Insulation Types for House Extensions: Which Is Best?

Insulation is one of the most cost-effective investments you make in an extension. Getting it right reduces heating bills for decades. Getting it wrong is expensive and difficult to remedy after construction. Here's a guide to the main options.

Why Extension Insulation Matters

New extensions must meet Building Regulations Part L (Conservation of Fuel and Power) minimum U-value requirements. But the minimum is rarely the optimal specification. Given that insulation materials are cheap relative to long-term energy savings, specifying above minimum is almost always worthwhile.

Key areas requiring insulation in an extension:

• Flat roof
• Walls (cavity or solid construction)
• Floor (below screed or floor deck)
• Junction details (where extension meets existing house)

Flat Roof Insulation

Flat roof extensions typically use one of two insulation approaches:

Warm roof (most common): Insulation is placed above the structural deck, keeping the roof structure warm and protected. PIR (polyisocyanurate) rigid board is the most common warm flat roof insulation — high performance in a slim thickness. Typical specification: 100–150mm PIR board achieving U-value of 0.15 W/m²K.

Cold roof: Insulation between the joists, below the deck. Cheaper but less thermally efficient and more prone to interstitial condensation. Less common in new extension roofs.

Wall Insulation

Cavity wall (new extension walls): Most new extension walls are cavity construction (inner and outer leaf with an insulated cavity). The cavity is filled with rigid board, full-fill mineral wool batts, or blown bead insulation. Typical total wall U-value specification: 0.18–0.28 W/m²K.

Solid wall extensions: Where brick or blockwork is a single solid leaf (less common in new extensions), insulation is applied internally (dry lining with insulated plasterboard) or externally. Internal insulation loses some floor area.

Floor Insulation

Floor insulation sits below the screed or structural floor deck. For an extension on a new concrete slab, rigid PIR board insulation (typically 100mm+) is installed before the screed is poured. For timber deck extensions, insulation fits between joists. Typical floor U-value target: 0.13–0.22 W/m²K.

Junction Detailing

Thermal bridges at junctions — where the extension meets the existing house, around window openings, at the wall-floor junction — are a significant source of heat loss in poorly designed extensions. Proper junction detailing using insulating profiles and careful specification reduces thermal bridging meaningfully.

Recommended Specifications

• Flat roof: 150mm PIR warm roof, U-value 0.13 W/m²K or better
• Cavity walls: 100mm full-fill cavity insulation, U-value 0.18 W/m²K or better
• Floor: 100mm rigid PIR under screed, U-value 0.15 W/m²K or better

RCB Design & Build

RCB Design & Build specifies insulation to exceed Building Regulations minimums as standard — considering thermal performance, junction detailing, and the long-term running cost impact of insulation choices.