Ceiling insulation guide: when you need it and how much it costs

Ceiling insulation vs loft insulation vs roof insulation

These terms are often confused:

• Loft floor insulation: Insulation on the floor of the loft (the ceiling of the rooms below). Keeps heat in the rooms. This is what most UK homes have.
• Roof insulation: Insulation on the underside of the roof (between or under the rafters). Keeps heat in the loft space. Use this if the loft is living space.
• Ceiling insulation: Usually means loft floor insulation. Sometimes means insulating a flat ceiling from below (cathedral ceiling with no loft above).

When to insulate the ceiling from below

You only insulate the ceiling from below (rather than the loft floor from above) if:

• You have a cathedral ceiling or vaulted ceiling with no loft space above.
• The loft is inaccessible (no hatch, too low to work in).

This is more expensive (£30 to £50 per m² vs £10 to £20 per m² for loft floor insulation) because you have to fit insulation boards to the underside of the ceiling joists, then plasterboard over them. You lose ceiling height.

Materials and cost

Loft floor insulation (the usual approach):

• 270mm mineral wool batts laid between and over the joists.
• Cost: £300 to £500 for a typical house. DIY materials: £150 to £250.
• Saves: £200 to £300 per year.¹

Ceiling insulation from below (rare):

• 100mm to 140mm PIR boards fixed to the underside of the ceiling joists, then plasterboard over.
• Cost: £30 to £50 per m² installed.
• You lose 120mm to 150mm of ceiling height.

Which should you do?

• If you have a loft: Insulate the loft floor. Cheaper, easier, better payback.
• If the loft is living space: Insulate the roof (between the rafters), not the ceiling.
• If you have a cathedral ceiling: Insulate the ceiling from below (or from the outside if you're re-roofing).

Related reading

• Is roof insulation worth it?
• Insulation board types explained