In climate zone 5, does a conditioned crawl space need insulation under the poured slab? Can we use rigid foam to insulate our CMU walls?

I read Martin’s article “Building an Unvented Crawl Space” https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space

Martin’s article says that rigid foam can’t be used to insulate stone and mortar walls – I assume he really means actual stones – and that rigid foam won’t work due to how rough/irregular those walls are.  Can you use rigid foam on CMU walls?  They aren’t as smooth as poured concrete but not as irregular as a stone wall.  If we use something like Thermax – does that need to be prepped for it to be an air barrier? Does it need to be adhered to the wall? 

I read “Info-512: Crawlspace Insulation”

https://buildingscience.com/documents/information-sheets/crawlspace-insulation

One line says, “In cold climates or in very low-energy buildings, installing insulation below the ground cover is recommended.”

I also saw the question “BSC crawl space insulation clarification” https://www.greenbuildingadvisor.com/question/bsc-crawl-space-insulation-clarification

Martin responded, “The main reason that continuous horizontal rigid foam is suggested for basement slabs is that the rigid foam reduces the chance of summertime condensation on the slab…”

After reading these articles, I’m still confused about whether or not we need insulation under our below grade poured concrete slab.

Thanks in advance for any responses.

Alan