Rigid foam board insulation — high R-value, low profile, moisture-resistant

Polyiso has the highest R-value per inch but costs more and loses some performance in very cold temperatures. XPS offers the best moisture resistance — extruded boards absorb no more than 0.3% water, versus 2-4% for expanded (EPS) boards — making it ideal for below-grade use. EPS is the most cost-effective for above-grade applications.

Its high moisture resistance makes foam board a strong choice in damp-prone areas: inside a basement against the foundation, on exterior foundation walls, in crawl spaces, and on exterior wall retrofits. It also complements batt insulation, acting as an air and vapor barrier in wall assemblies.

Rigid foam provides high insulating value at a relatively low thickness, which makes it easy to combine with other insulation types to leverage multiple properties at once. It also has very low water absorption — generally under 4%, and as low as 0.3% for extruded (XPS) boards — and stops thermal bridging when applied as continuous insulation.

Initial project cost typically runs $1,500-$10,000 depending on material, thickness, and area. Rigid foam costs more upfront than cellulose or batts but less than spray foam. Energize CT rebates of $2.00 per square foot can cover 75-100% of the cost on eligible projects.

Attics (against the underside of the roof), exterior and stud walls, basement foundation walls, ceilings in unconditioned spaces, crawl space walls and floors, around ducts, beneath floors over unheated garages, and on the exterior of foundation walls. Each area targets a specific source of heat loss or moisture intrusion.

Boards are measured and cut precisely to fit, then applied directly to foundation walls, framed walls, or between rafters and joists. All seams and edges are sealed with tape or spray foam to prevent air leaks, and in exterior applications the foam is covered with a weather-appropriate barrier to meet local building codes.

Yes. Rigid foam works well as a continuous layer over batt insulation, providing both an air/vapor barrier and additional thermal resistance without adding significant thickness — a common way to upgrade existing wall assemblies.

It is the recommended starting point. A Home Energy Solutions assessment identifies where rigid foam will cut the most energy loss and qualifies your project for Energize CT and IRA rebates. The assessment is a $40 service cost and is free if you are eligible.