Insulation in homes and buildings typically uses fiberglass, cellulose, mineral wool, foam, or reflective barriers, each suited to different spaces and budgets. The most common choice by far is fiberglass, sold as batts or rolls that fit between wall studs and ceiling joists. But the best material for any project depends on where it’s going, how much thermal resistance you need, and whether moisture is a concern.
Fiberglass, Cellulose, and Mineral Wool
These three materials make up the bulk of residential insulation. Fiberglass comes as flexible batts (pre-cut panels) or rolls that you lay between framing, or as loose fill that gets blown into attics and wall cavities. It provides about R-3.1 per inch in batt form, meaning a standard 3.5-inch wall cavity gets roughly R-11. Blown fiberglass performs slightly differently depending on where it’s installed: around R-2.2 per inch in open attic spaces and R-3.2 per inch when packed into walls.
Cellulose is primarily made from recycled newsprint, treated with fire retardants. It’s blown into place, which makes it especially good for filling irregular spaces and retrofitting older walls without tearing them open. At R-3.7 per inch in walls, it actually outperforms fiberglass in that application.
Mineral wool (also called rock wool or slag wool) is made from molten rock or industrial slag, with about 75% of it coming from recycled industrial material. It comes in both batts and loose fill, performs at roughly R-3.1 per inch, and has a natural advantage over fiberglass: it’s denser and more fire-resistant, and it holds its shape better when exposed to moisture.
Spray Foam Insulation
Spray foam is a liquid that expands on contact and hardens into a solid layer, sealing gaps and cracks that batts and loose fill can’t reach. It comes in two types. Open-cell foam is soft and flexible, with a density of about 0.5 pounds per cubic foot. Its cells are intentionally left open, which makes it lighter and less expensive, but it doesn’t block moisture. Closed-cell foam packs its cells tightly together so air and water can’t penetrate. It’s more than three times denser, at 1.75 pounds per cubic foot or more, and it adds structural rigidity to walls.
Foamed-in-place polyurethane (the chemistry behind most spray foams) delivers around R-6.25 per inch, making it one of the highest-performing options available. That means a 2-inch layer of closed-cell spray foam can match or beat 4 inches of fiberglass. The trade-off is cost: spray foam typically runs several times more per square foot than batts or blown-in materials.
Rigid Foam Boards
Rigid foam panels are lightweight, moisture-resistant sheets used on exterior walls, foundations, roofs, and below concrete slabs. Three types dominate the market:
- Expanded polystyrene (EPS) is the most affordable rigid foam, at about R-4 per inch. It’s widely used in insulated concrete forms and structural insulated panels, and it works in roofs, walls, floors, and below-grade applications.
- Extruded polystyrene (XPS) is denser and more moisture-resistant than EPS, delivering around R-5 per inch. It’s a common choice for walls and below-grade foundations where water contact is likely.
- Polyisocyanurate (polyiso) offers the highest R-value of any standard rigid board, about R-7.2 per inch when foil-faced. It’s most often used in commercial roofing but works well on residential exterior walls too.
Radiant Barriers
Most insulation works by slowing heat that moves through physical contact (conduction) or air movement (convection). Radiant barriers take a different approach: they reflect heat radiation, the same type of energy you feel radiating off a hot road in summer. They’re made of highly reflective aluminum foil applied to a backing material like kraft paper, plastic film, or oriented strand board.
Radiant barriers are most effective in hot climates, installed in attics where they reduce the amount of heat radiating from a sun-baked roof down into living spaces. They don’t have a traditional R-value because they work through a completely different mechanism, but they can meaningfully reduce cooling costs in southern regions.
Natural and Bio-Based Options
A growing segment of the insulation market uses plant-based or animal-based materials. Hemp insulation panels offer excellent breathability and moisture control, allowing walls to handle humidity without trapping dampness. Cork boards are highly durable, naturally resistant to moisture, and harvested from bark that regrows, making them one of the most sustainable options available. Straw panels provide strong thermal and acoustic performance for walls, floors, and roofs, and some products actually store more carbon than they emit during manufacturing. Sheep’s wool and cotton batts also exist as direct replacements for fiberglass in standard framing cavities.
These materials generally cost more than conventional insulation but appeal to builders focused on sustainability, indoor air quality, and reducing a building’s carbon footprint.
How R-Value Helps You Compare
R-value measures how well a material resists heat flow. Higher numbers mean better insulation. Here’s how common materials stack up per inch of thickness:
- Fiberglass batts: R-3.1
- Cellulose (wall): R-3.7
- Mineral wool batts: R-3.1
- Expanded polystyrene (EPS): R-4.0
- Extruded polystyrene (XPS): R-5.0
- Polyurethane spray foam: R-6.25
- Polyisocyanurate (foil-faced): R-7.2
These numbers matter practically because building codes require specific total R-values for walls, attics, and floors based on your climate zone. An attic in a cold northern climate might need R-49 or more, which means about 16 inches of fiberglass or just 7 inches of polyiso. Knowing the R-value per inch helps you figure out how thick your insulation needs to be, and whether a higher-performing material could save space in tight areas.
Industrial and High-Temperature Insulation
Homes aren’t the only buildings that need insulation. Factories, refineries, and power plants use specialized materials to wrap steam pipes, tanks, and equipment that operate at extreme temperatures. Calcium silicate is a common choice for high-temperature steam pipes because of its heat resistance, though it’s brittle and doesn’t handle moisture well. Cellular glass (foam glass) offers better resistance to fire, water, and chemical exposure, making it a go-to for harsh industrial environments like chemical plants.
At the cutting edge, aerogel is one of the most effective thermal insulators ever developed. Silica aerogel has a thermal conductivity as low as 0.017 watts per meter-kelvin, roughly two to three times better than standard foam insulation. It can withstand temperatures up to 900°C without its structure collapsing. In buildings, aerogel shows up as thin insulation mats used to eliminate thermal bridges (spots where heat leaks through framing) and as fillers in specialized double-pane glazing. Its high cost currently limits it to niche applications, but it’s increasingly used where space is tight and performance is critical.
Older Homes and Hazardous Materials
If your home was built before the 1980s, it may contain insulation materials that are no longer considered safe. Roughly one million homes in the United States still have vermiculite attic insulation, a pebble-like mineral that was a popular loose-fill product for decades. A major source of commercial vermiculite came from a mine in Libby, Montana, where the ore was contaminated with naturally occurring asbestos fibers known to cause serious lung disease. That mine is now closed and designated a Superfund cleanup site, but the insulation it produced remains in many attics. If you find granular, accordion-shaped pellets in your attic insulation, it’s worth having the material tested before disturbing it.
Electrical Insulation
The word “insulation” also applies to materials that block the flow of electricity rather than heat. Electrical wires are typically coated in organic materials like resin-based polymers and specialized paints that prevent current from escaping and causing shocks or short circuits. These polymer coatings work well at normal temperatures but break down under extreme heat. For high-temperature applications, ceramic coatings made from aluminum oxide offer far greater heat tolerance, with melting points around 1,500°C. You’ll find ceramic insulation in industrial motors, aerospace wiring, and other environments where plastic coatings wouldn’t survive.