Roofing Polyiso for Continuous Exterior Wall Insulation

A homeowner in Utah is planning to insulate the outside of a 2x6 wall (cellulose insulation inside, taped Zip sheathing, rigid foam board, then a rain screen gap and siding). They asked whether rigid foam board made for roofs, specifically polyiso sold for roofing use, could work on walls instead of foam labeled for walls. The roofing version costs roughly half as much. Two brands came up: Elevate Isoguard GL and Carlisle InsulBase. Both have a fiber facing rather than foil, which lets some moisture pass through, something the homeowner likes because it lets the wall dry out if it gets damp.

One reply pointed out that this swap has come up before in building circles. The facing on these fiber-faced boards is usually fiberglass-reinforced paper, and it does not hold up well once it gets wet, even when it sits behind a rain screen (the gap and furring strips that let air and water drain behind siding). People have used roofing polyiso on walls anyway. Workarounds mentioned include coating and taping the facing with primer to turn it into an air barrier, or adding a separate weather-resistant barrier over the foam to protect the facing from moisture.

For a homeowner, the takeaway is that cheaper roofing-grade foam board might work on a wall, but the facing's durability against moisture is the open question, and no firm verdict was reached in the discussion.