Abstract: A method of providing insulation between joists 31, 32 during building comprises unrolling a batt of mineral fibre which has, at least at its longitudinal edges, top 35, 45 and bottom 33, 43 layers. The bottom layer 33, 43 can be compressed to fit between joists, whilst the top layer 35, 45 extends across the joists, which adjacent strips 35, 45 abutting one another, to minimize cold bridging. The batt can be formed of two layers 33 and 35, 43 and 45 which may be manufactured by slitting a single web into two layers followed by winding the two layers together to form a roll.