How should a loft conversion be insulated in London?

Loft conversion insulation in London: warm roof, cold roof, and dormer insulation explained

Insulation is one of the most important performance decisions in a London loft conversion. Done correctly, a well-insulated loft room is comfortable year-round, has lower heating costs than any other room in the house (because heat rises), and will not suffer from condensation or overheating problems. Done incorrectly, a poorly insulated loft room is cold in winter, overheats in summer, and is vulnerable to condensation — all common complaints about poorly specified London loft conversions.

Building Regulations requirements (Part L)

Approved Document L (Conservation of Fuel and Power) sets maximum U-values for the insulated elements of a new loft conversion. The target U-values are:

• Roof slope and dormer flat roof: 0.18 W/m2K or better
• Dormer walls (cheeks): 0.18 W/m2K or better
• Floor (over unheated space): 0.18 W/m2K or better (not typically applicable in a loft conversion where the floor is over habitable space below)

These are relatively demanding values that require careful detailing, particularly on existing rafter sections where the depth available for insulation is limited.

The two main insulation approaches

1. Warm roof (insulation between and below rafters)

The warm roof approach places insulation:

• Between the rafters (in the rafter void)
• Below the rafters (on a continuous layer fixed to the underside of the rafters, then boarding over)

This creates a continuous warm insulated layer, eliminates the cold roof void above the insulation, and removes the risk of interstitial condensation in the roof structure.

This is the preferred approach for London loft conversions and the approach Building Control officers expect to see on Full Plans submissions.

Typical warm roof specification for a London loft conversion:

For a standard 100mm rafter (100x50mm sawn timber rafter at 400mm centres):

• 100mm PIR (polyisocyanurate rigid foam board, e.g., Kingspan K7 or Recticel Eurowall) between rafters, tight fitted
• 50mm PIR board continuous layer below rafters (fixed with screws through counterbattens)
• Total insulation depth: 150mm
• Achieved U-value: approximately 0.15–0.17 W/m2K (exceeds Part L minimum)
• Internal finish: 12.5mm plasterboard fixed to counterbattens below the PIR layer

For a deeper 150mm rafter (where rafters have been sistered or replaced):

• 100mm PIR between rafters + 50mm PIR below = 0.13–0.15 W/m2K

2. Cold roof (insulation at ceiling/floor level only)

The cold roof approach places insulation only at the loft floor level (the new habitable floor) and relies on ventilation of the cold void above to prevent condensation. The roof slope itself is uninsulated.

This approach is not suitable for a habitable loft room — it creates an uninsulated roof slope above the room, which will be cold in winter, prone to condensation on the cold inner face of the roof covering, and will result in a room that is extremely hot in summer (the roof acts as a radiator). Cold roof construction is only suitable for unheated loft storage, not for habitable use.

PIR vs mineral wool for loft conversion insulation

PIR (polyisocyanurate rigid foam boards) — Kingspan, Recticel, Quinn Therm:

• Higher thermal performance per mm (lambda value approximately 0.022–0.023 W/mK)
• Rigid, easy to cut and fit between rafters
• More expensive than mineral wool
• Must be fully encapsulated in plasterboard for fire resistance (PIR is combustible)
• The preferred choice in London loft conversions because the higher performance per mm matters in shallow rafter sections

Mineral wool (glass wool or rock wool) — Knauf, Rockwool, Isover:

• Lower performance per mm (lambda approximately 0.032–0.040 W/mK)
• Requires approximately 50% more depth to achieve the same U-value as PIR
• Suitable between deep rafters (175mm+) where depth is available
• Non-combustible (rock wool in particular has excellent fire resistance)
• Cheaper per m2 but requires more depth, which means more loss of internal height

Spray polyurethane foam (SPF):

• Applied by specialist contractor as a liquid that expands and hardens
• Not recommended for London loft conversions — it bridges the rafter voids and creates a cold roof configuration (no ventilation), and once applied it is very difficult to remove. It also causes problems at point of sale because mortgage lenders and surveyors frequently flag spray foam insulation in roofs as requiring removal before lending.

Dormer insulation

Dormer walls (cheeks) and the dormer flat roof must also meet Part L requirements:

Dormer flat roof (warm flat roof):

• Typically 150–200mm PIR or EPS above the structural deck (above rafters/joists)
• Covered with EPDM or single-ply membrane
• U-value target: 0.18 W/m2K achievable with 120mm PIR (Kingspan Thermaroof TR26 or similar)

Dormer walls (cheeks):

• 100mm PIR between studwork + 40mm PIR internal layer, or
• 150mm mineral wool between studwork + 40mm PIR internal layer
• External cladding (zinc, render board, or timber)
• U-value target: 0.18 W/m2K achievable with above specification

Ventilation and condensation risk

All habitable loft rooms require ventilation to control moisture. Key requirements:

• Background ventilation: trickle vents in all windows (minimum 5000mm2 equivalent area for a bedroom)
• Rapid ventilation: openable window area of minimum 1/20th of floor area
• Vapour control: where mineral wool is used in the warm roof build-up, a vapour control layer (VCL) on the warm side of the insulation is essential to prevent water vapour migrating into the cold zone and condensing

PIR boards, being closed-cell, act as their own vapour retarder when joints are taped with foil tape — but tape must be continuous and carefully applied at rafter intersections.

Typical insulation package cost for a London loft conversion (supply and fix, 2026)

Element	Cost range
Roof slope insulation (PIR warm roof, 40m2 slope area)	£3,500–£6,500
Dormer flat roof insulation (PIR warm flat roof, 15m2)	£1,200–£2,500
Dormer cheek insulation (PIR between studs, 10m2 each side)	£800–£1,800 per cheek
Vapour control layer (mineral wool configuration)	£300–£800
Total insulation package (typical rear dormer)	£6,500–£12,500

Note: costs include labour and materials for supply and fix but exclude plasterboard lining (separate trade).