Understanding Your Wall Type
The first question is whether your home has a cavity wall or a solid wall.
Cavity walls were introduced into mainstream residential construction in the 1920s and became standard from the 1940s onwards. They consist of two skins of brick (or a brick outer and block inner) with a gap of 50–100mm between them.
Solid walls are typical of Victorian and Edwardian properties, and some older inter-war houses. The outer and inner leaf are the same mass of brick or stone — there is no cavity.
A rough guide: if your house was built before 1920, it almost certainly has solid walls. If it was built after 1945, it almost certainly has a cavity. Properties from the 1920s–1930s may be either.
You can check by measuring the wall thickness at a window opening. A cavity wall is typically 300mm or more, with two visible brick courses. A solid wall is typically 220mm (9 inches — one full brick length).
Cavity Wall Insulation
How It Works
Cavity wall insulation fills the existing gap between the two skins of masonry. The most common materials are:
- Mineral wool fibre (blown in)
- EPS (polystyrene) beads (blown in)
- Polyurethane foam (injected)
The installer drills small holes through the outer leaf at intervals, blows in the insulation material, and then fills the holes with matching mortar.
Benefits
- Fast (typically 1–2 days for a semi-detached house)
- Low cost (often £500–£1,500 for a typical house)
- No change to external appearance
- No disruption internally
Limitations and Risks
Cavity wall insulation has been controversial due to a significant number of cases where it has caused or worsened damp problems:
- If the cavity is already damp (due to failed pointing, damaged roof, or high exposure), injecting insulation can cause moisture to bridge into the inner leaf
- In high-exposure areas (coastal, high rainfall, west-facing elevations), blown insulation can allow moisture penetration
- Incorrectly specified or installed insulation can cause serious damp problems
Before proceeding with cavity wall insulation:
- Ensure all defects in the external wall (pointing, render, copings) are repaired first
- Consider whether the property is in a high-exposure location
- Use an accredited installer (look for the National Insulation Association or CIGA warranty)
Is Your Cavity Already Insulated?
Many homes built from the 1990s onwards already have insulation fitted in the cavity as standard. Check your EPC or ask a specialist to assess the wall.
External Wall Insulation (EWI)
How It Works
Insulation boards are mechanically fixed and adhesively bonded to the outside of the building. A reinforced render coat is then applied over the insulation, followed by a decorative finish.
Benefits
- Can be applied to solid walls where cavity insulation is not possible
- Significant U-value improvement (from 2.0 to 0.3 W/m²K is achievable)
- Protects existing masonry from further weathering
- Can improve the appearance of tired renders or plain brick
Limitations
- Cost: £120–£250+ per square metre installed, making a whole-house EWI project expensive
- Changes external appearance — requires planning permission in Conservation Areas and for listed buildings
- Complicates window reveals, eaves, and door surrounds — careful detailing required
- Scaffolding is needed throughout
EWI for Rendered Properties
Many post-war rendered properties in London are well-suited to EWI because the render can simply be overcoated or replaced. Brick-faced properties are more complex because the visual character of the brickwork would be lost under a render finish.
Which Is Right for Your Home?
| Home Type | Best Option |
|---|---|
| Post-1945 cavity wall, brick | Cavity wall insulation (check condition first) |
| Pre-1920 solid brick (Victorian/Edwardian) | External wall insulation or internal wall insulation |
| Inter-war rendered property | External wall insulation |
| Listed building or Conservation Area | Internal wall insulation only (EWI likely refused) |
RCB Design & Build
RCB advises on appropriate insulation strategies as part of refurbishment projects. We work with energy assessors and specialist contractors to ensure specifications are correctly matched to the building type and condition.