There is no single best insulation material. The right choice depends on where you’re insulating, your climate zone, your budget, and whether you prioritize fire safety, soundproofing, moisture resistance, or pure thermal performance. Fiberglass remains the most popular option for its low cost and versatility, but spray foam, mineral wool, and rigid foam boards each outperform it in specific situations. Here’s how the major materials compare so you can match the right one to your project.
How Insulation Performance Is Measured
R-value is the number that matters most when comparing insulation. It measures resistance to heat flow: the higher the R-value, the better the material insulates. R-value is rated per inch of thickness, so a thinner material with a high R-value per inch can match or beat a thicker material with a lower one.
The Department of Energy recommends different R-values depending on your climate zone. In warmer southern states (zones 1 and 2), attic insulation of R-30 to R-60 is typical. In colder northern climates (zones 6 through 8), attics need R-60 and floors need R-38. Walls generally call for R-13 to R-20 of cavity insulation, often combined with continuous insulation on the exterior in colder zones. Knowing your target R-value narrows your material choices quickly.
Fiberglass: The Budget Standard
Fiberglass batts and rolls cost $0.30 to $1.50 per square foot, making them the cheapest option for most projects. They’re widely available, easy to handle as a DIY project, and fit standard stud cavities. Fiberglass provides roughly R-3.1 to R-3.8 per inch, which means a standard 2×4 wall filled with fiberglass batts delivers about R-13, and a 2×6 wall reaches R-19.
The tradeoff is that fiberglass doesn’t seal air gaps. It relies on being installed tightly against all surfaces, which rarely happens perfectly in real-world framing. It also loses effectiveness when it gets wet, and it can settle in attic applications over time. Under ideal conditions, fiberglass can last 80 to 100 years, but moisture or poor installation can degrade its performance within 15 to 20 years. Unfaced fiberglass is vapor-permeable, so it typically needs a separate vapor retarder in cold climates.
During installation, fiberglass causes skin, eye, and respiratory irritation. Gloves, long sleeves, and a dust mask are essential.
Spray Foam: Best Air Sealing
Spray foam comes in two types, and they perform very differently. Closed-cell spray foam delivers up to R-6.5 per inch, the highest R-value per inch of any common insulation. Open-cell spray foam provides about R-3.7 per inch, similar to fiberglass, but expands to fill every crack and gap in a wall cavity.
Open-cell foam is the better choice for standard wall cavities. It expands to fill the full depth of a 2×4 stud bay and trims flush easily. Closed-cell foam is so dense that installers typically stop at about 3 inches in a 3.5-inch stud cavity, leaving a gap behind the drywall. That exposed portion of the studs increases thermal bridging, meaning more heat escapes through the wood framing. Where closed-cell foam excels is in thin assemblies where you need maximum R-value in minimal space, or in areas prone to flooding, since it resists water absorption.
Spray foam costs more: open-cell runs $0.44 to $0.65 per board foot, while closed-cell costs $1 to $1.50 per board foot. Professional installation is essentially required. The chemical components include isocyanates, which OSHA identifies as the leading chemical cause of work-related asthma. Occupants should stay out of the home during application and for the curing period specified by the installer.
Mineral Wool: Best for Fire and Sound
If fire resistance or soundproofing is a priority, mineral wool (also called rock wool) is the standout. It has a melting point above 2,000°F, earns a flame spread rating of zero and a smoke developed rating of zero. No other common insulation comes close on fire performance. It also improves a wall assembly’s sound transmission class by up to 10 decibels, a noticeable reduction in noise transfer between rooms.
Mineral wool batts are denser and stiffer than fiberglass, which makes them easier to friction-fit into stud bays without sagging. They’re naturally moisture-resistant and won’t grow mold. The R-value per inch is about R-4.2, slightly better than fiberglass. The downside is cost: $1.40 to $4 per square foot, roughly two to three times the price of fiberglass. For interior walls between bedrooms, home theaters, or any shared wall where noise matters, mineral wool is worth the premium.
Rigid Foam Board: Best for Continuous Insulation
Rigid foam boards are used differently than batts or spray foam. Instead of filling stud cavities, they’re typically applied as a continuous layer over the exterior sheathing or on basement walls. This approach eliminates thermal bridging through the framing, which can account for a significant portion of heat loss in a typical wall.
Three types are common. Expanded polystyrene (EPS) is the most affordable at $0.25 to $0.35 per board foot, with an R-value around R-4 per inch. Extruded polystyrene (XPS) costs $0.40 to $0.50 per board foot and delivers about R-5 per inch. Polyisocyanurate offers the highest performance at roughly R-6 per inch but costs more. Both EPS and XPS are Class II vapor retarders, meaning they slow moisture movement without completely blocking it. This makes them useful in wall assemblies where some drying potential is needed.
Rigid foam is particularly effective on basement walls, where moisture control matters most. Basements are prone to water intrusion and humidity, and rigid foam resists moisture far better than fiberglass. Pairing rigid foam with spray foam on the interior side of basement walls is a common approach for finished basements.
Cellulose: Best Recycled Option
Cellulose insulation is made from recycled paper treated with borate for fire and pest resistance. It’s blown into wall cavities or attic floors, filling gaps and irregular spaces more completely than batts. At $0.60 to $2.30 per square foot, it sits between fiberglass and mineral wool on cost.
The main limitation is longevity. Cellulose typically lasts 20 to 30 years before settling reduces its effectiveness. It can also absorb moisture if not properly installed with appropriate vapor management. In attic applications where it’s blown loose over the floor, some settling is inevitable, so installers often add extra depth to compensate. Like unfaced fiberglass, cellulose is vapor-permeable and classified as a Class III vapor retarder.
Natural Materials: Sheep Wool, Hemp, and Denim
For homeowners prioritizing sustainability, several natural and recycled materials are available. Sheep’s wool batts deliver R-13 in a 2×4 wall and R-19 in a 2×6 wall, matching fiberglass. The wool is treated with borate to resist pests, fire, and mold, and it naturally absorbs and releases moisture without losing insulating performance. It costs significantly more than fiberglass, limiting it to buyers willing to pay a premium for a renewable material.
Hemp insulation offers R-values similar to other fibrous insulation types but remains uncommon in the United States. Recycled denim insulation, made from about 85% recycled cotton (often blue jean manufacturing scraps) and 15% plastic fibers, is another option. It’s also treated with borate and uses minimal energy to manufacture. These materials work well but are harder to source and more expensive than mainstream options.
Choosing by Location in Your Home
Your attic is usually the highest-impact place to insulate, since heat rises and escapes through the roof. Blown cellulose or fiberglass batts are the most cost-effective attic floor options. If you’re insulating between rafters in a finished attic, install vent baffles between the insulation and roof decking to maintain airflow, and consider high-density R-30 batts that fit 2×10 framing. Adding rigid foam under the rafters eliminates thermal bridging through the wood.
For exterior walls, the best approach in cold climates is cavity insulation (fiberglass, mineral wool, or open-cell spray foam) combined with continuous rigid foam on the exterior. This two-layer strategy addresses both the stud cavities and the thermal bridging through framing. In milder climates, cavity insulation alone is often sufficient.
Basements demand moisture-resistant materials. Spray foam applied directly to foundation walls is a strong choice for finished basements because it insulates and air-seals in one step. Rigid foam boards work well too. Avoid fiberglass batts against foundation walls, since poor perimeter drainage can saturate the insulation, leading to mold and R-value loss.
Cost Comparison at a Glance
- Fiberglass batts: $0.30 to $1.50 per square foot
- Cellulose (blown): $0.60 to $2.30 per square foot
- Open-cell spray foam: $0.44 to $0.65 per board foot
- Closed-cell spray foam: $1 to $1.50 per board foot
- Mineral wool: $1.40 to $4 per square foot
- EPS rigid foam: $0.25 to $0.35 per board foot
- XPS rigid foam: $0.40 to $0.50 per board foot
Professional installation typically runs $40 to $80 per hour on top of material costs. Fiberglass batts are the easiest DIY installation. Spray foam requires professional equipment and should not be a DIY project due to the chemical exposure risks. Mineral wool and rigid foam fall in between, manageable for experienced DIYers.
For most homeowners on a budget, fiberglass in the walls and blown cellulose or fiberglass in the attic delivers solid performance at the lowest cost. If you can spend more, mineral wool in interior walls and closed-cell spray foam in moisture-prone areas offer meaningful upgrades in comfort, noise reduction, and durability.