Glass wool

Glass wool is a thermal insulation material created by binding glass fibers into a wool-like texture. This structure traps countless tiny air pockets, and because gases conduct heat poorly compared to solids or liquids, these pockets give the material its insulating power. The insulation is further improved because the small air cells hinder natural convection—the bulk movement of gas driven by temperature differences—since their high surface area relative to volume creates viscous drag that slows airflow. Glass wool comes in rolls or slabs with varying thermal and mechanical properties, and it can also be produced as a spray-on or in-place material.

The same principle of trapping air in small cells is used in other man-made insulators like rock wool, Styrofoam, neoprene foam, and fabrics such as Gore-Tex and polar fleece, as well as in natural materials like down feathers and wool.

To manufacture glass wool, natural sand and recycled glass are mixed and heated to about 1,450 °C to form molten glass. The fiberglass is made by forcing this molten glass through a rapidly spinning metal cup with small holes; centrifugal force pulls the glass through the holes, and the fibers cool in the air. A binder is applied at each fiber intersection to cement them together, providing cohesion and strength. The mat is then heated to around 200 °C to polymerize the resin and calendered for stability, before being cut and packed into rolls or panels.

Glass wool is used as loose-fill material blown into attics, or sprayed with a binder onto surfaces like the underside of structures. It also comes in sheets and panels for insulating flat surfaces such as cavity walls, ceiling tiles, curtain walls, and ducting, as well as for piping and soundproofing. Batts are precut pieces, while blankets come in continuous rolls. Compressing the material reduces its effectiveness, and cutting it to fit obstructions can create air paths that bypass the insulation. For best results, batts can be installed in two perpendicular layers on an unfinished attic floor to prevent heat bridging, and blankets can cover both joists and the spaces between them. Gaps between batts can lead to air infiltration or condensation, so careful installation and vapor barriers are needed; adding a layer of cellulose loose-fill on top can also reduce air infiltration.

Health-wise, fiberglass can irritate the eyes, skin, and respiratory system, causing symptoms like sore throat, hoarseness, cough, and difficulty breathing. It has been linked to disease similar to asbestosis when used in appliances, though scientific evidence indicates it is safe to manufacture, install, and use if recommended work practices are followed to reduce temporary mechanical irritation. However, these practices are not always followed, and the American Lung Association advises that fiberglass insulation should never be left exposed in occupied areas. In 2011, the U.S. National Toxicology Program removed biosoluble glass wool used in home and building insulation from its Report on Carcinogens, and California’s Office of Environmental Health Hazard Assessment modified its Proposition 65 listing to include only inhalable and biopersistent glass wool fibers. The International Agency for Research on Cancer reclassified all fiberglass wools commonly used for thermal and acoustical insulation as not classifiable as to carcinogenicity to humans.