Insulating basement walls that are most below grade. Zone 5, and using a mix of rigid foam board and then framing 2x4 walls and insulating between the studs with R11 or R13 (whatever is on sale and easily obtainable) kraft-faced fiberglass insulation. To be safe should 1.5" of XPS be used before the 2x4 studs? 1.5" XPS is special order while 1" is in stock locally. Wondering if it's worth the wait and extra cost to order 1.5" thick foam before framing the walls out.
Everything I am reading is pointing towards R5 minimum and then framing out and getting mineral wool (way too expensive) or fiberglass installed after the R5 foam, while studies say that XPS loses some R value meaning it would drop below R5 in 20 years.