Three main materials are used for cavity wall insulation in the UK: polystyrene beads, mineral wool, and foam. Here's how they compare and which one most installers will recommend.
Polystyrene beads (EPS)
The most common choice for UK retrofits. Tiny expanded polystyrene beads mixed with adhesive, blown into the cavity through drilled holes.
Performance
Thermal performance: good (lambda value around 0.032-0.035 W/mK). The beads fill the cavity completely and don't settle over time.
Cost
Cheapest option. Most of the £800 to £1,500 quotes you'll get for cavity fill are for polystyrene beads.
Pros
- Low cost
- Good thermal performance
- Won't settle or degrade
- Water-resistant (doesn't absorb moisture)
Cons
- Lower fire resistance than mineral wool (though still meets UK regs)
- Made from plastic (not a natural material if that matters to you)
Mineral wool
Rock wool or glass wool fibres blown into the cavity. More expensive than beads but preferred in some situations.
Performance
Thermal performance: similar to beads (lambda around 0.032-0.037 W/mK). Fire performance is better (mineral wool is non-combustible).
Cost
About 10-20% more expensive than polystyrene beads. Expect to pay £900 to £1,800 for a typical semi.
Pros
- Excellent fire resistance (Class A1 non-combustible)
- Breathable (lets moisture vapour pass through)
- Natural material (if using rock wool)
Cons
- More expensive than beads
- Can settle slightly over time (though modern blown systems minimise this)
- Absorbs moisture if the cavity gets wet (dries out again, but it's a consideration)
Foam (urea-formaldehyde or polyurethane)
Liquid foam pumped into the cavity, where it expands and sets. Rarely used for new installs in 2026, but you might have it if your house was insulated in the 1980s or 1990s.
Urea-formaldehyde foam (UF)
Common in the 1980s, mostly phased out now. Concerns about formaldehyde off-gassing led to it falling out of favour. If your house already has it and it's in good condition (no shrinkage or gaps), leave it. Don't add more.
Polyurethane foam
Still used occasionally for specialist jobs (narrow cavities, partial fills around window reveals). Expands to fill awkward spaces. More expensive than beads or wool.
Which type should you choose?
For most UK homes, polystyrene beads are the best choice:
- Lowest cost
- Good thermal performance
- Long lifespan (25+ years)
- Won't settle or degrade
Choose mineral wool if:
- Your house is a flat or in a building where fire rating is critical
- You prefer natural materials over plastic
- You have a heritage building where breathability matters (stone walls, lime mortar)
Avoid foam unless a specialist installer recommends it for a specific reason (very narrow cavity, awkward access).
What installers usually recommend
Most installers will quote polystyrene beads unless you ask for something else. If you want mineral wool, say so when getting quotes. Expect to pay 10-20% more.
The thermal performance difference is minimal. Both beads and mineral wool will cut your heat loss through walls by around 60-70%. The choice comes down to cost, fire performance, and personal preference.
Sources
- Building Research Establishment (2018), "Cavity wall insulation materials: performance comparison", BRE Trust