Internal Wall Insulation: When and How to Use It

Internal wall insulation (IWI) is an increasingly important retrofit technology for UK properties that cannot accommodate external wall insulation (EWI). Understanding when and how to deploy IWI effectively is essential for retrofit coordinators working under PAS2035 guidelines.

Why Consider Internal Wall Insulation?

Internal wall insulation offers a practical solution for properties where external options are limited or unsuitable. Key scenarios include:

• Listed buildings or those in conservation areas with strict external appearance requirements
• Properties with solid brick or stone walls where EWI is impractical
• Terraced housing where party wall agreements complicate external work
• Buildings with complex external features or difficult access
• Properties where the external envelope is already recently refurbished

IWI typically improves U-values from 2.1 W/m²K (uninsulated solid wall) to around 0.3–0.4 W/m²K, depending on insulation thickness and material selection.

Key Considerations Before Installation

Moisture and Condensation Risk

The primary concern with IWI is moisture management. By insulating internally, you move the dew point inward, creating potential for interstitial condensation between the insulation and external wall. This requires:

• A continuous vapour control layer on the warm side of insulation
• Adequate ventilation in the property to manage internal moisture generation
• Assessment of existing damp conditions before work begins

Room Loss and Space Planning

IWI typically reduces usable floor area by 50–150mm per wall, depending on insulation thickness. Retrofit coordinators must work with homeowners to assess:

• Impact on ceiling heights and suitability for habitable rooms
• Effect on window reveals, door frames and skirting boards
• Changes to thermal mass and thermal behaviour

Selecting Appropriate Materials

Several insulation materials are suitable for IWI applications, each with distinct properties:

• Rigid foam boards (polyurethane, phenolic) – high performance, compact (80–150mm for target U-values), excellent vapour control
• Mineral wool – breathable, good acoustic properties, lower cost, typically requires 150–200mm thickness
• Aerogel composites – premium performance in slim profiles, suitable for listed properties, high cost
• Cork or woodfibre – natural alternatives with moderate performance, hygrothermal properties

Material selection must align with the building's characteristics, occupant needs, and performance targets set in the retrofit design stage.

Installation and System Design

Typical IWI System Components

1. Wall surface preparation and repair
2. Insulation boards or batts, typically 100–150mm
3. Continuous vapour control layer (polythene or specialist membranes)
4. Service cavity or counter-battens for services and final finishes
5. Plasterboard and skim or other wall finish

The vapour control layer is critical and must be continuous, sealed at junctions, and positioned on the warm (internal) side of the insulation. Gaps or tears compromise the entire system's effectiveness.

Key Installation Steps

• Detailed pre-works survey documenting wall condition, services, and moisture levels
• Remedial damp treatment if required (rising damp remediation or surface water management)
• Service location and relocation planning (electrical, plumbing, heating)
• Thermal bridging details at junctions with floors, ceilings, and party walls
• Air-tightness testing post-completion to verify vapour layer integrity

Thermal Bridging at Junctions

Effective IWI requires careful detailing at critical junctions to prevent thermal bridges that can undermine whole-house performance:

• Floor-to-wall junctions: Insulation should wrap around floor edges where feasible
• Party walls: Insulation typically stops at the party wall to avoid condensation risk in the uninsulated adjacent property
• Windows and doors: Reveals require careful treatment; recessed frames may need insulation behind or around frames
• Ceiling junctions: Upper floor ceilings need coordinated insulation strategy to minimise heat loss

Compliance with PAS2035

Under PAS2035, IWI installations must be part of a holistic retrofit design that considers whole-building performance, sequencing, and interactions with other building systems. Retrofit coordinators should ensure:

• Pre-retrofit building survey identifies suitability for IWI
• Retrofit design includes detailed thermal and moisture modelling
• Installation specifications meet current Building Regulations (including fire safety for materials)
• Post-works quality assurance confirms vapour control layer integrity and finish quality

Common Challenges and Solutions

Service Routing: Avoid routing new services within the insulation layer. Use surface-mounted trunking or a dedicated service cavity on top of insulation.

Moisture in Existing Walls: Test walls for damp using a moisture meter at multiple points. Address rising damp or penetrating damp before insulation installation.

Acoustic Performance: IWI often improves sound insulation incidentally; if acoustic improvement is a goal, specify materials and details accordingly.

Occupant Comfort: IWI can reduce thermal mass, leading to faster temperature swings in well-insulated properties. Educate occupants about ventilation and heating control adjustments.

When Not to Use Internal Wall Insulation

IWI is unsuitable for:

• Properties with active damp (rising, penetrating, or condensation)
• Buildings with solid stone walls where moisture movement is critical
• Properties with limited ceiling heights where space loss is impractical
• Situations where external appearance cannot be altered and EWI is already ruled out

In these cases, explore alternative retrofit strategies, including targeted external insulation to selected elevations, loft and basement insulation, or heating system upgrade combined with modest fabric improvements.

Internal wall insulation remains a valuable tool in the retrofit coordinator's toolkit when applied thoughtfully to suitable properties with proper design and installation practices.