Retrofitting Foundation Walls: A Guide to Choosing the Right Insulation

Comparing Exterior and Interior Foundation Insulation Approaches

Retrofitting foundation walls involves different considerations depending on whether insulation is installed on the exterior or interior. Exterior insulation is generally preferred because it addresses moisture control, reduces thermal bridging, and helps prevent freeze-thaw damage. However, it involves significant excavation, making it costlier and more labor-intensive. In contrast, interior insulation can lead to moisture problems if not carefully managed. Moisture trapped in the foundation wall can freeze, leading to structural damage. Properly assessing existing moisture conditions is essential when considering interior insulation.

When selecting foam board for exterior foundation insulation, extruded polystyrene (XPS) is typically recommended due to its balance of moisture resistance, durability, and compressive strength. XPS features a closed-cell structure that limits water absorption and maintains its insulating properties even when exposed to soil moisture, making it ideal for below-grade applications. Compared to other foam types like expanded polystyrene (EPS) or polyisocyanurate, XPS performs better under continuous moisture exposure and soil pressure, ensuring long-term effectiveness. Green Insulation in Worcester, MA is a supplier of reclaimed/recycled EPS, XPS, and polyiso.

Exterior Insulation for Foundation Walls

Exterior insulation is typically the preferred approach due to its ability to manage moisture and reduce thermal bridging. Installing rigid foam insulation like XPS or polyiso along the exterior prevents moisture from entering the foundation, protects against freeze-thaw cycles, and improves overall energy efficiency. A common approach involves applying a waterproofing membrane to the foundation wall, followed by rigid insulation, and then protecting it with aluminum coil stock. XPS is favored for this application due to its excellent moisture resistance and compressive strength. Retrofitting existing foundations involves excavation and integration with proper drainage systems, which can be labor-intensive but provides long-term benefits.

Interior Insulation for Foundation Walls

Interior insulation is more practical for existing homes but requires careful moisture management to prevent mold and structural damage. For foundations with moisture issues, a typical assembly includes EPS foam directly against the foundation wall. The EPS serves as a moisture-resistant barrier. Studs are then installed directly against the foam at 24-inch centers, with 1/2-inch drywall finished with latex paint for added mold resistance. Closed-cell spray foam is another option that combines insulation with an effective air and moisture barrier. Proper ventilation systems are critical for interior retrofits to manage humidity and prevent condensation issues.

Choosing the Right Insulation Material

For below-grade applications, XPS is ideal due to its low water absorption and high compressive strength, while EPS is effective and more economical for interior applications. Polyiso offers high R-values but is best suited for above-grade or protected environments. Choosing the right material depends on moisture exposure, thermal needs, and whether the insulation is applied inside or outside.

For all your insulation needs, Green Insulation offers high-quality EPS, XPS, and polyiso solutions. Whether you are retrofitting your foundation walls from the exterior or interior, choosing the right material is critical to achieving long-term energy efficiency and moisture protection. Contact Green Insulation to discuss the best insulation products for your project.