Summary

Internal wall insulation is chosen over external wall insulation (EWI) when external appearance cannot be changed — Conservation Areas, listed buildings, and properties with party walls on all sides where EWI would be impractical. It is also lower cost per m² than EWI, can be carried out room by room, and does not require scaffolding. However, it carries significantly higher technical risk, primarily from interstitial condensation at joist ends and at the masonry/insulation interface.

The fundamental physics problem with IWI is that it moves the dewpoint plane inward. Before IWI, the solid masonry wall has its outer face cold and its inner face relatively warm; condensation occurs within the masonry near the outside face, where it can dry outward. After IWI, the masonry is now cold throughout — the insulation has removed the heat that previously warmed it. The dewpoint may now fall at or near the masonry/insulation interface. If vapour from the warm interior reaches this cold zone, condensation can occur there. Joist ends embedded in the masonry wall are particularly vulnerable — they are now permanently cold, and any moisture that reaches them can cause wet rot decay.

These risks are manageable with correct design and detailing, but they cannot be ignored. PAS 2035:2019 classifies IWI on solid walls as a High Risk measure, requiring a Retrofit Coordinator to design and oversee the work on funded schemes.

Key Facts

  • IWI thickness — typically 50–100mm for a reasonable thermal improvement; 50mm PIR gives approximately 0.35–0.45 W/m²K on 225mm solid brick; 100mm PIR achieves approximately 0.25–0.30 W/m²K
  • Part L target (existing dwelling) — 0.30 W/m²K for solid walls where practically achievable; IWI at 50–70mm can achieve this
  • PIR board — polyisocyanurate (e.g. Celotex, Kingspan); λ ≈ 0.022–0.023 W/m·K; highest thermal performance per mm; forms a VCL in thin form; fire spread class needs checking
  • Mineral wool stud frame — rock or glass mineral wool between timber studs; λ ≈ 0.035–0.044 W/m·K; requires 70–100mm to match PIR at 50mm; vapour control layer essential on warm side
  • Cork — λ ≈ 0.040–0.045 W/m·K; vapour-open (breathable); does not require a VCL; suitable for breathable heritage construction; more expensive than PIR or mineral wool
  • Wood fibre board — λ ≈ 0.038–0.052 W/m·K; vapour-open; suitable for breathable construction; Steico Interior, Gutex Thermoroom; compatible with lime plaster finishes
  • Condensation risk — IWI on solid masonry is classified as High Risk under PAS 2035; a condensation risk assessment (Glaser or WUFI) is required for funded schemes
  • VCL requirement — essential for PIR and mineral wool IWI systems; must be on the warm side (inside face) of the insulation; foil-backed plasterboard is the standard VCL in domestic IWI
  • Joist end protection — floor joists bearing into the treated masonry wall must be assessed; where they will now sit in a cold zone, they must be insulated around the bearing pocket, or the joist end must be treated with preservative and ventilated
  • Planning implications — IWI does not change external appearance, so Permitted Development applies in most cases; listed buildings require Listed Building Consent for any alteration
  • Floor area reduction — typical 50–100mm IWI reduces internal floor area by approximately 0.1–0.2m² per metre of treated wall
  • PAS 2035:2019 — IWI on solid walls is Medium Risk for vapour-permeable (breathable) systems and High Risk for vapour-closed (PIR/mineral wool with VCL) systems; both require a Retrofit Assessor at minimum

Quick Reference Table

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System Type Lambda Typical Thickness VCL Required Condensation Risk Best For
PIR board (e.g. Celotex) 0.022 W/m·K 50–75mm Yes — critical High without VCL Maximum performance, thin build-up
Mineral wool stud frame 0.035–0.040 W/m·K 70–100mm Yes — critical High without VCL Standard retrofit; service integration
Cork slab 0.040–0.045 W/m·K 60–100mm No — breathable Low-medium Heritage; vapour-open construction
Wood fibre board (e.g. Steico) 0.038–0.050 W/m·K 80–120mm No — breathable Low-medium Heritage; breathable lime plaster finish
Aerogel blanket 0.015–0.017 W/m·K 20–30mm Yes Medium Space-constrained; high-value
Expanded cork board 0.040–0.045 W/m·K 60–80mm No Low Heritage; listed buildings

Detailed Guidance

PIR Board Dry-Lining

Rigid PIR boards (Celotex TB4000, Kingspan Kooltherm K118) are the highest-performance IWI option per millimetre. They are typically fixed to the masonry wall with dot-and-dab adhesive or proprietary adhesive ribbons, with foil-faced plasterboard as an over-board (or as a combined product — most PIR IWI boards have a foil face and an integrally laminated plasterboard outer face).

The foil face of the PIR board serves as the VCL if the board is installed with foil facing inward (toward the room). This is the correct orientation. However, this places the VCL on the inner face of the insulation, which is correct — but joints between boards must be taped with aluminium foil tape to maintain VCL continuity. Any gap in the foil continuity (at services, at floor and ceiling junctions, at window reveals) will allow vapour to penetrate the system and potentially condense on the cold masonry behind.

Services (sockets, light switches, pipes) must not be routed through or behind PIR boards unless a services zone is provided in front of the VCL. A 25–50mm services zone (a second batten layer between the PIR board and the plasterboard finish) keeps penetrations within the warm side of the VCL. Without this, every socket box creates a perforation in the vapour barrier.

At the base of the wall, the PIR board must be terminated above the floor and connected to the floor DPM via a horizontal DPC strip. Bridging across the floor/wall junction must be avoided — if IWI extends below finished floor level, it can bridge the DPC.

At window and door reveals, PIR board should be carried around the reveal as far as possible to reduce the cold bridge area at the frame. Minimum reveal insulation is 25mm, though more is better. Pre-formed corner beads provide a clean finish at inside and outside corners.

Mineral Wool Stud Frame

A timber stud frame (typically 50mm × 100mm studs at 400 or 600mm centres) is built against the masonry wall, insulated with mineral wool between studs, and boarded with foil-backed plasterboard as the inner face. This system is more common where:

  • Services integration is required (stud void allows wiring and plumbing to be run without penetrating the VCL)
  • The masonry surface is uneven (the stud frame bridges irregularities)
  • The wall is to receive a plastered finish (timber studs allow conventional plaster)
  • Ventilation of the void is desired (some designers specify a small gap between the stud insulation and the masonry face to allow limited drying)

The VCL (foil-backed plasterboard or separate PE sheet) must be on the warm side of the insulation — i.e., between the mineral wool and the room interior. This is typically the inner face of the stud frame, not at the masonry face.

Structural timber members (floor joists, lintels, plates) that bear into or through the treated masonry wall require careful detailing. Joist ends in masonry pockets will now be at masonry temperature — i.e., cold and vulnerable. Options are: cut the joist back 50mm from the masonry face, fill the pocket with insulation, and strap to a new metal hanger; or treat joist ends with borate preservative and detail to allow limited ventilation of the pocket.

Breathable Systems — Cork and Wood Fibre

Breathable IWI systems use vapour-open insulation materials — cork, hemp fibre, or wood fibre — in conjunction with lime plaster or other vapour-open finishes. Because the insulation is vapour-open, vapour from the interior can diffuse through the system and into the masonry; a conventional VCL is not appropriate and is typically not used.

The logic of this approach is that the hygroscopic materials in the system — lime plaster, wood fibre, masonry — can buffer moisture, absorbing it when vapour pressure is high and releasing it when conditions are drier. This avoids the accumulation of condensation at a cold interface because the moisture is distributed through the hygroscopic fabric rather than concentrating at a single interface.

This approach is recommended by Historic England for solid masonry walls in historic buildings, where maintaining the breathability of the fabric is important. It is also appropriate where the masonry is known to be subject to periodic dampness (for example, near the base of a solid wall with no DPC) — a vapour-open system allows any moisture that enters to evaporate, whereas a PIR system with a VCL would trap it behind the insulation.

The trade-off is that breathable materials have higher lambda values than PIR, so thicker insulation is needed for the same U-value improvement. Achieving 0.30 W/m²K with wood fibre IWI on 225mm brick typically requires 80–100mm, compared to 50–60mm for PIR.

Condensation Risk Assessment

For all IWI on solid masonry, a condensation risk assessment should be carried out. The Glaser method (BS EN ISO 13788) is the standard tool:

  • Input: material layers, thicknesses, thermal conductivities (λ), and vapour diffusion resistance factors (µ)
  • Method: calculate temperature and vapour pressure profiles through the construction for monthly UK conditions (typically using London or appropriate regional climate data)
  • Output: identification of any condensation plane; calculation of accumulated moisture; assessment of whether summer drying is sufficient to evaporate accumulated moisture

For PIR systems with a correctly installed VCL, the Glaser method typically shows minimal condensation risk because the VCL restricts vapour entry into the cold zone.

For breathable systems (cork, wood fibre), the Glaser method frequently flags apparent failures because it does not account for hygroscopic buffering. In these cases, WUFI dynamic modelling provides a more realistic assessment. A WUFI analysis carried out by a building physicist is recommended for breathable IWI on historic buildings.

PAS 2035 requires that for High Risk measures, the Retrofit Coordinator must specify, and the post-installation assessment must confirm, that moisture and condensation risks have been appropriately managed.

Frequently Asked Questions

Will IWI cause damp problems if the wall has no DPC?

It depends on the insulation type. Vapour-closed systems (PIR with VCL) can conceal rising damp behind the insulation, where it saturates the masonry and may eventually saturate the insulation and cause mould on the warm face. Vapour-open systems (wood fibre, cork) allow moisture to diffuse through; however, sustained rising damp will eventually exceed the buffering capacity of the system. In both cases, the moisture source should be investigated and addressed before IWI is installed. IWI does not treat damp — it can mask or worsen it if the source is not fixed.

How much floor area do I lose?

For 75mm PIR IWI on four walls of a 4m × 5m room, the floor area reduces from 20m² to approximately 18.8m² — a loss of 1.2m², or about 6%. The height of window sills reduces relative to the new wall face (they may need new profiles or infill). Skirting boards must be re-fixed, and light switches and sockets need to be brought forward on mounting boxes. These are significant disruptions that should be costed into the project.

Can IWI be installed without vacating the property?

Yes — IWI is typically carried out room by room, and residents can continue to occupy unaffected rooms. However, the room being treated needs to be fully cleared, and plaster and skirting removal is noisy and dusty. A typical 20–25m² room takes 2–4 days for a two-person team including preparation, insulation fixing, and boarding.

Does IWI affect the room's acoustics?

The addition of a substantial wall lining (typically 75–100mm total) can reduce airborne sound transmission from outside. This may be a welcome co-benefit in urban locations. The impact on internal room acoustics (reverberation) is minimal at the insulation thicknesses typical of IWI.

Regulations & Standards