Cavity Wall Insulation Types: Mineral Wool vs EPS vs PIR

Quick Answer: The three main cavity wall insulation types are mineral wool (rockwool/glasswool) blown in as loose fill or installed as batts, EPS (expanded polystyrene) bead blown in under BBA certificate, and phenolic (or PIR) rigid board installed during new-build cavity construction. For existing properties, blown mineral wool and EPS bead are the dominant retrofit options; phenolic board is generally only practical at new-build or during major rebuild. Building Regulations Part L1A (new dwellings) requires a wall U-value of 0.26 W/m²K (notional) and Part L1B (existing) targets 0.55 W/m²K for cavity-fill retrofits. All retrofit installations in England must comply with PAS 2030:2019 (installer competence) and PAS 2035:2019 (whole-building retrofit assessment) when government-funded, and installations must be registered with a competent-person scheme.

Summary

The cavity wall — two skins of masonry with a gap between them — became the standard UK house construction method from approximately the 1920s onwards, and by the 1970s it was all but universal in new housing. Cavities were originally designed to prevent rain penetration, not to insulate; most pre-1990 houses therefore have empty cavities that represent a major source of heat loss. Insulating those cavities is consistently one of the most cost-effective energy-efficiency measures available.

For retrofit (existing buildings), the two main methods are blown loose-fill insulation — either mineral wool (glass or rock fibre) or EPS bead — injected through small-diameter holes drilled through the outer brick or block leaf and plugged afterwards. Both are well-established, BBA-approved systems with a long performance history in the UK. For new-build, rigid board insulation (PIR or phenolic) built into the cavity during construction offers the highest thermal performance but is not retrofittable.

The selection decision is not as simple as picking the cheapest material. Cavity width, exposure zone (rainfall), masonry condition, cavity cleanliness (rubble, mortar snots), wall-tie condition, building age and type, and the presence of an existing DPC all affect which system is appropriate, whether it is safe to insulate at all, and what installer obligations apply. Getting this wrong — particularly in high-exposure zones or on walls with defects — can introduce or worsen damp problems and trigger disputes.

Key Facts

Quick Reference Table

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Insulation Type λ (W/mK) Method Cavity Width Best For Limitations
Mineral wool blown 0.034–0.044 Blown (retrofit) 50 mm+ Most UK housing, moderate exposure Not for narrow cavities; mortar snots can void fill
EPS bead (white) 0.036–0.040 Blown (retrofit) 50 mm+ Standard retrofit; fills irregular cavities well Glue binder needed; airflow through bead if poor bonding
EPS bead (grey/graphite) 0.031–0.034 Blown (retrofit) 50 mm+ Best retrofit thermal performance Higher material cost
PIR rigid board (partial fill) 0.022–0.026 New-build (built in) 75 mm+ New-build with high U-value target Not retrofittable; must maintain 50 mm residual clear cavity
Phenolic rigid board 0.018–0.022 New-build (built in) 75 mm+ Highest thermal performance new-build Most expensive; not retrofittable
Injected foam (UF foam) 0.031–0.040 Retrofit (specialist) 50 mm+ Narrow or complex cavities Mortgage lender concerns; see spray foam note

Detailed Guidance

Mineral Wool (Glass Wool / Rock Wool) Blown Fill

Mineral wool blown insulation consists of glass or rock fibre, typically in granulated or chopped fibre form, injected under controlled air pressure through holes (typically 22 mm diameter) drilled into the outer leaf of masonry at approximately 800–1000 mm centres. After injection the holes are filled with a mortar plug to match the existing pointing.

Glasswool vs Rockwool: Both are effective; glasswool has slightly lower density and λ at around 0.038–0.044 W/mK; rockwool (stone wool) is denser, typically 0.034–0.040 W/mK, and slightly more fire-resistant (classified A1 non-combustible). For standard CWI retrofit, both perform similarly and the choice is usually down to the installer's preference and the specific BBA certificate they hold.

Suitability and pre-installation survey: Before installation, a CIGA-registered assessor or the installer must assess: cavity width (minimum 50 mm); masonry condition (no significant cracking, spalling, or defective pointing that would allow water penetration); existing dampness (rising, penetrating, or condensation — must be resolved first); wall-tie condition; cavity cleanliness (mortar snots, debris, or original fill that could obstruct injection); and exposure zone. The Meteorological Office / BS 8215 exposure classifications define where full-fill is and is not appropriate.

High-exposure risk: In severe exposure zones (typically Scotland, Wales, and north and west-facing elevations in exposed areas), driven rain can penetrate the outer leaf and migrate through a full-fill cavity to the inner leaf, causing dampness that was not present before installation. In these locations, partial-fill or no-fill is often more appropriate. Assessors must use the Driving Rain Index (DRI) map to determine suitability.

Installation: A certificated installer uses a calibrated injection system with pressure gauge and flow meter to ensure correct fill density (typically 15–18 kg/m³ for glasswool blends; higher for rockwool). Under- or over-filling voids the BBA certificate and reduces performance. Thermal imaging is used by some insurers and surveyors post-installation to verify even fill.

EPS (Expanded Polystyrene) Bead

EPS bead CWI uses tiny expanded polystyrene balls mixed with an adhesive binder (typically a water-based polyisobutylene or polyurethane binder) to form a bonded mass within the cavity. The bead/binder mixture is blown in under pressure through holes in the outer leaf, similar to mineral wool injection.

Advantages over mineral wool: EPS bead is particularly good at filling cavities with obstructions — mortar snots, debris, irregular wall ties — because the small spheres flow around obstacles. Mineral wool fibres can bridge over debris and leave voids. EPS bead also has slightly lower water absorption and does not slump or settle after installation if correctly bonded.

Graphite/silver EPS bead: Grey bead incorporating graphite particles achieves λ ≈ 0.031–0.034 W/mK compared to 0.036–0.040 W/mK for standard white bead. On a 50 mm cavity, the improvement in U-value is modest (approximately 0.03–0.05 W/m²K) but may be material to achieving a compliance target.

Binder quality and voids: The adhesive binder must coat the beads sufficiently to bond them together. If the binder ratio is incorrect or the injection system is poorly set up, the bead can remain loose and allow air convection within the mass, degrading thermal performance. Ensure the installer's BBA certificate specifically covers the binder system being used.

EPS and moisture: EPS itself has very low water absorption (≤2% by volume in most grades) and does not lose thermal performance when damp. However, a saturated bead mass can wick moisture if the cavity is persistently wet — the moisture cause must be resolved before installation, as with all CWI types.

Rigid Board — PIR and Phenolic (New-Build and Full Rebuild)

Rigid board insulation in a cavity is installed during new-build construction as each course of blockwork is laid. Boards are placed against the inner leaf (or clipped/supported mid-cavity in a partial-fill arrangement) and the outer brick leaf is built up around them, leaving a residual cavity of at least 50 mm (measured from the face of the board to the inside face of the outer leaf) to prevent wind-driven rain bridging to the insulation.

PIR (polyisocyanurate) board: λ ≈ 0.022–0.026 W/mK. Common brands include Kingspan Kooltherm K8 and Celotex CW4000. A 90 mm PIR board in a 140 mm cavity (50 mm residual clear cavity) achieves a wall U-value of approximately 0.19–0.22 W/m²K depending on wall leaf construction — comfortably meeting Part L1A's notional value.

Phenolic board: λ ≈ 0.018–0.022 W/mK. The most thermally efficient rigid board; used where cavity width is constrained but a very low U-value is required. Common brands include Kingspan Kooltherm K8 and Recticel Eurowall. Phenolic boards are more fragile than PIR and require careful handling to avoid face damage.

Full-fill rigid board: Some manufacturers produce rigid boards designed to fill the entire cavity (e.g. 100 mm full-fill PIR in a 100 mm cavity). These boards have drainage channels on the outer face to direct any penetrating moisture downwards and out via weep holes at the base of the wall. This system eliminates the residual cavity and achieves the highest thermal performance, but requires careful specification and is not suitable in severe exposure zones without manufacturer confirmation.

Not retrofittable: Rigid board installation requires access to the cavity from above (during construction) or full reconstruction of the outer leaf. It cannot be retrofitted into an existing wall without demolition.

PAS 2030 and PAS 2035 — Grant-Funded Work

Any cavity wall insulation funded under government schemes — ECO4 (Energy Company Obligation), the Great British Insulation Scheme (GBIS), or local authority schemes — must comply with PAS 2030:2019 (installer competence standard) and PAS 2035:2019 (whole-dwelling retrofit assessment standard).

Under PAS 2035, a Retrofit Assessor (holding the relevant TrustMark-registered qualification) must assess the property before installation. The assessor identifies whether the property is suitable for CWI, flags any risk factors (moisture, ventilation, structural defects), and produces an Improvement Opportunity Report. The installer must follow the Retrofit Coordinator's medium-term improvement plan. A Retrofit Coordinator must oversee funded projects.

For non-funded installations, PAS 2030/2035 compliance is not legally mandated but CIGA-registered installers will typically follow similar assessment protocols to protect their guarantee obligations.

Spray Foam and Mortgage Lender Concerns

Injected (blown) UF (urea formaldehyde) foam CWI is a variant of cavity insulation that has caused significant problems in the UK housing market. While it was a popular retrofit option in the 1980s and 1990s, UF foam has been associated with moisture entrapment, wall-tie corrosion, and structural concerns. More critically: mortgage lenders — including most high-street banks — will not lend on properties with spray foam insulation in cavities (or in lofts) until a specialist assessment has been completed and often not at all without removal. This has rendered affected properties unmortgageable and caused significant financial hardship.

Blown mineral wool and EPS bead are distinct from spray foam and do not carry these concerns. However, if a survey or thermal imaging reveals evidence of foam in an existing cavity, flag this to the client immediately and advise specialist assessment before proceeding.

Frequently Asked Questions

How do I know if a house is suitable for cavity wall insulation?

The property needs a pre-installation survey by a CIGA-registered assessor. Key checks: the cavity must be at least 50 mm wide (borescope inspection); the masonry must be in good condition; there must be no evidence of current dampness; the exposure zone must be appropriate; and the wall ties must be in acceptable condition. Houses built before 1920 and solid-wall properties (no cavity) are not candidates for retrofit CWI. Many post-1990 properties already have partial-fill insulation — a survey will determine whether additional fill is feasible and beneficial.

Will cavity wall insulation cause damp?

Correctly specified and installed CWI in an appropriate property in a suitable exposure zone does not cause damp. However, incorrectly assessed properties — high-exposure zones, walls with pre-existing cracks, defective DPCs, or water ingress from above — can experience damp that is attributed (correctly or incorrectly) to the CWI. The key safeguard is a thorough pre-installation survey and choosing an installer with a valid CIGA guarantee. If damp problems emerge post-installation, report them to CIGA, which will arbitrate and, if necessary, fund remedial works including extraction of the insulation.

What U-value will I achieve with EPS bead in a 1960s cavity wall?

A typical 1960s wall is two leaves of 102.5 mm brickwork with a 50 mm cavity. Uninsulated, the U-value is approximately 1.4–1.6 W/m²K. After full-fill with EPS bead (λ = 0.038 W/mK) in a 50 mm cavity, the U-value improves to approximately 0.5–0.55 W/m²K — a reduction of approximately 65% in heat loss through the wall. With graphite bead (λ = 0.032 W/mK) the value improves slightly to approximately 0.47–0.50 W/m²K. Run a full U-value calculation using actual wall construction data to get a precise figure.

Can I insulate a cavity that already has partial-fill rigid board?

Potentially yes — if the residual clear cavity is ≥50 mm, blown insulation (mineral wool or bead) can be injected to fill it. However: the board must be checked for continuity (gaps between boards are common), the cavity must be surveyed by borescope, and the installer must confirm their BBA certificate covers this retrofit application. The thermal gain from filling a 50 mm residual cavity is significant — reducing U-value from approximately 0.30–0.40 W/m²K to approximately 0.18–0.22 W/m²K.

Regulations & Standards