Insulation for London House Extensions: What's Required, What Types Work, and How to Achieve Compliance

**What does Building Regulations Part L require for a London extension?**

Approved Document L (Conservation of Fuel and Power) sets out the energy performance requirements for extensions to existing dwellings in England. Part L for domestic extensions (introduced in its current form in 2021/2022) sets minimum U-values (thermal transmittance — the rate of heat loss through a building element per unit area per degree of temperature difference) for each element of the new extension.

**Minimum Part L U-value requirements for a new extension to a London house (2025)**:

| Building element | Maximum U-value (W/m²K) | |---|---| | External walls (new extension walls) | 0.28 | | Flat roof (warm deck or cold deck) | 0.15 | | Pitched roof (insulation at ceiling level) | 0.16 | | Pitched roof (insulation at rafter level — warm roof) | 0.15 | | Ground floor (new extension floor slab) | 0.22 | | Windows and external doors (including glazed doors) | 1.60 (whole unit — frame + glass) | | Roof lights and lanterns | 1.60 | | New opening through existing external wall (into existing dwelling) | Existing wall U-value (no improvement required to the existing wall, but the opening should not be uninsulated) |

These are the maximum U-values permitted for each element. Lower U-values (better insulation performance) are permitted and may be required where the SAP (Standard Assessment Procedure) energy compliance calculation for the extension demands it — for example, where the glazing area of the extension exceeds 25% of the new floor area, the non-glazed elements must perform better to compensate.

**The 25% glazing limit and SAP calculation**:

Part L for extensions imposes a limiting condition: where the total area of new windows, doors, and roof windows in the extension does not exceed 25% of the new floor area, the individual elemental U-values above are sufficient and no SAP calculation is required.

Where the glazed area exceeds 25% of the floor area (common for large open-plan extensions with extensive bi-fold doors and roof lanterns), a 'notional building' SAP calculation is required — demonstrating that the total energy performance of the extension is no worse than a notional extension of the same size with standard elemental U-values and 25% glazing. In practice, this means that where glazing exceeds 25%, the wall and roof insulation must be improved above the elemental standard to compensate for the additional heat loss through the extra glazing.

**What about the existing house — must it be insulated when the extension is built?**

Under Part L for extensions, there is no requirement to insulate the existing house as part of the extension project (unlike Part L for new-build dwellings, which applies to the whole dwelling). However, where an existing cavity wall is being exposed or opened up as part of the extension works (e.g., where a structural opening is formed in the original rear wall of the house to connect the extension to the kitchen), the opportunity to insulate the exposed element should be taken — it is far cheaper to install cavity wall insulation while the wall is open than to do it retrospectively.

Where the existing external wall of the original house is used as an internal wall within the extension (i.e., the original rear wall is now inside the building envelope), it loses its insulation function — the new external walls of the extension are now the thermally significant elements. The existing wall does not need to be insulated to a new U-value in this situation, but it should be checked for damp penetration and treated if necessary before it becomes an internal surface.

**PIR (Polyisocyanurate) rigid insulation board — the workhorse of London extension insulation**:

PIR (commonly known by trade names such as Kingspan Kooltherm, Recticel Eurowall+, Xtratherm, and Celotex) is a rigid foam insulation board with a highly reflective foil facing on both sides. It is the most commonly used insulation product in London residential extensions, for all three main elements (floor, wall, and flat roof), for the following reasons:

*Performance (thermal conductivity)*: PIR achieves a thermal conductivity (λ — lambda) of 0.022–0.023 W/mK, compared to 0.037 W/mK for standard mineral wool. This means PIR is approximately 60% more efficient per millimetre of thickness — allowing the same U-value to be achieved with significantly less insulation thickness, which is critical in London extensions where overall build thickness matters (it affects the internal footprint and the structural zone available).

• *Applications in a London rear extension*:
• •Floor insulation: PIR laid on top of the DPM (or in a warm floor sandwich below the screed). Typically 100–120mm PIR to achieve ≤ 0.22 W/m²K floor U-value
• •Flat roof insulation (warm deck): PIR boards laid above the structural decking, below the waterproof layer. Typically 150–180mm PIR to achieve ≤ 0.15 W/m²K flat roof U-value
• •Wall insulation (internal insulation on existing solid walls, or as part of a composite cavity): PIR in various thicknesses depending on the existing wall U-value and the target

• *Limitations of PIR*:
• •PIR is not breathable — in a tight enclosure without adequate ventilation, it can promote condensation in the wrong position if the vapour control layer (VCL) is not installed correctly
• •PIR must be installed in contact with the surrounding construction (gaps at edges reduce performance significantly — thermal bridging at the junction between PIR boards and adjacent structure can be a significant proportion of the total heat loss if not detailed carefully)
• •PIR is a petrochemical product with a higher embodied carbon than natural insulation alternatives (see below)

**Mineral wool (glass wool and rock wool/stone wool)**:

Mineral wool insulation (glass wool trade names: Isover, Knauf; rock/stone wool trade names: Rockwool, Superglass) is the traditional insulation medium for UK buildings — widely used in pitched roof insulation (between and over rafters), in cavity walls, and in timber-frame constructions.

*Performance*: Thermal conductivity typically 0.033–0.040 W/mK — lower performance per mm than PIR, meaning more thickness is needed for the same U-value. However, mineral wool is cheaper per m² for a given R-value (thermal resistance) than PIR at equivalent thicknesses.

• *Applications in a London rear extension*:
• •Pitched roof insulation at ceiling level: typically 100–200mm between ceiling joists/rafters + 100–200mm over, cross-lapped, to achieve ≤ 0.16 W/m²K
• •Cavity wall — partial-fill or full-fill cavity: partial-fill batt (75mm PIR or mineral wool in 100mm cavity) or full-fill batt (100mm mineral wool in 100mm cavity, suitable only where cavity is protected from driving rain)
• •Between timber studs in a dormer frame: 100–150mm mineral wool between 100–150mm CLS studs at 400mm centres — used where PIR is too rigid for irregular stud bays

*Limitations*: Mineral wool sags and settles if incorrectly installed (particularly in sloped applications). It must be kept dry — saturated mineral wool loses most of its insulating value. Not suitable as the primary insulation in an exposed flat roof deck (PIR or EPS preferred for flat roof applications).

**EPS (Expanded Polystyrene) and XPS (Extruded Polystyrene)**:

EPS (white expanded polystyrene board) and XPS (extruded polystyrene board — trade name Styrofoam, Jackodur) are used in specific applications:

*EPS/XPS under the floor slab (ground-bearing floor)*: XPS has the advantage of being moisture-resistant — it can be laid beneath the concrete slab (where PIR may absorb moisture from damp ground conditions). Thermal conductivity: XPS 0.034–0.037 W/mK; EPS 0.030–0.038 W/mK. Typically 100–150mm XPS beneath the ground-bearing slab to achieve ≤ 0.22 W/m²K floor U-value.

*EPS in thermal blocks (Marmox, Jetfloor) at perimeter*: At the perimeter of a ground-bearing floor slab, the junction between the wall and the floor is a thermal bridge. Proprietary insulated concrete blocks (Marmox, Foamglas, Jetfloor) or EPS thermal liners placed at the inner edge of the floor-to-wall junction reduce the thermal bridge effect at this critical junction.

**Natural and alternative insulation products**:

• For clients seeking lower-embodied-carbon alternatives to petrochemical insulation:
• •*Wood fibre (e.g., Pavaflex, Steicoflex, Pavatex)*: thermal conductivity 0.038–0.045 W/mK; breathable; suitable for timber-frame walls and rafters; higher thickness needed than PIR
• •*Hemp insulation (e.g., Tradical Hempcrete, Hemp-lime)*: breathable; very low embodied carbon; good acoustic as well as thermal properties; used as structural fill in timber-frame walls; thermal conductivity approximately 0.060–0.070 W/mK
• •*Cork*: thermal conductivity 0.040–0.045 W/mK; excellent acoustic properties; used in internal and external applications

Natural products are increasingly specified for London extensions where the client has sustainability goals — they typically require additional build thickness compared to PIR, which must be accommodated in the design.

**Thermal bridging — what it is and why it matters**:

Thermal bridging (also called 'cold bridging') occurs where a thermally conductive element (structural steel, concrete ring beam, mortar joint, timber stud, window frame) bypasses the insulation layer and provides a faster path for heat to conduct from warm to cold. In a well-insulated London extension:

• *Common thermal bridges*:
• •Steel beam bearing on the external wall — the steel conducts heat from the internal warm side to the external wall surface (and thus to the outside)
• •Window frame junction with the wall — the outer edge of the window frame is warmer than the wall surface behind the insulation
• •Concrete ring beam or lintel above windows — concrete is significantly more thermally conductive than the surrounding insulation
• •Cavity wall wall ties (though modern plastic wall ties have much lower conductivity than older galvanised steel ties)
• •The external wall base where the floor insulation meets the wall insulation (junction thermal bridge)

• *How to address thermal bridging in a London extension*:
• •Use proprietary thermal break pads (e.g., Halfen, Schöck Isokorb) at structural steel bearing points where the steel penetrates the insulation layer
• •Ensure the insulation layers are continuous at junctions (window jamb insulation wrapping behind the window frame; wall insulation continues behind the ring beam with an insulated lintel above)
• •Specify insulated cavity closers at all window and door openings in a cavity wall extension
• •Use insulated concrete lintels (or insulated steel lintels with thermal break, such as Catnic or Teplo) above window and door openings

Thermal bridges are often not visible in the finished construction but have a material impact on the actual energy performance of the extension — particularly at junctions where condensation can form in the cold months.

**A complete insulation strategy for a standard single-storey London Victorian terrace rear extension**:

The following build-ups are typical for a standard quality London rear extension using cavity masonry external wall construction and a flat roof, achieving Part L compliance:

**Floor build-up (ground-bearing concrete floor, warm floor approach)**:

1. Existing ground / hardcore sub-base (compacted, blinded with sand) 2. DPM (150 micron polythene, lapped up at perimeter to DPC height) 3. PIR insulation boards (100mm, Kingspan/Celotex or equivalent — laid tight, joints staggered) 4. Perimeter EPS thermal block or Marmox liner at wall base 5. Concrete screed (65mm sand-cement screed, or 40mm anhydrite screed over UFH pipes) 6. Floor finish (tiled, engineered wood, vinyl — client-supplied)

U-value achieved: approximately 0.19 W/m²K with 100mm PIR (better than the 0.22 requirement — 10% margin)

**External wall build-up (cavity masonry, 100mm cavity)**:

1. Outer leaf: 102mm brickwork (to match existing property elevation — typically London stock brick for the side walls, brick or render for the rear elevation) 2. 100mm cavity (with stainless steel wall ties at 600mm horizontal, 450mm vertical staggered — Teplo-BV type for thermal performance; or standard stainless steel ties) 3. 65mm partial-fill PIR insulation batt (Kingspan or equivalent) fixed to the inner face of the outer leaf with clips 4. 35mm remaining cavity (providing an unventilated inner zone) 5. Inner leaf: 100mm lightweight concrete block (e.g., Durox 7, Thermalite Turbo, or similar; thermal conductivity ≤ 0.15 W/mK preferred for improved whole-wall U-value) 6. 12.5mm plasterboard or 15mm sand-cement plaster finish

U-value achieved: approximately 0.22–0.26 W/m²K with 65mm PIR partial-fill in 100mm cavity and lightweight block inner leaf. To achieve closer to 0.20 W/m²K (a better margin on Part L), increase PIR to 90mm in a wider 125mm cavity, or use full-fill mineral wool in a 100mm cavity.

**Flat roof build-up (warm deck — the standard for London single-storey extension flat roofs)**:

A 'warm deck' flat roof places the insulation on TOP of the structural roof deck, beneath the waterproof layer. The structural deck (typically 18mm OSB or 18–22mm structural plywood on timber joists at 400mm centres) is always above ambient temperature (never cold enough to attract condensation) because the insulation is above it.

1. Ceiling boarding / plasterboard (12.5mm to the underside of joists; 15mm in fire protection zones) 2. Timber joists (typically C24 grade; 200mm or 220mm deep — check SE specification for span; at 400mm centres) 3. Vapour control layer (VCL — typically 1200 gauge polythene or proprietary vapour check plasterboard on warm side of joists, taped at all joints) 4. Structural roof deck (18mm OSB3 or 18mm structural plywood, nailed to joists) 5. Bonding layer (bituminous primer for felt systems; no primer for EPDM systems) 6. PIR flat roof insulation boards (150mm minimum; 180mm for a better margin — total insulation thickness typically 150–180mm PIR in a warm deck flat roof achieves ≤ 0.15 W/m²K) 7. Waterproof layer: EPDM (single-ply rubber membrane, fully adhered or mechanically fixed); or GRP (glass reinforced polyester, laid wet-on-site); or single-ply TPO or PVC-U 8. Falls: minimum 1:80 falls (1:40 preferred) to outlet(s) — falls created by tapered PIR boards (tapered insulation boards, pre-cut to profile) or by the timber joist structure

U-value achieved: approximately 0.13 W/m²K with 150mm PIR warm deck (better than 0.15 requirement).

**Practical guidance for clients on insulation choices**:

1. *Always specify Part L-compliant U-values from the outset*: Do not assume the contractor will automatically achieve Part L compliance — specify the required U-values in the contract and ensure Building Control is appointed before works start to inspect at the insulation stages

2. *Vapour control layers (VCLs) must be continuous and sealed*: The VCL in the flat roof and floor prevents warm moist air from the interior migrating into the insulation layer and condensing on the cold deck. Gaps and unsealed joints in the VCL are the most common cause of flat roof condensation failure — ensure the VCL is taped at all joints and lapped at all perimeters

3. *Thermal bridging at the wall-floor junction*: The junction between the new extension external wall (cold external element) and the new extension floor slab (warm insulated slab) is the most significant thermal bridge in the typical London extension. Specify insulated concrete blocks or a Marmox/Jetfloor proprietary thermal break at this junction

4. *Building Control inspection at insulation stage*: Building Control must inspect the insulation before it is covered over. This is the commencement inspection for the floor (before the screed is poured) and the roof inspection (before the waterproof membrane is laid). Ensure the Building Control inspector is booked and has confirmed attendance before covering the insulation