Insulation and soundproofing
Rigid insulation panels: which ones to use for different applications
...foam panels was compared to the same assembly using 2" of EPS foam and was found to be losing 30% more heat in this study (see below)!
XPS - Extruded Polystyrene Foam Panels - The blue, pink or green ones (eg. Styrofoam):
XPS is Rated at R5 per inch, but it will off-gas and lose some insulation performance over time - ...