Insulating the loft rafters
You need to insulate the rafters only if you’re converting your loft into a habitable room. You can use most of the materials that you use on the loft floor but the easiest to use are the sheet materials, which the roofer can secure across the rafters and the blanket materials, which can be pushed between the rafters and held in place with timber
Insulate the loft at ceiling level or Insulate the loft at rafter level
Insulating pipework in the loft
If the pipes lie within your loose-fill or under your blanket insulation there’s no need to lag them separately, but if they’re positioned above you’ll need to wrap them with pieces of blanket insulation, ready-made mineral fibre rolls, or proprietary pre-formed pipe insulation. To prevent electric cables overheating you shouldn’t cover them with insulation. Attach them to the side of a joist or, if you’re using blanket material, lay them on top. If your wiring is the old rubber-insulated sort you’d be wise to replace it. Another point to watch now that the loft will be much colder is the greater risk of condensation in the loft space.
This can be a serious problem, which can rot the roof timbers and soak the insulation, making it useless. The way to avoid this is to ensure there’s sufficient ventilation in the loft space by leaving gaps at the eaves equivalent to 10mm all the way round. Don’t fill the gaps with insulation. At the same time, make sure that the gaps around pipes and the loft hatch are well sealed to keep moisture out.