Home Insulation Guide: Types, Standards, and What to Specify for Extensions and Renovations

**What U-values are and why they matter**:

A U-value (thermal transmittance) measures the rate of heat transfer through a building element (wall, roof, floor) — lower U-value = better insulation. U-values are measured in W/m²K (Watts per square metre per degree Kelvin). The Building Regulations Part L (Conservation of Fuel and Power) sets minimum U-value requirements for new elements in new construction and material alterations, including extensions.

**Current minimum U-values for extensions and conversions under Part L (2021 and the Future Homes Standard trajectory)**:

| Building element | Minimum U-value (new build / extension) | Best practice target | |---|---|---| | External wall (cavity) | 0.18 W/m²K | 0.15 W/m²K | | External wall (solid / framed) | 0.18 W/m²K | 0.13 W/m²K | | Ground floor | 0.13 W/m²K | 0.10 W/m²K | | Pitched roof (between/over rafters) | 0.16 W/m²K | 0.13 W/m²K | | Flat roof | 0.18 W/m²K | 0.15 W/m²K | | Rooflights | 1.4 W/m²K (whole unit) | 1.2 W/m²K | | Windows (whole unit) | 1.4 W/m²K | 1.2 W/m²K | | Doors (where significantly glazed) | 1.4 W/m²K | 1.2 W/m²K |

*Note*: The 2021 Part L update strengthened U-value requirements. The Future Homes Standard (expected from 2025 onwards) will require further improvements. New extensions are subject to the Part L requirements current at the time of Building Regulations application.

**How U-values are calculated**:

U-value calculations are typically carried out by the architect or building energy assessor. The U-value of a wall depends on: the construction (brick, block, cavity, internal board, plaster); the insulation type and thickness; and the thermal performance of all layers including surface resistances. A 100mm cavity filled with rigid polyurethane (PUR) foam gives approximately 0.25 W/m²K — below the minimum 0.18 W/m²K; a 150mm cavity with 100mm rigid insulation and 50mm unfilled outer leaf achieves approximately 0.18 W/m²K. Achieving 0.15 W/m²K requires either a wider cavity or better-performing insulation.

**SAP (Standard Assessment Procedure) and extensions**:

For larger extensions (over a certain threshold of floor area relative to the existing house) or where the extension creates a new dwelling, a full SAP calculation may be required to demonstrate overall energy compliance — not just individual element U-values. The SAP calculation assesses the building's energy performance holistically. A SAP assessor carries out the calculation and produces an EPC for the extended building.

**The four principal insulation categories and their characteristics**:

**1. Mineral wool (glass wool and rock wool)**:

*What it is*: Inorganic fibrous insulation made from glass or rock (basalt). The most widely used insulation material in UK domestic construction. Brands: Knauf, Rockwool (rock wool), Isover (glass wool), Superglass (glass wool).

*Thermal performance*: λ (lambda, thermal conductivity) = 0.032–0.044 W/mK depending on product and density. Higher-density rock wool products perform better than standard glass wool batts.

• *Where used*:
• •Loft / attic insulation between and over rafters (blown glass wool or mineral wool rolls)
• •Cavity wall insulation (injected mineral wool slabs)
• •Internal wall insulation (mineral wool boards between studwork)
• •Between floor joists for thermal and acoustic separation
• •Flat roof insulation (high-density rock wool boards above the roof deck)

*Strengths*: Low cost, non-combustible (fire-safe), good acoustic performance (particularly rock wool), easy to cut and fit, widely available *Limitations*: Lower thermal performance per mm than rigid boards; can retain moisture if incorrectly detailed (vapour control layer required for some applications); must be protected from mechanical damage in exposed situations

*Cost*: Low — mineral wool rolls for loft insulation: £3–£6/m² for 200mm depth (material only)

**2. Rigid insulation boards (PIR, PUR, EPS, XPS)**:

*What it is*: A family of rigid or semi-rigid foam board insulation materials used where space is limited and a high performance per millimetre is needed.

• *Types and performance*:
• •PIR (Polyisocyanurate) boards — λ = 0.022–0.026 W/mK — the best-performing rigid insulation in UK domestic construction. Brands: Kingspan Kooltherm, Recticel Eurowall. Used for wall insulation, roof insulation, floor insulation.
• •PUR (Polyurethane) boards — λ = 0.022–0.028 W/mK — similar performance to PIR. Brands: Recticel Eurowall, Xtratherm.
• •EPS (Expanded Polystyrene) boards — λ = 0.032–0.038 W/mK — lower performance than PIR/PUR but lower cost; suitable for below-slab floor insulation (80–100mm EPS under a concrete slab achieves good ground floor U-values)
• •XPS (Extruded Polystyrene) boards — λ = 0.030–0.038 W/mK — moisture-resistant; used for below-slab and inverted flat roof (warm flat roof) insulation where moisture resistance is essential

• *Where used*:
• •Flat roof insulation (PIR or XPS above the waterproof layer in an inverted warm flat roof)
• •Cavity wall insulation (PIR boards in full-fill or partial-fill cavity)
• •Floor slab insulation (EPS or XPS below concrete slab)
• •Pitched roof insulation (PIR boards between and below rafters)

*Strengths*: Very high performance per mm (allows thinner wall/roof build-up); rigid and dimensionally stable; moisture-resistant (particularly XPS) *Limitations*: Higher cost than mineral wool; combustible in some forms (fire stopping at all penetrations required); some PIR/PUR boards have high embodied carbon

*Cost*: Medium-high — 100mm Kingspan K3 flat roof board: £15–£22/m²; 100mm Knauf DriTherm 32 cavity board: £8–£12/m²

**3. Spray polyurethane foam (SPF)**:

*What it is*: Two-component foam applied as a liquid spray that expands and cures in place. Can be open-cell (soft, flexible, lower performance) or closed-cell (hard, rigid, high performance).

• *Where used*:
• •Loft conversions — sprayed between rafters to insulate the roof plane
• •Irregular roof spaces where batt insulation cannot be cut to fit
• •Air sealing applications

*Performance*: Closed-cell SPF: λ = 0.022–0.028 W/mK

*Caution*: Spray foam insulation in lofts has been associated with mortgage refusal in some cases — some mortgage lenders refuse to lend on properties with spray foam insulation in the loft because it makes inspection and repair of the roof structure difficult. Before specifying spray foam for a loft conversion that may be sold, confirm the position with a mortgage broker. Spray foam applied without adequate vapour control detail can also trap moisture in timber rafters, leading to rot.

**4. Natural and sustainable insulation materials**:

*Wood fibre boards (e.g., Steico, Gutex)*: λ = 0.038–0.048 W/mK; excellent acoustic performance; high vapour permeability (allows the building to 'breathe'); lower embodied carbon than PIR; used for external wall insulation and roofs.

*Sheep's wool*: λ = 0.035–0.040 W/mK; natural, breathable, moisture-regulating; appropriate for traditional or solid-wall buildings where vapour permeability is important; more expensive than mineral wool.

*Hempcrete*: A mix of hemp hurds and lime binder that provides structure and insulation in one material; used in new build and traditional renovation contexts; λ = 0.060–0.090 W/mK (lower performance than PIR but excellent breathability); not suitable for high-performance extensions without additional insulation.

**Cavity wall extension — the standard London approach**:

The most common external wall construction for a London domestic extension is a cavity wall: outer leaf of facing brick (matching or similar to the existing house), 100mm cavity (full-fill or partial-fill insulation), 100mm dense aggregate concrete block inner leaf, and internal plasterboard (12.5mm or 15mm, on dabs or on 25mm metal furring strips).

*Full-fill cavity*: The entire 100mm cavity is filled with cavity insulation — either injected mineral wool, blown cellulose, or a PIR board cut to width. Full-fill achieves a better U-value but can be problematic if the cavity is exposed to wind-driven rain (risk of moisture bridging to the inner leaf). Full-fill is appropriate for sheltered sites; partial-fill is safer for exposed or high-rainfall sites.

*Partial-fill cavity*: A rigid insulation board (typically PIR, 75–100mm) is fixed to the inner leaf, leaving a 25–50mm clear residual cavity. Partial-fill is the standard approach for most London extensions — it maintains the cavity's drainage function while achieving good U-values.

A standard 100mm cavity with 70mm PIR partial-fill board + 30mm residual cavity + 100mm dense blockwork + 12.5mm plasterboard achieves approximately 0.18–0.20 W/m²K — meeting the Part L minimum. To achieve 0.15 W/m²K, increase to 100mm PIR partial-fill with 25mm residual cavity, or use a higher-performing PIR board such as Kingspan K8.

**Flat roof insulation — warm vs. cold deck**:

*Warm flat roof (the correct standard)*: The insulation is placed on top of the structural roof deck (OSB or plywood), above the vapour control layer. The waterproof membrane goes on top of the insulation. This is the correct approach for all new flat roof extensions.

*Cold flat roof (avoid)*: The insulation is placed below the structural deck, between the joists. This creates a risk of interstitial condensation within the roof structure, which is a principal cause of flat roof failure in UK buildings. Cold deck roofs should not be specified for new construction.

*Required insulation thickness for a warm flat roof to achieve 0.18 W/m²K*: approximately 120mm PIR, or 150mm mineral wool.

**Pitched roof insulation (loft conversion)**:

For a loft conversion where the roof space becomes habitable, the insulation must be at the slope (between and below the rafters), not at ceiling level.

*Standard approach*: PIR boards between rafters (cut to fit between 47×150mm or 47×200mm rafters at 400mm centres) + 25–50mm service void below rafters + 12.5mm or 15mm plasterboard. A combined between-and-below rafter assembly with 100mm PIR between rafters and 50mm below achieves approximately 0.16 W/m²K.

*Better approach (Passivhaus-influenced)*: 100mm PIR between rafters + 50–75mm PIR continuous insulation board below rafters (no thermal bridging at rafter positions) achieves 0.13–0.15 W/m²K.

**Ground floor insulation**:

*Suspended timber floor in an extension*: 100–150mm mineral wool semi-rigid batts between joists (supported by netting), with air bricks retained below the joist void to maintain sub-floor ventilation. Achieves approximately 0.20 W/m²K. Does not meet the 0.13 W/m²K target without additional continuous insulation below the floor boards.

*Concrete slab with EPS insulation below*: The standard approach for a concrete ground-floor slab extension. 75–100mm EPS boards below the concrete slab (on the blinded hardcore), plus 50–75mm PIR boards above the slab (below the screed) achieves approximately 0.10–0.13 W/m²K — comfortably meeting the Part L minimum.