Rigid foam comes in pre-cut panels (usually XPS or EPS). Unlike spray foam which is blown wet and expands, rigid foam is purchased in sheets like plywood. It is heavily utilized during new construction but holds specific secrets for home renovations as well.
Continuous Insulation (CI)
Wood wall studs are terrible at blocking heat (thermal bridging). If you only insulate between the studs, the wood itself transfers heat outside. Rigid foam is nailed on the outside of the house framing to create "Continuous Insulation," completely stopping the stud bridging.
Basements & Crawlspaces
Because XPS rigid foam panels are 100% waterproof, they are the ideal thermal barrier for damp environments like pier-and-beam floors where blown-in fiber would instantly mold.