Hydronic radiant under subfloor against cold rim joist

This is a builder's forum question, not a news story, so there's no program update or rebate change here. A homeowner in a cold climate (climate zone 7, which includes parts of Alaska and the northern U.S.) is dealing with a construction detail: the rim joist, the band of framing at the outer edge of a floor where it meets the foundation. About 15 joist bays were blocked with engineered wood cutoffs, then sprayed with closed-cell foam insulation, but only about 7/8 inch of foam ended up between the block and the rim joist.

The complication is that the homeowner is also installing hydronic radiant heat, a system that runs hot water through tubing under the subfloor to warm the room above, using a "staple up" method. That system needs insulation batts pushed up against the tubing, and in this case that insulation would sit right against the thin foam layer near the rim joist. The concern is whether adding that heat and insulation in those spots could trap moisture and cause condensation problems inside the wall assembly, since thin foam layers in cold climates don't always block enough moisture from migrating outward.

There's no fix or answer here yet, no product recommendation, and no rebate or program tied to this. It is simply a technical question posted for other builders to weigh in on. For a homeowner, the practical lesson is that rim joist details and insulation thickness matter a lot when combining foam insulation with in-floor radiant heat, especially in very cold regions.