Moisture Trap Between Layers of Rigid Foam Insulation

A homeowner is planning a vented cathedral ceiling using three layers of 2-inch polyiso rigid foam board (a foam insulation with a facing on each side), stacked below a 2-inch air gap that runs from the soffit vents to the roof ridge. Below the foam would sit unfaced fiberglass batts and drywall. The panels he wants to use have small manufactured grooves running the length of each sheet, spaced about every 6 inches. His question: if the grooves in one layer line up with the grooves in the layer below it, they create a tiny enclosed channel between the layers. He's asking whether that hidden gap could trap moisture inside the wall assembly, or whether he's overthinking a minor manufacturing detail.

This is a builder's forum question, not a finished project or product recall. No one has answered it yet, so there's no verdict on whether the grooves actually pose a moisture risk. For a homeowner, the relevant point is more general: rigid foam insulation used in layered assemblies, especially in roofs, needs to be installed with attention to how layers meet so that air and moisture can't get trapped between them. If you're insulating a cathedral ceiling or similar assembly with foam board, it's worth understanding how the specific product's surface features and the assembly's air gaps and venting work together, since small design details like this can matter for long-term durability even when they seem trivial at first glance.