more "solid" feel -- filled walls act as "bass traps" and give the impression of being more massive than they are.
For the internal walls I've used Bradford Soundscreen New Generation R2.5 -- the "new generation" bit refers to their move from a rockwool product (their Victorian factory closed down) to a higher density glass fibre batt. The Soundscreen batts are reportedly 24-26kg/m3, whereas the regular yellow batts are 6-7kg/m3 (this information comes from an email conversation I had with one of their service people). Frankly, I'm dubious: there are 6 Soundscreen batts in a pack, and 8 in a regular yellow pack... but the yellow pack is noticeably heavier. If the Soundscreen batts were four times as dense, their pack should be considerably heavier. Dubious indeed...
Cost wise, I calculated that the Soundscreen batts are around 62% more expensive than the standard yellow Bradford batts (this is using m3 coverage costs, and my trade pricing -- thank you owner/builder benefits!). Having 6 batts in a Soundscreen pack, versus 8 in a regular R2.5 pack, means they take up more space for delivery/cartage too.
Now, onto the effectiveness. Initially I was extremely sceptical about the glass fibre, having read that rockwool is vastly superior for acoustic properties. But... I was insulating a built-in robe in the upstairs bedroom, and as I placed the last piece the radio went off. It was only when I rounded the corner to the doorway that I realised the batts had blocked the sound completely... awesome.
Which leads me to the key for this stuff: you have to fill every gap, or sound will still get through and propagate. Think of waves on the water, going through a gap -- they hit the gap, then spread out on the other side... radiating much like a speaker cone. If you've got gaps, you'll still get sound transfer.
So, is Soundscreen worth it? For now I say yes, but can't say for sure until I actually move into the house (around Sept 2013 I reckon). However, if I wasn't doing the insulating and wasn't super-pedantic, I'd call it a waste of money. Unless you fill Every Single Gap, it's not going to be effective.
Hope that helps!