Why wall type matters
Most UK homes have either cavity walls or solid walls. Identifying which you have is the first step because the products, install method, disruption and costs differ.
How to tell what you have (quick checks)
- Build era: post-1920 homes are often cavity and pre-1920 homes are often solid.
- Brick pattern: stretcher bond usually indicates a cavity and visible headers in a regular pattern can indicate solid.
- Wall thickness at a reveal: ~260–300 mm often suggests a cavity and ~220 mm can suggest solid.
If unsure, ask a professional or use a borescope inspection.
What is cavity wall insulation?
Cavity wall insulation fills the gap between the inner and outer masonry leaves with materials such as blown mineral wool, polystyrene beads or foam to reduce heat loss through external walls.
Advantages
- Usually quick to install with low disruption
- Meaningful bill savings over time
- Suits many post-1920 properties with suitable cavities
- Can help improve EPC ratings
What is solid wall insulation?
Solid walls have no cavity, so you insulate either internally with boards such as PIR or insulated plasterboard or externally with an insulation layer plus render or cladding.
Advantages
- Can dramatically cut heat loss and remove cold spots
- Makes older buildings more comfortable and consistent room to room
- Often the right solution for pre-1920 properties and heritage walls subject to consents
Cavity vs Solid Wall Insulation — side-by-side
| Factor | Cavity Wall Insulation | Solid Wall Insulation (Internal / External) |
|---|---|---|
| Typical property age | Post-1920 with suitable cavities | Pre-1920 or any solid masonry |
| Install method | Blown mineral wool, beads or foam into the cavity | IWI: PIR or insulated boards on the inside. EWI: insulation plus render or cladding |
| Disruption | Low to moderate | Moderate (IWI) to high (EWI) |
| Indicative upfront cost | Lower (often hundreds per home, property dependent) | Higher (often several thousands; IWI typically lower than EWI) |
| Thermal impact | High for suitable cavities | Very high when well designed and installed |
| Aesthetics / space | No change to internal space or façade | IWI reduces room size slightly; EWI changes external appearance |
| Payback (indicative) | ~3–5 years in many cases | ~5–10+ years depending on system and bills |
| Best for | Homes with clear, dry cavities | Older or solid walls, hard-to-heat homes, severe cold-spot issues |
Notes: figures are guides only. Actual costs, savings and payback depend on property size, condition, fuel type, tariffs and installation quality.
Which should you choose?
- Newer homes with suitable cavities: cavity wall insulation is usually the simplest win.
- Older or hard-to-heat homes: solid wall insulation delivers the largest thermal upgrade when moisture and detailing are handled correctly.
- Tight spaces or target U-values: PIR boards help reach performance with less thickness on internal linings and roofs.
Good practice and safeguards
- Check walls for moisture issues before insulating and maintain ventilation routes
- Use qualified installers and follow guidance on fixings, vapour control and detailing around openings
- Combine with loft and floor upgrades for the biggest whole-house benefit
Where to buy wall insulation
At Buy Insulation we stock solutions for both cavity and solid walls, including mineral wool, PIR boards and multifoil options.
Ready to make your home more efficient?
Choosing the right wall insulation lowers bills, raises comfort and can improve value. Browse our full range today and take the first step towards a warmer, quieter and more energy-efficient home.
Final thoughts
Understanding the difference between cavity and solid wall insulation helps you select the best route for your property. With the right products and careful installation you will see long-term gains in comfort and efficiency.
