Installed performance is what matters. The polyiso insulation vs XPS comparison depends on temperature exposure, moisture conditions, facer selection, and interface requirements.

Thermal performance is rarely a single number. EPS insulation R-value reflects how expanded polystyrene performs as thickness and density change, and how those variables carry through into real assemblies.

Framed walls lose thermal performance in common transition points. Studs, plates, and rim conditions interrupt cavity insulation and create faster heat flow paths through wood members, and those losses accumulate across an elevation as higher heating and cooling demand.