Mineral Wool

Mineral wool is manufactured from volcanic rock (predominantly silica, with alumina and magnesium oxide) which is blended with coke and limestone and fused at 1500°C in a furnace. The melt runs onto a series of rotating wheels which spin the droplets into fibres; they are then coated with resin binder and water-repellent mineral oil. The fibres fall onto a conveyor belt, where the loose mat is compressed to the required thickness and density, then passed into an oven where the binder is cured; finally, the product is cut into rolls or slabs. Mineral wool is non-combustible, water-repellent, rot-proof and contains no CFCs or HCFCs.

Mineral wool is available in a range of forms dependent on its degree of compression during manufacture and its required use:

• loose for blown cavity insulation;

• mats for insulating lofts, lightweight structures and within timber-framed construction;

• batts (slabs) for complete cavity fill of new masonry;

• semi-rigid slabs for partial cavity fill of new masonry;

• rigid slabs for warm pitched roof and flat roof insulation;

• rigid resin-bonded slabs for floor insulation;

• weather-resistant boards for inverted roofing systems;

• dense pre-painted boards for exterior cladding;

The mats and board materials may be faced with aluminium foil to enhance their thermal properties. Roof slabs may be factory cut to falls or bitumen-faced for torch-on sheet-roofing systems. Floor units are coated with paper when they are to be directly screeded. A resilient floor can be constructed with floor units manufactured from mineral wool slabs, with the fibres orientated vertically rather than horizontally, bonded directly to tongued and grooved flooring-grade chipboard.

(The thermal conductivity of mineral wool products for internal use ranges typically between 0.031 and 0.039 W/m K at 10°C, although products for external use have higher conductivities.)

Mineral wool can be used effectively to attenuate transmitted sound. In lightweight construction, acoustic absorbent quilts are effective for reducing transmitted sound through separating walls when combined with double plasterboard surfaces and a wide airspace, as well as in traditional timber joist floors when combined with a resilient layer between joists and floor finish. Pelletised mineral wool can be used for pugging between floor joists to reduce sound transmission, and is particularly appropriate for upgrading acoustic insulation during refurbishment.

Mineral wool, due to its non-combustibility, is used for manufacture of fire stops to prevent fire spread through voids and cavities, giving fire resistance ratings between 30 and 120 minutes. Mineral wool slabs give typically between 60 minutes' and 4 hours' fire protection to steel. Similar levels of protection can be achieved with sprayed-on mineral wool which may then be coated with a decorative finish.

Ceiling tiles for suspended ceilings manufactured from mineral wool typically provide Class 1 Spread of Flame to BS 476 Part 7 (1997) and Class 0 to Part 6 (1989) on both their decorative and back surfaces. The thermal conductivity of mineral wool suspended ceiling tiles is typically within the range 0.052-0.057 W/m K. Sound attenuation of mineral wool ceiling tiles usually lies within the range 34-6 dB, but depending upon the openness of the tile surface, the sound absorption coefficient may range from 0.1 for smooth tiles, through 0.5 for fissured finishes to 0.95 for open-cell tiles overlaid with 20 mm mineral wool.

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