Spray Foam
Spray foam is a two-part liquid plastic that expands and cures in place when applied. There are two types: open-cell (~R-3.5 per inch, low-density, vapor-permeable, lower cost) and closed-cell (~R-6.5 per inch, high-density, vapor-retarder, higher cost, structural). Spray foam excels at air sealing — a 2-inch closed-cell layer is both insulation and air barrier in one step. The downsides are very high embodied carbon (closed-cell can be 12+ kg CO2e per sq ft of typical wall coverage), reliance on fluorinated blowing agents, and the need for trained applicators.