Insulation Types for Extensions and Loft Conversions

Polyisocyanurate (PIR) rigid insulation — sold under brand names like Kingspan Koolthus, Recticel Eurowall, and Quinn Therm — is the dominant insulation choice for modern extensions. It offers the highest thermal performance per millimetre of any commonly available insulation type.

**How PIR works**: PIR boards consist of a closed-cell foam core bonded to aluminium foil facings. The foil facings provide a vapour control layer and reflective thermal resistance. Thermal conductivity (lambda value): typically 0.022–0.023 W/mK, compared to 0.035–0.044 for mineral wool.

• **Where PIR is used in extensions**:
• •Flat roof — fully supported flat roofs typically use a warm roof construction with 100–150mm PIR boards above the structural deck, tapered where required for drainage falls
• •Pitched roof — PIR cut between and below rafters (hybrid system) to achieve the required U-value within the rafter depth
• •Cavity walls — PIR boards blown into or installed in the cavity during construction
• •Floor — PIR boards below a concrete slab or above an existing floor (with a raised floor system)

**Why PIR is specified for extensions**: Modern extensions must achieve a U-value of 0.18 W/m²K for roofs and 0.28 W/m²K for walls under Part L 2021. In a typical 150mm rafter loft, achieving 0.18 W/m²K with mineral wool alone would require an unachievable depth — PIR achieves this in 100–120mm.

• **PIR costs (supply only, 2025)**:
• •100mm PIR (flat roof / floor): £25–£40 per m²
• •70mm PIR (between rafters): £18–£28 per m²
• •150mm PIR (warm flat roof): £40–£60 per m²

Mineral wool — available as glass wool (Knauf, Isover) and rock wool (Rockwool, Superglass) — is the most widely used insulation type in UK residential construction. It is less thermally efficient than PIR per millimetre but has important advantages: it is vapour-open, non-combustible, and significantly cheaper.

**How mineral wool works**: Mineral wool consists of spun glass or rock fibres. Unlike PIR, it is breathable (vapour-open) — it allows moisture vapour to pass through rather than blocking it. This makes it the preferred choice for breathable wall construction and for areas where damp management matters.

**Thermal performance**: Lambda value typically 0.035–0.044 W/mK — so to achieve the same thermal resistance as 100mm of PIR, mineral wool requires approximately 140–170mm.

• **Where mineral wool is used in extensions**:
• •Cavity walls (partial or full fill) — rock wool slabs or glass wool batts blown or pressed into the cavity during masonry construction
• •Timber frame walls (between studs) — mineral wool batts friction-fitted between 89mm or 140mm studs
• •Between rafters in a loft conversion (first layer, with PIR below for the hybrid system)
• •Suspended timber floors (below floorboards, friction-fitted between joists)

**Non-combustibility advantage**: Rock wool (stone wool) is non-combustible (Euroclass A1 or A2) — making it the preferred insulation for areas requiring fire resistance, and for many flat roof constructions where fire performance is specified.

• **Mineral wool costs (supply only, 2025)**:
• •100mm glass wool batts (between rafters / studs): £6–£10 per m²
• •100mm rock wool cavity slab: £8–£14 per m²
• •150mm glass wool roof batts: £9–£14 per m²

Spray polyurethane foam — applied as a two-component liquid that expands on contact — is the third major insulation type. It can be used in new-build extensions but is most commonly discussed in the context of retrofitting insulation into existing roofs or walls.

**How spray foam works**: PUR/SPF is sprayed directly onto the surface to be insulated, expanding to fill cavities, gaps, and irregular surfaces. Closed-cell PUR achieves thermal conductivity of approximately 0.024 W/mK — similar to PIR.

• **Two types**:
• •**Open-cell PUR**: lower density, vapour-permeable, lower thermal performance (~0.038 W/mK), cheaper, softer
• •**Closed-cell PUR**: higher density, vapour barrier, high thermal performance (~0.024 W/mK), rigid and structural, used in new-build applications

• **Where spray foam is appropriate**:
• •Infilling irregular cavities or complex geometry where rigid boards are impractical
• •Insulating the underside of a roof deck (warm roof undersides in industrial/commercial settings)
• •Airtightness applications where the foam seals gaps as well as insulating

**The mortgage problem with spray foam in existing lofts**: Spray foam applied to the underside of existing roof tiles has caused significant problems in the UK housing market. Mortgage lenders frequently refuse lending on properties with spray foam in the roof because: — It prevents inspection of the roof structure — It can cause condensation and timber decay by trapping moisture — Removal is expensive and often damages tiles If you are considering spray foam in a loft, confirm with your mortgage lender before proceeding. For new extensions and loft conversions, PIR or mineral wool are the standard and lender-accepted choices.

• **Spray foam PUR costs (installed, 2025)**:
• •Open-cell PUR (100mm thickness): £15–£25 per m²
• •Closed-cell PUR (100mm thickness): £30–£50 per m²

**Flat roof extension (warm roof)**: PIR rigid boards above deck — 100–150mm tapered or flat, depending on required U-value and drainage design. The standard for London single-storey rear extensions.

**Pitched roof extension or loft conversion**: Hybrid system — mineral wool between rafters (full rafter depth), PIR rigid boards below rafters (cut and fit, 50–70mm), plasterboard finish. Achieves Part L compliance within the rafter depth without exposed PIR.

**Extension cavity walls (standard 300mm cavity construction)**: Full-fill cavity insulation — rock wool slabs (non-combustible, preferred by many BCOs) or PIR cavity boards pushed into the cavity as the wall is built. Achieves 0.18–0.22 W/m²K depending on thickness.

**Timber frame extension walls**: Mineral wool between studs (89 or 140mm), PIR board fixed to inside face of studs (50mm) for enhanced performance and thermal bridging reduction.

**Extension floor slab**: PIR boards (100mm) on sand blinding below the concrete slab — standard specification for insulated ground floor slabs in London extensions.

• **Summary — what to expect your contractor to specify**:
• •Flat roof: 100–150mm PIR warm roof
• •Loft rafters: hybrid (mineral wool + PIR below)
• •Cavity walls: full-fill rock wool or PIR cavity boards
• •Ground floor slab: 100mm PIR under slab
• •Timber frame: 140mm mineral wool + 50mm PIR service zone