Insulating existing walls from the outside is a practical solution for improving energy efficiency without major interior disruptions. The most common methods involve injection foam, blown-in cellulose, or rigid foam board installed over the exterior sheathing, each offering distinct advantages depending on the home’s structure and climate. Injection foam (like RetroFoam) is particularly popular for its minimal invasiveness, as it can be installed by drilling small holes through the siding or mortar, while rigid foam board provides continuous insulation when added as an exterior layer. The process varies by siding type—aluminum, vinyl, brick, or stucco—but typically requires professional installation to ensure proper air sealing and moisture control.

Key findings from the sources:

• Injection foam (e.g., RetroFoam) is installed by drilling 2½-inch holes into wall cavities, often without removing drywall or siding ^[3].
• Blown-in cellulose is an eco-friendly, cost-effective option for retrofitting, installed through drilled holes in exterior walls ^[4][7].
• Rigid foam board (e.g., polyiso) can be added as a continuous layer over existing sheathing, achieving R-values up to R-20 and reducing thermal bridging ^[6][10].
• Siding removal may be necessary for some methods, such as installing a vapor barrier or replacing old insulation ^[5].
• Professional assessment is recommended to determine existing insulation levels and avoid moisture issues ^[8].

Methods for Insulating Existing Walls from the Outside

Injection Foam Insulation: Minimal Disruption, High Efficiency

Injection foam is a leading choice for retrofitting insulation into existing walls because it requires minimal structural changes and provides superior air sealing. This method involves drilling small holes—typically 2½ inches in diameter—into the exterior wall cavities, then injecting expanding foam that fills gaps and seals air leaks. RetroFoam, a common brand, highlights that the process can often be completed in a single day, with the foam expanding to fill voids around wiring, plumbing, and framing irregularities ^[3].

The installation approach depends on the siding material:

• Aluminum/vinyl siding: A row of siding is temporarily removed to access the wall cavities for drilling ^[3].
• Brick/stucco: Holes are drilled directly into the mortar joints, which are later patched to maintain aesthetics ^[3].
• Wood siding: Holes may be drilled through the siding itself or by removing a section temporarily ^[3].
• Concrete block walls: Cores are drilled into the blocks to inject foam, improving thermal resistance ^[3].

Advantages of injection foam include:

• High R-value per inch: Closed-cell foam achieves R-6 to R-7, while open-cell ranges from R-3.6 to R-3.8 ^[4].
• Air sealing: Foam expands to fill cracks, reducing drafts and energy loss more effectively than fiberglass or cellulose ^[7].
• Moisture resistance: Closed-cell foam acts as a vapor barrier, reducing condensation risk in walls ^[4].
• Longevity: Properly installed foam insulation can last the lifetime of the home without settling or degrading ^[3].

However, professional installation is critical, as improper application can lead to over-expansion, which may warp siding or drywall. Costs vary but are generally higher than blown-in cellulose, though the energy savings often justify the investment ^[4].

Exterior Rigid Foam Board: Continuous Insulation for Maximum Efficiency

Adding rigid foam board to the exterior of existing walls is a highly effective method for achieving continuous insulation, which eliminates thermal bridging through studs and framing. This approach is detailed in sources like GreenBuildingAdvisor and The Build Show, where experts demonstrate its application over existing sheathing ^[6][10]. The process involves:

1. Assessing the existing wall assembly: Ensuring the sheathing is structurally sound and addressing any moisture issues before installation ^[10].
2. Installing a weather-resistive barrier (WRB): A layer like Tyvek is applied over the sheathing to prevent water intrusion while allowing vapor to escape ^[10].
3. Attaching foam boards: Panels (commonly polyiso or XPS) are fastened to the sheathing with adhesive or mechanical fasteners, overlapping seams to prevent air leaks ^[6].
4. Adding a vented rainscreen: Furring strips create a gap between the foam and new siding, promoting drainage and drying ^[10].
5. Reinstalling or replacing siding: The final layer is attached over the furring strips, completing the assembly ^[6].

Key benefits of this method include:

• High R-value per inch: Polyiso foam boards typically offer R-5.6 to R-6.5 per inch, significantly boosting wall insulation ^[4].
• Thermal bridge elimination: Continuous insulation over studs reduces heat loss by up to 20% compared to cavity-only insulation ^[6].
• Durability: Foam boards resist moisture, mold, and pests, extending the life of the wall assembly ^[10].
• Compatibility with existing structures: Can be applied without removing interior drywall, making it ideal for retrofits ^[6].

Challenges include:

• Cost: Materials and labor for exterior foam board are more expensive than injection foam or blown-in cellulose ^[4].
• Thickness considerations: Adding 2–4 inches of foam may require extending window trim and roof overhangs ^[10].
• Professional installation: Precision is needed to avoid gaps and ensure proper WRB integration ^[6].

This method is particularly recommended for climates with extreme temperatures, where continuous insulation provides the greatest energy savings. For example, the Build Show video demonstrates a system achieving R-20 in a cold climate, with the foam’s aluminum facer also providing a radiant barrier in hot conditions ^[6].