Should I run my PEX loops into the thickened edge of an Alaskan slab?

Pull your tubing back about 8 to 12 inches from the slab edge. Running PEX all the way out into that thickened edge without horizontal wings is just dumping money into the ground, especially in deep winter.

You don't need active tubing directly in the haunch to stop frost jacking. Concrete conducts heat relatively well on its own, so heat from your interior loops will naturally migrate into that perimeter mass. As long as your vertical EPS is continuous and your site drainage keeps meltwater from pooling along that shallow bedrock, the slab mass will keep the ground beneath the footing safely above freezing. Remember, water causes frost heave, not cold concrete alone. If the base stays drained, it can't heave. There is also a huge mechanical reason to hold back: framing. You're going to be drilling anchor bolts or shooting pins along that entire perimeter for your bottom plates. Running loops into that outer zone turns every anchor hole into Russian roulette with a hammer drill. Slicing an active PEX run inside cured concrete is a nightmare you relly don't want to deal with. Keep the loops 10 to 12 inches off the exterior form, but drop your tube spacing down to about 6 inches on the first run right where the floor transitions into the haunch. That gives you a solid **thermal buffer** against the outside edge without cooking the dirt or risking a punctured line during framing.

Honestly, I'd be way more worried about that 2-inch EPS up in Alaska than the tubing offset. Two inches is pretty light for deep freeze country, especally with shallow ledge around. Bedrock's basially a giant heat sink—if your concrete touches bare stone anywhere without a gravel break or foam, it'll suck BTUs right out of that floor 24/7. Make sure you've got clean crushed rock breaking contact with that rock, and definately keep the air gauge on your manifold while you're pouring so you know right away if someone nicks a pipe.

Total newbie here so forgive the dumb question, but how do you keep the PEX from shifting around during the pour? If you're trying to hit that exact 10-inch setback from the forms, does the weight of the wet mix push it out toward the edge anyway? Also, does tightening the spacing down to 6 inches on that edge run make that part of the floor way hotter than the middle, or do you just balance that out at the manifold?

Watch your rebar detailing across that haunch transition, because maintaining a significant temperature delta between an actively heated five-inch interior plate and an unheated, massive perimeter introduces localized differential thermal expansion stresses right along that taper. If you haven't already tied continuous transverse #4 bars trough that slope to reinforce the re-entrant zone where the thickness changes abruptly, the expanding interior concrete will likely induce a longitudinal crack parallel to your edge, regardless of how well you cut your control joints.

Back when we were embedding copper pipe in these floors, you lived and died by your pressure gauge on pour day. Don't just lay that PEX down and forget it untill the boiler guy shows up next month. Hook a manifold up and pump the whole layout to 50 or 60 PSI with compressed air before the ready-mix truck shows up. Leave that gauge right where you can eyeball it during the pour. If somebody gets careless with a rake or a vibrator and nicks a line, you want to hear that hiss immediatly. Fixing a leak while the mud is wet takes five minutes. Fixing it after it kicks takes a jackhammer.