Insulation is made from a wide range of materials, from spun glass and recycled newspaper to petroleum-based foams and natural fibers like sheep’s wool. The specific composition depends on the type of insulation, and each material offers different levels of heat resistance, durability, and environmental impact. Here’s what goes into the most common types you’ll find in homes today.
Fiberglass
Fiberglass is the most widely used insulation material in North America. It’s made by melting sand and recycled glass, then spinning the molten mixture into extremely fine fibers, similar to how cotton candy is spun from sugar. These thin glass strands trap pockets of air, which is what actually slows heat transfer. A binding resin holds the fibers together in their familiar pink or yellow batt form.
Fiberglass batts provide about R-3.14 per inch of thickness. (R-value measures how well a material resists heat flow; higher is better.) That means a standard 3.5-inch wall cavity filled with fiberglass batts gives you roughly R-11, while a 6-inch attic batt reaches around R-19. Fiberglass is also available as loose-fill that can be blown into attics and wall cavities.
Cellulose
Cellulose insulation is 80% post-consumer recycled newsprint by weight. The remaining 20% consists of borate compounds, which are non-toxic mineral salts that serve triple duty: they make the paper resistant to fire, repel insects, and inhibit mold growth. The newspaper is shredded and fiberized into a dense, fluffy material that’s typically blown into attics or dense-packed into wall cavities.
Performance-wise, cellulose is nearly identical to fiberglass in an attic setting, at about R-3.13 per inch. When dense-packed into walls, it performs slightly better at R-3.70 per inch, partly because the tightly packed fibers reduce air movement more effectively. Its high recycled content makes it one of the more environmentally friendly conventional options.
Spray Foam
Spray foam insulation is a chemical product created when two liquid components are mixed on-site. One component (called the “A side”) contains isocyanate, a reactive chemical compound. The other (the “B side”) contains a polyol resin. When these two liquids meet at the tip of a spray gun, they react instantly, expanding into a rigid or semi-rigid foam that hardens in seconds and bonds directly to the surface it’s sprayed on.
There are two main types. Open-cell spray foam is softer and spongier, with tiny bubbles that aren’t fully sealed, giving it a lower R-value. Closed-cell spray foam is denser and more rigid, with fully sealed air pockets that deliver roughly R-6.25 per inch, about twice the insulating power of fiberglass. Closed-cell foam also acts as a moisture barrier and adds structural rigidity to walls.
Rigid Foam Boards
Rigid foam boards come in three main varieties, all petroleum-based but manufactured differently.
Expanded polystyrene (EPS) starts as tiny resin beads that resemble table salt. These beads contain about 5% pentane by weight, a gas that acts as a blowing agent. When heated with steam, the beads soften and expand dramatically, then get fused together in a mold. The result is that familiar white foam you see in coffee cups and packaging, but in denser, more uniform sheets for building use.
Extruded polystyrene (XPS) uses the same base resin but takes a completely different manufacturing path. The polystyrene pellets are melted, mixed with a blowing agent and color dye under high pressure, then pushed through a die that shapes them into continuous boards. This process creates a more uniform, closed-cell structure. XPS boards are typically blue, pink, or green depending on the manufacturer.
Polyisocyanurate (polyiso) boards use a chemistry similar to spray foam. They’re made from a thermoset plastic foam sandwiched between foil or fiber facers. Polyiso offers the highest R-value per inch of the three rigid board types, typically around R-5.7 to R-6.5 per inch, making it popular for roof and exterior wall applications where space is limited.
Mineral Wool
Mineral wool insulation comes in two forms: rock wool and slag wool. Rock wool is made by melting basalt (a volcanic rock) at temperatures above 2,900°F and spinning it into fibers, much like fiberglass production. Slag wool uses a byproduct of steel manufacturing instead of natural rock. Both types are naturally fire-resistant without needing added chemicals, and they’re denser than fiberglass, which gives them better soundproofing properties. Mineral wool typically provides R-3.3 to R-4.2 per inch.
Natural and Recycled Alternatives
A growing category of insulation uses agricultural and animal byproducts. Sheep’s wool insulation takes advantage of wool’s natural ability to trap air and absorb moisture. One research approach combines recycled sheep’s wool with hemp fiber in a 50/50 blend, using the wool’s own keratin protein as a natural binder. A soda solution treatment causes the wool fibers to release keratin, which acts like a glue that bonds the wool and hemp together without any synthetic adhesives, creating a 100% natural, recyclable panel.
Denim insulation repurposes post-consumer jeans and cotton scraps into batts that look and install much like fiberglass but without the skin irritation. Cotton fibers are cleaned, treated with a borate-based fire retardant (similar to cellulose), and bound with synthetic fibers to hold their shape. Hemp insulation uses the fibrous outer portion of the hemp stalk, which makes up about 20 to 25% of the plant, and can be formed into batts, boards, or loose-fill.
Radiant Barriers
Radiant barriers work differently from all the insulation types above. Instead of slowing heat conduction through trapped air pockets, they reflect radiant heat, the same type of energy you feel radiating from a hot stove across the room. They consist of a thin layer of highly reflective aluminum foil applied to one or both sides of a substrate material. That substrate can be kraft paper, plastic film, cardboard, oriented strand board, or a specialized air barrier material. Radiant barriers don’t have a traditional R-value because they reduce heat gain through reflection rather than resistance. They’re most effective in hot climates, installed in attics where they reflect solar heat back upward before it can warm the living space below.
How R-Value Compares Across Materials
If you’re choosing between insulation types, R-value per inch is a useful starting point. Here’s how the most common materials stack up:
- Fiberglass batts: R-3.14 per inch
- Blown cellulose (attic): R-3.13 per inch
- Blown cellulose (wall): R-3.70 per inch
- Mineral wool: R-3.3 to R-4.2 per inch
- EPS foam board: R-3.6 to R-4.2 per inch
- XPS foam board: R-4.5 to R-5.0 per inch
- Polyiso foam board: R-5.7 to R-6.5 per inch
- Closed-cell spray foam: R-6.25 per inch
Higher R-value per inch means you need less thickness to achieve the same level of insulation. That matters most in tight spaces like existing wall cavities or low-clearance crawl spaces, where spray foam or rigid boards can deliver significantly more insulation in less room than fiberglass or cellulose. For open attics where depth isn’t a constraint, the cheaper per-square-foot cost of blown cellulose or fiberglass often makes them the more practical choice.