Wall insulation affects your comfort and energy bills more than most homeowners realize. Walls account for roughly 35 percent of a home’s total thermal envelope, and under-insulated walls bleed conditioned air year-round. Choosing the best type of insulation for walls depends on whether you are working with open framing in new construction, retrofitting finished walls, or adding continuous insulation to the exterior. Each scenario calls for a different material and installation approach.
Fiberglass Batt Insulation
Fiberglass batts dominate residential wall insulation because they are affordable, widely available, and straightforward to install in open stud bays. Standard R-13 batts for 2×4 walls cost $0.25 to $0.40 per square foot. R-19 batts for 2×6 walls run $0.35 to $0.55 per square foot. Pre-cut widths of 15 and 23 inches match standard framing spacing.
Installation quality matters enormously with batts. Compressing a batt to fit around wiring or pipes reduces its R-value because fiberglass works by trapping air in its fibers. Split the batt around obstacles so it maintains full thickness on both sides of the pipe or wire. Gaps as small as 5 percent of the cavity area can reduce effective insulation performance by 25 percent, according to building science research from Oak Ridge National Laboratory.
Mineral Wool (Rockwool) Insulation
Mineral wool batts, sold under brands like Rockwool (formerly Roxul), offer R-15 in a 3.5-inch batt for 2×4 walls and R-23 in a 5.5-inch batt for 2×6 walls. That is a meaningful upgrade over standard fiberglass at a modest price increase of $0.50 to $0.80 per square foot. Mineral wool is denser and more rigid, which means it friction-fits into stud bays without sagging or stapling.
Fire resistance is mineral wool’s standout advantage. It resists temperatures up to 2,150 degrees Fahrenheit without melting, compared to roughly 1,100 degrees Fahrenheit for fiberglass. Mineral wool also repels water rather than absorbing it, maintaining its R-value even in damp conditions. For exterior walls in humid climates or walls adjacent to bathrooms, mineral wool provides an extra layer of moisture resilience.
- R-value per inch: 4.0 to 4.3 (higher than fiberglass at 3.0 to 3.8)
- Sound reduction: STC rating of 45 compared to 39 for standard fiberglass
- Cuts with a serrated bread knife or specialty insulation knife
- Does not itch like fiberglass, though a dust mask is still recommended
Spray Foam Insulation
Open-cell spray foam fills wall cavities completely, expanding to seal every gap, crack, and void around framing, wiring, and plumbing. It provides R-3.6 to R-3.9 per inch and costs $0.35 to $0.55 per board foot installed professionally. A standard 2×4 wall cavity filled with open-cell foam reaches roughly R-13.
Closed-cell spray foam delivers R-6.0 to R-7.0 per inch but costs $1.00 to $1.50 per board foot. At 2 inches thick in a 2×4 wall, it provides R-12 to R-14 while also acting as a vapor barrier and adding structural rigidity to the wall assembly. Closed-cell is the preferred choice for below-grade basement walls and areas prone to flooding.
Spray foam’s air-sealing properties make it the best-performing option in real-world conditions. While fiberglass batts rely on a separately installed air barrier to reach their rated R-value, spray foam is the air barrier. This combined function often makes spray foam walls outperform higher-R-value batt assemblies that have air leakage paths.
Rigid Foam Board Insulation
Rigid foam boards, including XPS (extruded polystyrene), EPS (expanded polystyrene), and polyisocyanurate, are used as continuous insulation on the exterior of wall sheathing. This application eliminates thermal bridging through wood or steel studs, which can reduce wall R-value by 15 to 25 percent.
XPS boards (the pink or blue panels) provide R-5 per inch at $0.30 to $0.60 per square foot. Polyisocyanurate offers R-6 to R-6.5 per inch at $0.40 to $0.80 per square foot. Adding 1 to 2 inches of continuous rigid foam to exterior walls during re-siding or new construction is one of the most cost-effective wall insulation upgrades available.
Blown-In Insulation for Existing Walls
Retrofitting insulation into finished walls without tearing out drywall requires blown-in material. Contractors drill 2-inch holes through the exterior siding or interior drywall at the top of each stud bay, insert a fill hose, and blow dense-pack cellulose or fiberglass into the cavity. The holes are plugged and patched afterward.
Dense-pack cellulose blown into existing 2×4 walls achieves approximately R-13 at a cost of $1.50 to $3.00 per square foot installed. Dense-pack fiberglass achieves similar R-values at a comparable cost. This method provides both insulation and improved air sealing because the densely packed material resists airflow through the wall cavity.
Expect to pay $2,000 to $5,000 to insulate all exterior walls in an average 1,500 square foot home using the drill-and-fill method. The payback period is typically four to seven years in heating-dominated climates based on energy savings of $300 to $700 per year.
Choosing the Right Insulation for Your Project
New construction with open walls gives you the most options. Mineral wool batts offer the best balance of performance, fire safety, and ease of installation. If budget is tight, fiberglass batts remain a solid choice as long as installation is careful. Spray foam makes sense when air sealing is a priority, such as in homes with complex framing or in extreme climates.
Retrofit projects are more constrained. Blown-in dense-pack cellulose or fiberglass is the standard approach for existing finished walls. Adding exterior rigid foam during a re-siding project addresses thermal bridging that cavity insulation alone cannot fix. For the most comprehensive upgrade, combine dense-pack cavity fill with exterior continuous insulation.
Code Requirements and R-Value Targets
The International Residential Code (IRC) sets minimum wall insulation requirements by climate zone. Zones 1 and 2 require a minimum of R-13. Zones 3 through 5 require R-20 or R-13 cavity plus R-5 continuous. Zones 6 through 8 require R-20 plus R-5 continuous or R-13 plus R-10 continuous.
Meeting code minimums is the baseline, not the goal. Going beyond code by one R-value step, for example from R-13 to R-20 in a zone 3 home, adds $0.30 to $0.50 per square foot to wall insulation costs but reduces heating and cooling loads for the life of the building. Given that walls are sealed behind drywall and siding, upgrading insulation later is expensive and disruptive. Invest in the right level now while the walls are accessible.
