A subset of green builders have always been grumpy about foam. Such builders look at rigid foam panels and spray foam as suspect products: they are made from petroleum, laced with mysterious chemicals, and impermeable to vapor flow.
Not all green builders agree with this analysis, however. Here’s the opposing view: most new homes include many products derived from petroleum, including drain pipes, bathtubs, window frames, and the plastic [no-glossary]sheathing[/no-glossary] on Romex wiring — so why focus only on insulation? Moreover (the argument goes), there’s nothing wrong with using petroleum-derived insulation; in fact, converting oil into insulation makes more sense than burning the stuff to drive our cars.
Regardless of whether you are a “foam is evil” builder or a “foam is a useful product” builder, you probably agree that it’s handy to have a range of available products to solve construction quandaries. That’s why it’s good news for builders that mineral wool insulation can be installed on the exterior side of wall sheathing, just like polyiso.
Almost everyone agrees: it’s a good idea to keep wall sheathing warm
The thermal performance of a wall is greatly improved when wall sheathing is protected by an exterior layer of insulation. Exterior wall insulation keeps wood warm, keeps wood dry, reduces the condensation potential, reduces thermal bridging, and probably increases the durability of the wall.
However, some builders worry about the use of rigid foam to keep stud bays warm — not only because “foam is evil,” but because foam does not allow the wall assembly to dry to the exterior. (Although this concern is unwarranted, it exists.) Such builders have often wondered whether rigid mineral wool panels (for example, insulation panels made by Roxul or Thermafiber) could be sandwiched between wall sheathing and vertical…