Mineral wool or polystyrene beads: which cavity fill?

Blown mineral wool is non-combustible and usually a little cheaper; bonded polystyrene bead flows into narrow and uneven cavities more evenly and some bead systems are certified for higher driving-rain exposure. Neither is best everywhere: the wall, its exposure and the building's use decide.

By the Cavity Wall Insulation UK editorial team · checked against the official sources below · last reviewed 19 October 2026

Cutaway sample of a cavity wall filled with bonded white polystyrene beads between brick and block

Side by side

	Blown mineral wool	Bonded polystyrene bead
What it is	Water-repellent glass or stone fibre	EPS beads coated with a binder
Fire	Non-combustible	Combustible; certificate sets limits
Narrow or uneven cavities	Less forgiving	Flows around obstructions
Exposure	Common in sheltered and moderate zones	Some systems certified for higher exposure
Cables in the cavity	No special precautions	PVC cables checked and protected
Typical price	A little lower	A little higher

What about foam?

Old urea-formaldehyde foam is no longer a mainstream choice, and some foam materials need an assessment of formaldehyde emissions under the Building Regulations. Modern polyurethane foam has specialist uses, such as stabilising failing wall ties, rather than general cavity filling.

Related services

Sources: Planning Portal: cavity wall insulation and building regulations; Energy Saving Trust: cavity wall insulation.

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