Mineral Wool Roof Insulation

Mineral wool roof boards are rigid insulation made from fibres spun from molten rock (stone wool) or industrial slag, bonded with a binder and compressed to a high density. Glass wool belongs to the same broad family but is rarely used as rigid flat-roof board.

Manufacture and history

Stone wool is produced by melting rock at about 1,600 C (2,900 F) and spinning or blowing the melt into fibres typically 2-6 micrometres thick. Slag wool was made in Wales by Edward Parry in 1840, and commercial production began in Germany in 1871. Rock wool insulation was being produced in Ontario, Canada, by the mid-1930s.

Properties

• Thermal conductivity: about 0.030-0.045 W/(m.K) for mineral wool insulation in general; dense roof boards sit at the higher end.
• Fire: stone wool withstands about 700-850 C (1,300-1,560 F), far above foam plastics, and is used where non-combustible roof build-ups are required.
• Acoustics: the fibrous structure absorbs sound, useful on lightweight steel-deck roofs.
• Stability: boards do not shrink or expand significantly with temperature and are vapour-open.

Use on low-slope roofs

Dense stone wool boards are laid over steel or concrete decks under single-ply, bitumen and liquid membranes, either as the full insulation layer or as a fire-resistant layer and cover board over foam. Tapered stone wool schemes are available for creating falls. Boards with a bitumen-compatible or torchable upper layer are made for bituminous systems.

Considerations

Mineral wool is heavier than foam insulation for the same thermal value, so more thickness or weight is needed. It must be kept dry during installation, and boards need adequate compressive strength for foot traffic. The IARC classifies insulation stone wool and glass wool in Group 3, not classifiable as to carcinogenicity in humans.

Fire toxicity comparisons

Research on insulation fire behaviour has compared mineral wools with foam plastics. A 2011 University of Central Lancashire study, for example, found that polyisocyanurate released considerably more toxic combustion products than stone wool and glass wool, among other materials, which is one reason specifiers consider mineral wool where fire performance is a priority.