well, the lower the thermal conductivity the better....
theres a product from moy insulation called metac. It claims a TC value of 0.034 which is very good for a quilted material. [broken link removed]
Rockwool have a product called rockwool flexi which has a TC value of 0.037.. .this has the added advantage of being semi rigid and can be cut to fit between roof timbers. [broken link removed]
the typical roofing insulation is products such as moy isover plus. this has a TC value of 0.044. [broken link removed]
when comparing TC values you can see that the metac product would perform approx 30% better than the isover product.