I am bidding on a grade beam 4' tall and 1'6" thick on helical anchors and lots of rebar, horizontal and stirrups. Among the usual benefits I would need to insulate it after concrete placement this winter.

I don't know of any ICFs with a 18" core, does anyone? or other ideas to make it easier?

18" core can be achieved with either Eco Block or Nudura, you need loose panels, clips (webs) and joiners, available from both companies.

Word of advice....bring it up slow, like maybe 12-16" per lift or diaphragm the hell out of it with 1 1/8 ply and threaded rod

Integra Spec is also a possibility.

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Formtech and PolyPro claim to be able to also, I can not find any illustrations. Did you find any details? Which do you think would work best/ stronger? Nudura would be less desirable because the 18" height does not come out to 4'.

For Nudura you rip the top block in half and use a height adjuster, wahlaa, 4' wall

You realize no matter whose block you use you are fabricating your own corner blocks

According to Lite-Form's FAQ, "Forming ties are currently available to produce concrete walls 4", 6", 8", 10", 12", 16", 18" and 24" thick."  And the XPS insulation planks are 2" thick, 8" tall, and 8' long, so forming a 4' tall wall/beam is easy.