When to insulate the roof rather than the loft floor
- The loft is, or will be, a room.
- Tanks and pipes freeze in a cold loft.
- There is no loft floor — vaulted ceilings, flat roofs, a bungalow with rooms in the roof.
- You are re-roofing and can build a warm roof from outside.
The routes
| Route | When | Materials | Cost band |
|---|---|---|---|
| Between and under rafters, from inside | Tiles staying on | PIR between, multifoil or PIR under | £45 – £110 per m² |
| Warm roof, from outside | Re-roofing | PIR over rafters, breathable membrane | Priced within re-roof |
| Room-in-roof | Existing loft room | Sloping ceilings, dwarf walls, flat ceiling | £2,500 – £7,000 |
| Flat roof | Extensions, dormers | PIR warm deck when re-covering | Priced within re-cover |
The ventilation rule
A pitched roof must either ventilate from eaves to ridge above the insulation, or have a breathable membrane with a designed gap. Every roof insulation failure we hear about — wet rafters, black felt, a damp smell — is a blocked ventilation path. Insist on seeing it in the quote.
Warm loft, loft room, or re-roofing?
Check my homeSix questions and we show the likely route, materials and cost before any contact details.
Roof guides
- Foil and multifoil roof insulation
- Rafter insulation
- Room-in-roof insulation
- Loft conversion insulation
Questions homeowners ask
Is roof insulation better than loft insulation?
Neither is better; they do different things. For most homes the loft floor is the right first move. Insulate the roof when the loft needs to be warm.
Can I have both loft floor and roof insulation?
You can, but you do not need both. Insulating the slope makes the floor layer largely redundant.
Sources and when they were last checked
- GOV.UK — Room in roof insulation best practice — checked 2 September 2026
- Energy Saving Trust — Roof and loft insulation — checked 2 September 2026