Vapor Issue w/ EPS & Mineral Wool in Attic

This isn't a news story, it's a homeowner's question posted to a building-science forum, so there's no update here that affects your house directly. But the situation is a common one worth knowing about if you're ever thinking about finishing attic space.

The person posting owns a 1950s house in Massachusetts and wants to turn part of an unheated attic into a small office before a bigger roof renovation happens in a few years. Because the rafters are shallow, they're planning to combine mineral wool batt insulation with rigid foam board on both the inside and the outside of that insulation. The worry is that foam board often comes with a foil facing that blocks moisture from passing through, and having that kind of barrier on both sides of the mineral wool could trap moisture inside the wall or ceiling assembly instead of letting it dry out. That matters because trapped moisture in a roof or wall cavity can eventually lead to mold or wood rot.

For most homeowners, the lesson isn't the technical fix, it's the principle: insulation layers need a way for moisture to escape, usually by making sure at least one side of an assembly stays "vapor open" rather than sealed on both faces. If you're ever finishing an attic, especially an older one with limited ventilation, this is the kind of detail worth asking a contractor or energy auditor about before work begins, since getting it wrong can cause hidden damage long after the drywall goes up.