Insulation for floors, walls and roofs is usually rigid PIR (polyisocyanurate) lightweight foam, mineral wool, sheep wool, cellulose (recycled paper) , wood fibre (soft-board). Usually, the lower the 'U' value, the better the level of insulation and energy performance that may be expected.
Rigid PIR (polyisocyanurate) foam insulation has a better thermal efficiency per mm than many other insulation materials. It also has an A+ rating when compared to the BRE Green Guide, a low environmental impact, and a Class O fire performance. It is ideal for creating (comparatively) thin floors, walls and roofs.
Some materials, like Rockwool and sheep wool, need to be twice as thick as foams like PIR, with PIR actually providing the lowest cost for equivalent insulation.