I'm staying with my brother in St Ives, Cambs. He has a flat in a listed building, and is insulating the loft over his bedroom. After a lot of kerfuffle with the planning officers (more to do with staff turnover than anything else), he's finally got permission to go into the loft and insulate it. He's used a recycled plastic fibre, which you use like rockwool insulation.
1) it's very nice to use. In fact, it's like a fleecy blanket, not at all itchy.
2) it's 90% recycled bottles.
The disadvantage is that it's still plastic, and 10% new plastic. But, it keeps the old bottles out of landfill, and out of incinerators, and means that the CO2 from the plastic doesn't get into the atmosphere - at least not for another few decades.
I'd be interested to know what the real environmental costs are relative to the other options for loft insulation. Rockwool seems pretty benign, as does sheep's wool. But all three have their manufacturing costs, and sheep's wool comes with methane emissions and land use issues.