Solid wall insulation is essential for retrofitting pre-1920s properties and many buildings constructed up to the 1980s. The choice between external wall insulation (EWI) and internal wall insulation (IWI) affects thermal performance, building fabric, costs and project delivery. This guide outlines the key considerations for PAS2035 retrofit coordination.
Solid masonry walls—without cavity insulation—account for significant heat loss in UK housing stock. Unlike cavity walls where insulation can be blown in, solid walls require either surface-applied systems or interior interventions. The decision impacts energy performance, moisture management, structural considerations and long-term maintenance requirements.
EWI involves applying insulation board directly to the external surface of the wall, then finishing with render, cladding or other weatherproof coatings. The insulation layer sits outside the thermal mass of the building, with the masonry remaining warm.
EWI systems typically use expanded polystyrene (EPS), mineral wool or polyurethane boards (80–150mm thickness). Render finishes must be breathable to allow moisture escape. Mechanical fixing combined with adhesive ensures durability. Detailing around openings, eaves and service penetrations is critical for airtightness and weather resistance.
Key point: EWI is particularly effective for detached and semi-detached properties where external access is straightforward. Terraced properties require neighbours' involvement, making coordination essential under PAS2035 whole-house assessment principles.
IWI applies insulation to the interior surface of external walls, typically using rigid boards, semi-rigid batts or blown-in materials. A vapour control layer prevents moisture migration into the insulation and masonry.
IWI requires rigorous vapour control and airtightness detailing. Materials include rigid phenolic or polyisocyanurate boards, mineral wool or cellulose batts. Vapour barriers must be continuous and properly sealed at junctions. Internal surfaces need careful design to avoid condensation, particularly in bathrooms and kitchens. Service routing (electrics, pipes) must be planned to avoid notching insulation.
EWI is inherently lower risk for moisture in UK maritime climates. IWI demands stricter vapour control design and is better suited to drier internal conditions.
Detached/semi-detached: EWI generally preferred where access and planning permit. Terraced/mid-terrace: IWI avoids neighbour coordination; EWI requires shared decision-making. Listed/conservation: IWI often more acceptable to local planning, though sympathetic EWI design may gain consent.
EWI has higher capital cost but better long-term energy savings. IWI suits tighter budgets and staged delivery but may not achieve equivalent performance gains.
Under PAS2035, the decision should reflect whole-building assessment findings. Coordinated retrofit should consider interactions with ventilation strategy, heating system upgrades and airtightness. EWI typically aligns with whole-fabric approaches; IWI requires careful integration with humidity management and controlled ventilation.
Both systems demand experienced contractors and rigorous quality control. Workmanship directly affects thermal and moisture performance. Thermal imaging, airtightness testing and post-installation inspections verify standards. PAS2035 coordination should include clear specification, staged inspections and defect rectification protocols.
EWI offers superior thermal performance and lower moisture risk but demands higher investment and planning coordination. IWI suits budget constraints and phased delivery but requires meticulous vapour control design. The optimal choice depends on property type, planning constraints, budget, moisture climate and whole-house retrofit strategy. Early decision-making within PAS2035 assessment ensures compatibility with wider retrofit objectives.
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