The Icynene insulation is too expensive to use everywhere, so it's only on the roof and everywhere else in the house has fiberglass insulation. In this picture of the attic you see where the untrimmed Icynene is used for maximum insulation value, fiberglass for economy, and trimmed Icynene where the cathedral ceiling is.No, it's not much of a Cathedral, that's the term of art for a room's ceiling that is also an exterior wall, and it's always an insulation challenge. In this case, that part of the roof is framed with 2x6 rafters, so there's really not enough insulation between us and the 160-degree roof shingles baking in the summer sun (the roof design is unvented).
The plan is to implement what I call a double radiant barrier below the Icynene. The Icynene will be covered with perforated aluminum foil (actually an aluminized housewrap-like product called RadiantGuard). Then 1" strapping will be applied, then another layer of non-perforated foil underneath the sheetrock. The two foil surfaces have high reflectivity and low emissivity - they reflect radiant heat, and they don't radiate the heat they absorb. This is the same principle as a Thermos bottle, except that the surfaces are separated by air not vacuum.
It's hard to translate the potential benefit of this insulation approach to conventional metrics. Some analyses suggest that a single radiant barrier makes an R20 wall perform like an R30 wall, which is about what we're hoping for. We'll be able to measure the surface temperatures on the foam versus the drywall, and get a sense of its performance.
Addendum: performance measurements in warm weather