Most insulation in homes and buildings is made from one of a handful of core materials: spun glass fibers, recycled newspaper, volcanic rock, plastic foam, or increasingly, natural fibers like wool and hemp. What separates them is not just the raw ingredient but the binders, fire retardants, and chemical additives that turn those raw materials into something that slows heat transfer. Here’s what actually goes into each type.
Fiberglass: Recycled Glass and Sand
Fiberglass insulation starts with glass. Up to 85% of the raw material is recycled glass, known in the industry as cullet. The remaining 15% is a mix of sand, limestone, soda, borax, and dolomite. These ingredients are melted together and spun into extremely fine fibers, which trap pockets of still air to resist heat flow.
The fibers alone would fall apart without something holding them together. Traditionally, manufacturers use a binder made from phenol-formaldehyde resin, mixed with urea, silicone compounds, and ammonium sulfate. Formaldehyde-based binders have drawn scrutiny over the years, and many producers have shifted toward alternatives based on acrylic acid or other plant-derived compounds. If you see a brand marketed as “formaldehyde-free,” it typically uses one of these newer acrylic binders instead.
Fiberglass batts deliver roughly 3.1 to 4.0 R-value per inch, depending on the product density. Loose-fill fiberglass blown into an attic settles lower, around 2.2 per inch, while wall-cavity applications reach about 3.2 per inch.
Cellulose: Shredded Newspaper With Fire Retardant
Cellulose insulation is ground or shredded newspaper. It’s one of the simplest insulation products to manufacture, and it meets EPA criteria for recycled-material content. The obvious problem with using paper in walls and attics is fire, so every cellulose product is treated with a fire retardant before it’s sold.
Boric acid has been the standard retardant since 1978, when Congress set minimum flammability requirements for cellulose insulation. It’s effective at preventing flame spread, and it also discourages insects and mold. Some manufacturers add ammonium sulfate as a secondary retardant, though boric acid remains dominant.
Cellulose is almost always installed as loose fill, blown into attics or dense-packed into wall cavities. It delivers about 3.1 R-value per inch in open attic installations and up to 3.7 per inch when packed into walls.
Mineral Wool: Volcanic Rock and Steel Slag
Mineral wool comes in two varieties that are often sold interchangeably. Rock wool is made from natural volcanic rock like basalt or diabase. Slag wool is made from blast furnace slag, a byproduct of steel production. In practice, many products blend both.
Manufacturing is intense. The raw materials are loaded into a furnace with coke fuel and heated to a molten state at roughly 3,000°F. The liquid is then spun into fibers, similar to the process for fiberglass but at much higher temperatures. Because those fibers won’t melt below 2,000°F, mineral wool is inherently fire-resistant without needing added chemical retardants. This makes it a popular choice for fire-rated wall assemblies and areas around chimneys or flues.
Thermal performance is comparable to fiberglass, ranging from about 3.0 to 3.1 R-value per inch. Mineral wool batts are denser and stiffer than fiberglass, which makes them easier to friction-fit into framing cavities without sagging.
Spray Foam: A Two-Part Chemical Reaction
Spray foam insulation is polyurethane plastic created on-site through a chemical reaction. It arrives as two separate liquids. Side A contains isocyanates, highly reactive compounds that form the backbone of the foam. Side B contains a polyol (an alcohol-based compound that reacts with the isocyanates), along with catalysts, flame retardants, blowing agents, and surfactants. When the two sides mix at the spray gun tip, they react within seconds to expand and harden into foam.
The blowing agents are what create the tiny gas-filled cells that give spray foam its insulating power. Older formulations used hydrofluorocarbons, but many manufacturers have switched to hydrofluoroolefins, which have a much lower impact on global warming. Closed-cell spray foam traps these gases in rigid, sealed bubbles and delivers the highest R-value of any common insulation, roughly 6.0 to 6.25 per inch. Open-cell foam uses water as a blowing agent, producing softer foam with a lower R-value of about 3.5 to 3.7 per inch.
Because the chemical reaction produces volatile compounds including amine catalysts, residual blowing agents, and flame retardant vapors, spray foam requires careful ventilation during and after installation. Manufacturers provide specific re-occupancy times for residents, which toxicologists determine based on off-gassing data. This is one of the few insulation types where the installation process itself poses meaningful air-quality concerns for the people living in the building.
Natural Fibers: Wool, Hemp, and Denim
A growing segment of the insulation market uses plant and animal fibers. Sheep’s wool, hemp, and recycled cotton denim are the most common. These products appeal to builders looking for low-embodied-energy materials, though they cost more than conventional options.
Sheep’s wool contains a protein called keratin that can act as a natural binder. In composite panels made from wool and hemp, manufacturers treat the wool with a mild soda solution that causes it to release keratin, essentially gluing the wool and hemp fibers together without synthetic adhesives. Processing is kept minimal to reduce energy use. Like cellulose, natural fiber insulation is typically treated with borates for fire and pest resistance.
Recycled denim insulation follows a similar model: post-consumer cotton fibers are cleaned, shredded, and formed into batts with a borate treatment. Thermal performance for natural fiber products generally falls in the 3.0 to 3.8 R-value per inch range, competitive with fiberglass and cellulose.
Aerogel: Silica at the Extreme End
Aerogel insulation sits at the high-performance end of the market. It’s made from silica, the same compound found in glass and sand, but structured in a way that’s radically different. The manufacturing process replaces the liquid in a silica gel with air, leaving behind a solid that is over 90% empty space. The result is one of the lightest solid materials that exists.
That extreme porosity is what makes aerogel such an effective insulator. Its thermal conductivity is 0.03 watts per meter-kelvin or less, which puts it on par with or better than the gas trapped inside closed-cell spray foam. Aerogel insulation is used in thin blankets for pipe cladding, building retrofits where space is limited, and even thermal protection systems for spacecraft. The tradeoff is cost: aerogel insulation runs several times the price of fiberglass or mineral wool per square foot, which limits it to applications where every fraction of an inch matters.
Airborne Fibers and Installation Safety
Fiberglass and mineral wool both release tiny fibers into the air during handling. These synthetic vitreous fibers cause irritation of the eyes, nose, throat, and lungs on contact. They can also irritate exposed skin. According to the CDC’s Agency for Toxic Substances and Disease Registry, these effects are reversible and disappear shortly after exposure stops. However, animal studies show that prolonged, repeated exposure to high concentrations can lead to inflammation and eventually scarring (fibrosis) in lung tissue.
OSHA sets a workplace exposure limit of 5 milligrams per cubic meter for the respirable fraction of these fibers. For anyone installing fiberglass or mineral wool in a home project, long sleeves, gloves, safety glasses, and an N95 mask are standard precautions. Cellulose insulation also generates heavy dust during blowing, making respiratory protection equally important. Spray foam installation carries chemical exposure risks that go beyond dust, which is why it’s almost always handled by trained contractors with specialized ventilation equipment.