NOTE Nº 0099 · REV B

Wall Insulation Materials: Cavity, Internal & External

Cavity, internal and external wall insulation compared: materials, U-values, costs per m² and condensation risk, for retrofits and for new build.

By The Renovation Notes Team · 11 min read · Published 30 June 2025 · updated within the last month · Sources checked 30 September 2026

Close-up view of a classic red brick wall creating a textured pattern.

In short

Filling an existing 100 mm cavity costs £15–£45/m² installed and typically brings the wall to around 0.30–0.35 W/m²K; internal wall insulation (IWI) runs £50–£110/m² and EWI £80–£160/m², both reaching 0.15–0.30 W/m²K depending on thickness and material.

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.¹ 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. Whether full fill is acceptable depends on exposure to wind-driven rain: Approved Document C gives the maximum recommended exposure zone for each insulated cavity wall construction, and the fill must suit the site’s zone.²

Partial fill leaves a residual clear cavity between the outer leaf and the insulation board, which Approved Document C asks to be at least 50 mm wide.² 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

Cavity fill in an existing building should use a system with current third-party certification, usually a British Board of Agrément (BBA) certificate, fitted by an installer registered with CIGA (the Cavity Insulation Guarantee Agency). The independent 25-year guarantee comes from CIGA rather than the installer.³ Filling a cavity is building work in its own right, so it has to be notified to building control unless the installer is registered with a competent person scheme that covers it.⁴⁵ UKAS suspended the BBA’s accreditation on 26 February 2026, so while that stands the BBA cannot issue new accredited certificates, and a renewal visit falling inside the suspension may not be supportable. A certificate issued before that date remains valid, and an equivalent from another UKAS-accredited certification body, or a European Technical Assessment, carries the same weight.⁶

For new build, the wall has to beat the limiting U-value in Approved Document L Volume 1, which is 0.26 W/m²K (Table 4.1), and then contribute to the dwelling’s target emission and primary energy rates. The 0.18 W/m²K figure everyone quotes for new build is the wall in the notional dwelling of Table 1.1, the reference building those targets are calculated from.⁷ Build to 0.26 and you are lawful but will have to make the difference up elsewhere in the design.

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).⁸ 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.

Vapour-open systems, such as woodfibre board finished with lime plaster, are common in heritage contexts: on older lime-pointed brick or rubble-stone walls, a breathable build-up is often preferred so the wall can keep drying and rising or penetrating moisture is not trapped.⁹

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.⁹ A vapour control layer (VCL) on the warm side of the insulation is essential in most cases. BS 5250:2021 Management of moisture in buildings sets out how to run a hygrothermal analysis, by the Glaser method or by dynamic simulation. It was retitled in 2021; control of condensation in buildings is the old name and belongs to the withdrawn 2011 edition.¹⁰ It is a code of practice rather than a regulation, so what makes the analysis necessary is Requirement C2, which asks that interstitial and surface condensation not cause damage.² 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.⁸ The main constraint is aesthetic: it alters the external appearance and may require planning permission, particularly in Conservation Areas. On article 2(3) land, which includes conservation areas, National Parks and Areas of Outstanding Natural Beauty, cladding any part of the outside of a house in stone, render, timber, plastic or tiles is not permitted development, so it needs planning permission.¹¹

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 and head details must be extended to accommodate the added thickness, this is a significant labour and material cost often underestimated in initial quotes.

BS 8414 is often described as something that kicks in above 11 m. It is not a requirement at any height; it is an alternative route. Approved Document B paragraph 10.3 gives external walls two ways to comply: follow the material provisions in 10.5 to 10.9, or meet the BR 135 performance criteria using full-scale test data from BS 8414-1 or BS 8414-2.¹² Two other heights do the real work. At 11 m or more with a residential purpose, paragraph 10.7 wants insulation and filler material at class A2-s1,d0 or better.¹² At 18 m, regulation 7(2) turns that into a statutory ban on anything below A2-s1,d0 or A1 in the external wall, and the BS 8414 route closes, because paragraph 10.3 applies only to buildings other than those regulation 7(4) describes.¹³

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 EN 13162, 13163 and 13165 are the product standards for the insulants themselves, listed below. There is no current British Standard code of practice for cavity insulation: BS 8208-1, the guide to assessing whether a cavity wall was suitable for filling, was withdrawn in 2009 and never replaced, which is why suitability now rests on the installer’s system certification.¹⁴ BS 8215 is sometimes named here in error; it is the code of practice for damp-proof courses¹⁵
BS 5250:2021, management of moisture in buildings. Note the title: the old control of condensation in buildings belongs to BS 5250:2011+A1:2016, which is withdrawn¹⁰
BS EN 13162: Mineral wool insulation products¹⁶
BS EN 13163: EPS insulation products¹⁷
BS EN 13165: PIR insulation products¹⁸
Approved Document L Volume 1 (2021 edition): the limiting wall U-value is 0.26 for a new dwelling (Table 4.1) and 0.18 for a new wall in an existing dwelling, an extension included (Table 4.2). 0.30 is what an existing solid wall has to reach when it is renovated with internal or external insulation (Table 4.3), and an existing cavity wall being filled has to reach 0.55.⁷ None of those is a consequential improvement, which is a separate duty that bites on larger buildings.¹⁹ 0.18 is also the wall value in the notional dwelling of Table 1.1, which is why it gets quoted for new build even though the limit there is 0.26
PAS 2035: Retrofitting standard for domestic buildings, whole-house assessment and risk management required for ECO4-funded work, as it was for the Great British Insulation Scheme until that closed in March 2026²⁰²¹²²

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.

References (22)

Figures in this note were checked against the sources below on 30 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

1technicalfairEnergy Saving Trust, guidance.RN-54KWR6How to identify a cavity wall and what filling it costs. About 33% of all heat lost from an uninsulated home escapes through the walls. Houses built after the 1920s usually have cavity walls; pre 1920 houses are usually solid; houses from the 1990s onwards usually already have wall insulation. Identification: cavity walls show an even stretcher bond with all bricks laid lengthways, solid walls show an alternating pattern with header ends visible; a brick wall 260mm to 350mm thick is likely a cavity wall, narrower is likely solid, stone walls are solid up to about 500mm thick and earth walls run 300mm to over a metre. Typical installed cost around £2,200 in Great Britain and around £1,000 in Northern Ireland, with payback usually five years or less. Unsuitable where the wall is exposed to extreme weather or flooding, in poor condition, at risk of overheating, or where the cavity is narrower than 50mm. External repairs to brickwork, pointing, roofs, gutters and downpipes must be completed first or damp can be trapped. Underfloor grilles, airbricks, wall vents and trickle vents must not be blocked. Post installation damp is chased through the installer, then the Cavity Insulation Guarantee Agency guarantee or an independent insurance backed guarantee.
2technicalvery strongMinistry of Housing, Communities and Local Government, guidance.RN-YZL3J2The Approved Document for Part C. It sets out how a suspended timber ground floor must be built and ventilated, including the size of the ventilation openings, the requirement for a free air path between opposing walls, and the clearances beneath the floor.
3technicalstrongCIGA, guidance.RN-GCH26LCIGA's home page, stating it provides independent 25-year guarantees for cavity wall insulation fitted by registered installers.
4technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-BP3IWGSchedule 3 item 12 lets registered installers (APHC, BESCA, Certsure, HETAS, NAPIT) self-certify sanitary installations in a dwelling that involve no shared or underground drainage.
5technicalvery strongThe National Archives, legislation.RN-ZCMRHVRegulation 3 of the Building Regulations 2010, the meaning of building work. Lists what counts: erecting or extending a building, providing or extending a controlled service or fitting, material alteration, work required by a material change of use, and inserting insulation into a cavity wall. If a job is not on this list it is not building work, so this is the first place to check whether approval is needed at all.
6technicalstrongUnited Kingdom Accreditation Service.RN-BNGAIDThe live UKAS register of accredited bodies currently under sanction. It records the British Board of Agrement under imposed suspension from 26 February 2026 across all four of its accreditations: ISO/IEC 17065 product certification and ISO/IEC 17021 management systems under reference 0113, ISO/IEC 17020 inspection under 4345, and ISO/IEC 17025 testing under 0357. This is the page to check before repeating any advice that treats a BBA certificate as accredited third party assurance.
7technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
8technicalfairEnergy Saving Trust, guidance.RN-RV8Z6HSolid wall insulation costs and the quality problem. Typical installation £12,000 to £15,000, split as around £15,000 for external wall insulation and around £12,000 for internal. Around 33% of the heat lost from an uninsulated home goes through the walls. Homes built before 1920 are more likely to have solid walls, of brick, stone, earth or timber frame with panels. A registered installer should guarantee the work for 25 years, and any TrustMark solid wall insulation company offers a 25 year guarantee backed either by an insurer or through the Solid Wall Insulation Guarantee Agency. Records that in October 2025 the National Audit Office reported on quality failures in wall insulation under government schemes, and that 98% of homes fitted with external wall insulation under ECO4 and the Great British Insulation Scheme have problems needing fixing, with affected households sent an Ofgem letter carrying a Unique Measure Reference Number in early 2026 and a remediation line on 0333 555 1234 option 1.
9technicalvery strongHistoric England, guidance.RN-P1GORFHistoric England's technical advice on internal wall insulation for solid walls. It warns that warm internal vapour can condense on the cold side of the insulation or on the wall behind it.
10technicalstrongBSI, a standard.RN-8KZI9QBS 5250:2021, the code of practice for managing moisture in buildings. The UK standard covering ventilation, vapour control layers, roof space and wall construction moisture risk, and the interaction between insulation and condensation. The 2021 revision reorganised the guidance considerably, so an older source quoting BS 5250 may be describing a superseded structure.
11technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Z0XQ8VParagraph A.1 sets the householder extension limits: buildings may not cover more than 50 per cent of the curtilage (A.1(b)); a single-storey rear extension may not extend more than 4 metres beyond the rear wall of a detached house or 3 metres for any other house, nor exceed 4 metres in height (A.1(f)), rising to 8 metres and 6 metres under the larger home extension route (A.1(g)); an extension of more than one storey may not extend more than 3 metres beyond the rear wall or be within 7 metres of the rear boundary (A.1(h)); eaves within 2 metres of a boundary may not exceed 3 metres (A.1(i)); a side extension may not exceed 4 metres in height, have more than one storey or be wider than half the width of the original house (A.1(j)). Paragraph A.2 removes cladding and side or multi-storey rear extensions on article 2(3) land such as conservation areas. A.3 requires materials of similar appearance and obscure-glazed, non-opening upper-floor side windows unless the opening part is more than 1.7 metres above the floor, and A.4 sets the neighbour consultation (prior approval) procedure with its 42-day determination period.
12technicalvery strongMinistry of Housing, Communities and Local Government.RN-J0UZDBApproved Document B Volume 1, fire safety in dwellings, 2019 edition with the 2020, 2022 and 2025 amendments and the forthcoming 2026 and 2029 changes collated in. Covers means of warning and escape, internal fire spread through linings and structure, external fire spread and access for the fire service. This is the document behind escape window sizes, protected stairways, the 4.5 metre rule and the fire doors a loft conversion needs.
13technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Z3KFVUThe regulation behind the combustible cladding ban. Regulation 7(2) requires materials in the external walls or specified attachments of a relevant building to achieve European Classification A2-s1, d0 or A1 for reaction to fire, and regulation 7(1A) prohibits relevant metal composite materials in external walls. Regulation 7(4) defines a relevant building as one with a storey at least 18 metres above ground level, excluding roof-top plant areas and storeys consisting only of plant rooms, containing one or more dwellings, an institution, or a room for residential purposes. Regulation 7(3) lists the exempt components: cavity trays between masonry leaves, parts of roofs connected to external walls, door frames and doors, electrical installations, fibre optic cables, insulation and waterproofing below ground or up to 300mm above it, intumescent and fire-stopping materials, membranes, seals, gaskets, fixings, sealants and backer rods, solar shading device components other than the primary shade element, thermal break materials used for Part L compliance, window frames and glass, and balcony floor layers classified A1fl or A2fl-s1 over an imperforate substrate.
14technicalstrongBSI, guidance.RN-X4CJG1the withdrawn guide to assessing a cavity wall's suitability for filling, which Approved Document C still names
15technicalstrongBSI (BSI Knowledge catalogue), a standard.RN-EU4DHIDated 28 February 1991, this contains recommendations for the selection, design and installation of damp proof courses in both solid and cavity masonry construction, and is the code behind DPC practice referenced alongside Approved Document C. Its scope notes mark the boundaries clearly: BS 8215 does not cover waterproofing of underground structures such as basements where water is under hydrostatic pressure (see BS 8102), does not deal with the installation of chemical DPCs (see BS 6576), does not cover control of condensation (see BS 5250) and does not cover surface treatment of masonry with water repellents (see BS 6477). Those cross references are useful for a note that needs to say which standard governs which damp problem.
16technicalstrongBSI (British Standards Institution), a standard.RN-L1P8GVThe product standard for rock and glass mineral wool, the material behind loft roll, cavity batts and acoustic quilt. Published 28 Feb 2013, current but under review, and carries Designated status meaning it is a designated standard for UKCA marking. It specifies requirements for factory made mineral wool products with or without facings or coatings used for the thermal insulation of buildings, manufactured as mat blankets, boards or slabs, and covers product characteristics, testing, evaluation of conformity, marking and labelling. Three limits are worth citing: it does not specify what level of any property a product must reach for a particular application, which is set by regulations instead; products with a declared thermal resistance below 0.25 m2K/W or a declared thermal conductivity above 0.060 W/mK at 10 degrees C fall outside it; and it excludes in situ insulation, covered instead by EN 14064 parts 1 and 2, and building equipment insulation, covered by EN 14303. It also states that the performance of a system incorporating these products is not covered, which is why a full wall build-up needs its own assessment.
17technicalstrongBSI (British Standards Institution), a standard.RN-21YSR6The product standard for EPS, the white bead board used for most external wall insulation systems and under many floor slabs. Published 28 Feb 2013, current but under review. It specifies requirements for factory made expanded polystyrene products with or without rigid or flexible facings or coatings used for the thermal insulation of buildings, made as boards, rolls or other preformed shapes including tapered, tongue and groove, shiplap and profiled, and notes the same products are used for sound insulation and in prefabricated systems and composite panels. It sets classes of dimensional tolerance for length, width, thickness, squareness and flatness, the EPS designation code, sampling and conditioning for thermal tests, and the Assessment and Verification of Constancy of Performance regime of type testing plus factory production control. Products with a declared thermal resistance below 0.25 m2K/W or a declared thermal conductivity above 0.060 W/mK at 10 degrees C are outside its scope, as are in situ products, building equipment and industrial insulation, civil engineering applications and beam and block floor systems.
18technicalstrongBSI (British Standards Institution), a standard.RN-03453VThe product standard for PIR and PUR boards, the rigid foil faced insulation sold under brand names such as Kingspan and Celotex. Published 28 Feb 2013, listed as Current with Designated status for UKCA marking. It specifies requirements for factory made rigid polyurethane foam products with or without facings or coatings used for the thermal insulation of buildings, made in board form, and the page states explicitly that PU includes both PIR and PUR products, which settles the common confusion about whether PIR has its own standard. It covers multi-layered PU insulation products, product characteristics, testing, evaluation of conformity, marking and labelling, dimensional tolerance classes for length, width, thickness, squareness and flatness, the designation code, and the AVCP regime of type testing plus factory production control. Products with a declared thermal resistance below 0.25 m2K/W or declared thermal conductivity above 0.060 W/mK at 10 degrees C are excluded, as are in situ insulation products and insulation of building equipment and industrial installations.
19technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-8CNXVLRegulation 28 of the Building Regulations 2010. Consequential improvements apply only to existing buildings over 1,000 m2 of useful floor area, so they do not reach ordinary houses.
20technicalvery strongOfgem.RN-FPPUSLECO4 is the fourth phase of the Energy Company Obligation across Great Britain. It applies to measures installed from 1 April 2022 and runs until 31 December 2026 following a nine-month extension; ECO3 closed on 31 March 2022 and ECO4 came into force in July 2022. The scheme places a Home Heating Cost Reduction Obligation on medium and large energy suppliers to promote measures that improve the ability of low-income, fuel-poor and vulnerable households to heat their homes, funding insulation and heating upgrades for both homeowners and tenants. Ofgem also lists the Great British Insulation Scheme as a separate obligation on this page.
21technicalvery strongOfgem.RN-RDOIJRThe scheme, formerly known as ECO+, ran from April 2023 to 31 March 2026 and is now closed. It obliged medium and large energy suppliers to deliver measures reducing energy use in the least energy-efficient homes in Great Britain. Eligibility was homes with an EPC rating of D to G in Council Tax bands A to D in England, or A to E in Scotland and Wales, plus low-income and vulnerable households. Unlike ECO4, which funds whole-house packages, GBIS mostly delivered a single insulation measure per home. This is the page to cite when a note has to say a grant route has closed rather than leave a reader chasing it.
22technicalstrongBSI, guidance.RN-DCE677the combined retrofit specification that publicly funded energy efficiency work has to be installed under