Choosing the best insulation for exterior walls depends on factors like R-value requirements, installation method, moisture resistance, and budget. The most effective options balance thermal performance with practical considerations such as ease of installation and long-term durability. For new construction, fiberglass batts and rigid foam boards are popular due to their cost-effectiveness and consistent R-values (typically R-13 to R-23 for exterior walls). In existing walls, blown-in cellulose and injection foam excel because they can be installed without major renovations, filling gaps completely to create an air seal. Spray foam, while more expensive, offers superior air sealing and moisture control, making it ideal for high-performance homes.

Key findings from the sources:

• Fiberglass batts remain the most common choice for new construction, with R-13 to R-15 for 2x4 walls and R-19 to R-21 for 2x6 walls ^[1][7].
• Spray foam (open-cell) and injection foam are top performers for existing walls, providing air sealing and moisture resistance without requiring drywall removal ^[2][3].
• Rigid foam boards (e.g., polyisocyanurate) are recommended for continuous exterior insulation, reducing thermal bridging when installed with staggered seams ^[6].
• Blown-in cellulose is a cost-effective, eco-friendly option for retrofits, though it may settle over time ^[1][2].

Best Insulation Options for Exterior Walls

New Construction: Fiberglass Batts and Rigid Foam Boards

For newly built homes, fiberglass batts and rigid foam boards are the most practical and widely used insulation types. Fiberglass batts fit snugly between wall studs, providing consistent R-values at a lower cost, while rigid foam boards add a continuous insulation layer that minimizes thermal bridging. The Department of Energy emphasizes that proper installation—ensuring no gaps or compression—is critical for achieving the labeled R-value ^[5].

Fiberglass batts are particularly popular due to their affordability and ease of installation. For standard 2x4 walls, R-13 or R-15 batts are recommended, while 2x6 walls can accommodate R-19 or R-21 for higher thermal resistance ^[7]. However, fiberglass can sag over time if not installed correctly, and it requires a vapor barrier in colder climates to prevent moisture issues ^[8]. Rigid foam boards, such as polyisocyanurate or extruded polystyrene (XPS), are often added as an exterior layer to boost R-values and improve air sealing. In the YouTube video featuring builder Scott True, polyisocyanurate foam boards with staggered seams and a foil face are highlighted for their radiant barrier properties and moisture management benefits ^[6].

Key advantages of these options:

• Fiberglass batts:
• Cost-effective, with prices ranging from $0.50 to $1.50 per square foot ^[9].
• Non-combustible and resistant to mold when kept dry ^[5].
• Widely available and compatible with standard wall framing ^[7].
• Rigid foam boards:
• Provide continuous insulation, reducing thermal bridging by up to 40% when installed correctly ^[6].
• Can achieve R-values of R-4 to R-6.5 per inch, depending on the material ^[9].
• Often used in conjunction with other insulation types for maximum efficiency ^[1].

One limitation is that fiberglass batts alone may not achieve the highest R-values needed for passive house standards without additional layers. Rigid foam boards, while effective, require careful sealing at joints to prevent air leakage ^[2].

Existing Walls: Blown-In Cellulose and Injection Foam

Retrofitting insulation into existing exterior walls presents unique challenges, as accessing wall cavities without removing drywall is often necessary. Blown-in cellulose and injection foam are the most effective solutions for these scenarios, as they can be installed through small holes drilled into the wall. Blown-in cellulose, made from recycled paper treated with borate for fire and pest resistance, is favored for its eco-friendliness and ability to fill cavities completely ^[1]. However, it may settle over time, reducing its R-value by up to 20% if not densely packed ^[3].

Injection foam, particularly RetroFoam, is often considered the best option for existing walls because it expands to fill gaps, creates an air seal, and does not degrade over time ^[2]. Unlike cellulose, injection foam adheres to the wall cavity, preventing settling and improving long-term performance. Open-cell spray foam, another popular choice, offers similar air-sealing benefits but requires professional installation due to its expansion properties ^[5].

Key considerations for retrofitting:

• Blown-in cellulose:
• R-value of approximately R-3.5 per inch, comparable to fiberglass but with better soundproofing ^[8].
• Can be installed without major renovations, though it may create dust during installation ^[2].
• More susceptible to moisture absorption if exposed to leaks ^[1].
• Injection foam:
• Achieves R-values of R-3.6 to R-4.0 per inch, with superior air-sealing properties ^[2].
• Does not settle or degrade, ensuring long-term performance ^[3].
• Higher upfront cost (approximately $1.50 to $3.00 per square foot) but offers greater energy savings over time ^[9].

For homeowners prioritizing energy efficiency without extensive remodeling, injection foam is the top recommendation due to its air-sealing capabilities and durability ^[2]. However, those on a tighter budget may opt for blown-in cellulose, provided it is densely packed to minimize settling ^[3].