Wall insulation is one of the highest-impact retrofit measures available on UK housing stock, yet it is also one of the most frequently mis-specified. Getting the system wrong can trap moisture, reduce living space, cause interstitial condensation, and create expensive remediation. This guide covers the three main approaches — cavity fill, internal wall insulation (IWI) and external wall insulation (EWI) — with the materials, costs, U-values and condensation considerations relevant to each.
Cavity Wall Insulation
Cavity walls, typical of UK construction from roughly 1920 to the present, have a gap (cavity) of 50–100 mm between the inner and outer leaf. This gap can be either partially or fully filled with insulation.
Full Fill vs Partial Fill
Full-fill packs insulation across the entire cavity width. It achieves lower U-values for a given wall thickness but places the insulation in contact with both leaves, which raises the risk of moisture bridging from the wet outer leaf to the inner leaf in exposed locations. Full-fill materials must have a water-absorption resistance appropriate to the exposure zone per BS EN 13162 / BS 8415.
Partial fill leaves a residual clear cavity of at least 25 mm (50 mm recommended by NHBC for Zone 3 very severe exposure) between the outer leaf and the insulation board. The board is held against the inner leaf by retaining wall ties. Partial fill is specified in more exposed locations and gives slightly higher U-values for the same wall thickness.
Cavity Fill Materials
| Material | Lambda (W/mK) | Typical U-value (100 mm cavity, full fill) | Cost installed |
|---|---|---|---|
| Mineral wool (blown fibre) | 0.033–0.040 | 0.32–0.36 W/m²K | £15–£30/m² |
| EPS bead (bonded) | 0.033–0.038 | 0.30–0.35 W/m²K | £18–£35/m² |
| Injected polyurethane foam | 0.025–0.030 | 0.25–0.30 W/m²K | £25–£45/m² |
| Partial-fill PIR board (75 mm) | 0.022–0.025 | 0.20–0.25 W/m²K | £25–£50/m² |
All cavity fill installations in existing buildings must use a system holding a current British Board of Agrément (BBA) certificate and be installed by a CIGA (Cavity Insulation Guarantee Agency) registered installer. The installer provides a 25-year guarantee.
For new build, cavity design must achieve the elemental U-value target in Approved Document L (Conservation of Fuel and Power): 0.18 W/m²K for walls in a notional dwelling under the 2021 Part L revision.
Internal Wall Insulation (IWI)
IWI is the retrofit method for solid walls (pre-1920 brick or stone) or where cavity fill is not feasible (e.g. cavity too narrow, wall ties corroded, exposed location). It is fixed to the inner face of the external wall and loses some floor area — typically 60–100 mm per wall. It also relocates the thermal mass of the wall to the outside of the insulation layer, which can affect thermal comfort and is a consideration in overheating assessments.
IWI Materials
Rigid insulation boards (PIR, EPS, mineral wool rigid board) are dot-and-dab or mechanically fixed to the masonry, then plasterboarded. They offer the highest thermal performance per unit of thickness.
Insulated dry-lining systems combine a rigid insulation layer factory-bonded to a plasterboard face. They are quick to install but must be cut precisely at reveals and window returns.
Wooden-framed systems (a timber studwork frame with mineral wool or cellulose fill) are flexible, accommodate services within the frame, and are preferred where airtightness detailing is critical. They take up 100–140 mm of floor width.
Mineral-fibre slabs in a metal furring system are common in heritage contexts where a breathable build-up is required: on older lime-pointed rubble-stone walls, a vapour-open system is often preferred to avoid trapping rising or penetrating moisture.
Condensation Risk and Vapour Control
IWI moves the dew point into or behind the insulation layer, creating the risk of interstitial condensation on the masonry surface. A vapour control layer (VCL) on the warm side of the insulation is essential in most cases. BS 5250 Code of practice for control of condensation in buildings requires a hygrothermal analysis (Glaser method or dynamic simulation) to confirm no problematic condensation occurs. For solid stone or soft-brick walls, a vapour-open (hygroscopic) system may be preferable, and a specialist assessment is advisable.
External Wall Insulation (EWI)
EWI wraps the outside of the building with an insulation layer, followed by a render or cladding finish. It avoids internal space loss, keeps the thermal mass inside the insulation envelope (beneficial for thermal comfort and summer overheating), and allows refurbishment of weathered external walls simultaneously. The main constraint is aesthetic: it alters the external appearance and may require planning permission, particularly in Conservation Areas.
EWI Systems
| Insulation layer | Lambda (W/mK) | 100 mm thickness U-value (solid 225 mm brick wall) | Typical installed cost |
|---|---|---|---|
| EPS (expanded polystyrene) | 0.033–0.038 | 0.28–0.33 W/m²K | £80–£120/m² |
| Mineral wool slab | 0.033–0.040 | 0.30–0.36 W/m²K | £90–£140/m² |
| Phenolic foam board | 0.020–0.023 | 0.20–0.24 W/m²K | £110–£160/m² |
| Woodfibre board | 0.038–0.050 | 0.35–0.45 W/m²K | £100–£150/m² |
EWI systems should carry a BBA or ETA (European Technical Assessment) certificate. The system includes adhesive/mechanical fixing of the insulation, reinforcing mesh embedded in base coat, and the finish coat or cladding. Window sills, heads and cills must be extended to accommodate the added thickness — this is a significant labour and material cost often underestimated in initial quotes.
EWI on new build must be designed to BS 8414-1/2 (fire testing of external cladding) if the building is over 11 m in height, following the changes to Approved Document B and the Building Safety Act 2022.
Choosing the Right Approach
| Factor | Cavity fill | IWI | EWI |
|---|---|---|---|
| Wall type | Cavity wall | Solid wall (or narrow cavity) | Any wall type |
| Space loss | None | 60–140 mm per wall internally | None internally |
| Thermal performance | Good | Very good | Very good |
| Disruption | Low (injected externally) | High (strips internal finishes) | Moderate (external scaffolding) |
| Planning risk | Low | Low | Higher in sensitive areas |
| Moisture risk | Low–moderate (full fill in exposed zones) | Moderate (VCL required) | Low (if system certified) |
| Cost/m² installed | £15–£50 | £50–£110 | £80–£160 |
UK Standards Summary
- BS 8215: Code of practice for design and installation of cavity insulation
- BS 5250: Condensation control
- BS EN 13162: Mineral wool insulation products
- BS EN 13163: EPS insulation products
- BS EN 13165: PIR insulation products
- Approved Document L 2021: Target wall U-value 0.18 W/m²K (new build), 0.30 W/m²K (existing buildings — consequential improvement)
- PAS 2035: Retrofitting standard for domestic buildings — whole-house assessment and risk management now required under ECO4 and Great British Insulation Scheme funded works
