Expert members and others – GreenBuildingAdvisor

If the air space is confined, I don’t think it would affect the ratios. An area of dead air, such as between two layers of batts, will act as a combination of poor R value per inch (relative to whatever the batts are), and potential convective air flow, but that convective air flow will only act to reduce the effective R value of the air space IF that air space is confined. That means that if you have two layers of R15 batts with a 2″ air space in the middle, even if we don’t cound that air space as any extra R value, the batts themselves are still additive for R30 total in terms of how much heat can get through the assembly as a whole.

If the air space is NOT confined, such as is the case with a vent channel open to the outdoors, then the air space on the interior (towards the living space) acts as building insulation, but the air flow through the vent channel “cancels out” any R value on the exterior side of the air space. Maybe in the summer that exterior R value would help limit attic heating a bit, but that’s about the best you’d ever see from it.

I’ll look at that post to see if it’s a more unique situation.

Bill