Mineral Wool Loft Insulation

What mineral wool is

Mineral wool is the material most people picture when they think of loft insulation: soft rolls or batts of fine fibres that trap air and slow the movement of heat. It comes in two main forms — glass mineral wool, spun from recycled glass and sand, and rock (stone) mineral wool, made from volcanic rock. Both perform the same basic job; rock wool is denser, which can help with sound and fire performance, while glass wool is lighter and often lower cost.

Where it works well

Mineral wool is the default choice for the great majority of loft-floor installations. It is widely available, well understood by installers, and offers strong thermal performance for the money. It suits a straightforward installation in most modern and mid-century homes, and it is the usual material for a top-up over existing wool that is still in good condition.

How it is installed

For a loft floor, the first layer is laid between the joists, then a thicker layer is laid across them at right angles to reduce the heat lost through the timber. Together these usually build up to the 270mm benchmark for wool, though the precise depth depends on the product. Clearances are kept around downlights and flues, the eaves are left clear for airflow, and cold-water pipes and tanks are insulated so they do not freeze once the warm air below is held back.

Thermal, moisture and fire behaviour

Mineral wool is a breathable, non-hygroscopic material — it does not readily soak up water, but if it becomes wet through a roof leak or persistent condensation it loses performance and should be dried out or replaced. It is non-combustible or, in the case of some glass wools, has a very high resistance to fire; the manufacturer’s data sheet gives the classification for a specific product. Good ventilation matters: insulation and airflow are considered together so moisture is not trapped at the eaves.

Boarding, handling and equipment

Wool must not be squashed. If you want to store belongings in the loft, a raised boarding system keeps the boards above the insulation so its depth — and its performance — is preserved. During handling, fine fibres can irritate skin, eyes and airways, so installers use gloves, eye protection and a mask, and ventilate the space.

Sustainability and cost

Glass mineral wool typically contains a high proportion of recycled content. It is among the most cost-effective loft insulation materials, which is a large part of why it is so widely used. As with any material, the installed cost depends on the loft area, access and build-up — see the cost guide — and an assessment gives you an accurate written quotation.