Any rigid foam installed on the exterior side of wall sheathing needs to be thick enough to keep the sheathing above the dew point during the winter.
2012 IRC still maintains antiquated vapor barrier requirements: Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4.
If you’re building a wall with exterior rigid foam, the wall can no longer dry to the exterior; it needs to be able to dry to the interior.
An interior Class I vapor barrier prevents inward drying, and a Class II vapor retarder reduces inward drying to a considerable extent.
Class III vapor retarder — in other words, ordinary latex paint
The code establishes a minimum prescriptive requirement for walls that mandates a layer of rigid foam that is too thin to keep the wall sheathing above the dew point during the winter. If builders choose to follow this minimum code requirement, wall sheathing can accumulate moisture and eventually rot.
To minimize potential problems, building codes should do a better job of regulating the airtightness of building envelopes.
We still had to structurally account for this vertical load. We designed a 4-foot-on-center wood bracket system at the first-floor rim joist that could support the metal siding by fastening a small steel angle to the face of the wood bracket.
We screwed a perforated metal shelf with a nylon insect screen to support and protect the bottom edge of the rock wool insulation while providing ventilation and weep openings for the rainscreen design
Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior.
If you are building a wall w/Rigid Foam, you should not include interior polyethylene or vinyl wallpaper, nor should you install any closed-cell spray foam between the studs.
If you plan to install a thicker-than-usual layer of fluffy insulation, you'll also need to install a thicker-than-usual layer of rigid foam (to make sure that the proper ratio of rigid foam to fluffy insulation is maintained).
Prevent your sheathing from getting cold enough for “condensation” (moisture accumulation) to be a problem