What is perlite?

Perlite is a volcanic rock heated to around 1,000 °C, at which point the water locked inside it turns to steam and expands the rock to many times its original volume. The result is a white, extremely light granule full of air cells — expanded perlite.

Because it is a purely mineral product, it does not rot, does not burn, and offers no food source for mould or vermin.

Properties

• Thermal conductivity. The lambda value of loose-fill perlite is broadly between 0.040 and 0.060 W/m·K; the exact figure depends on grain size, bulk density and any water-repellent treatment. That makes it a poorer insulator than PIR or closed-cell foam, but comparable with many loose mineral insulation materials.
• Non-combustible. Purely mineral granules sit in the highest reaction-to-fire class.
• Light. The low bulk density suits it to lightweight mortars and fills without adding much weight.
• Moisture behaviour. Perlite is non-capillary; treated grades absorb little water and dry back well. Untreated perlite can hold moisture.

Applications in construction

1. Retrofit cavity wall insulation. The granules are blown into the cavity through drilled holes. The advantage over organic fills is that it is non-combustible and rot-proof. It requires a clean cavity free of mortar droppings and an outer leaf that does not let rain through.
2. Filling voids behind dry linings, in joist zones and around chimneys and flues, where combustibility would be a problem.
3. Perlite concrete and perlite mortar. Lightweight insulating falls and levelling layers, for example on roofs.
4. Perlite plasterboard and insulating plasters, in which the granules act as a lightweight aggregate.
5. As a drainage layer under floors and in roof gardens, thanks to the low weight.

Points to watch

• Settlement. Loose fill can settle over the years; with cavity insulation the blowing technique and achieved density are therefore decisive. Ask the installer about the installed density and the guarantee.
• Dust. Fine dust is released during handling; wear a dust mask and eye protection.
• Not a substitute for board insulation where you need a high insulation value within a thin build-up.

Related terms

• Insulation value
• Cavity wall insulation
• Retrofit insulation
• EPS
• Capillary action

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