©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
First, this is for residences in Climate zone 5. We have seen several insulation companies recommend a flash and batt system to insulate a roof system in an unfinished attic space. The recommendation is for 3" of CC foam (R-20 to R-23) covered by an R-21 unfaced fiberglass batt for a total nominal R value of ~ R-40. The fiberglass batt is visible from the unfinished attic and according to the insulation company meets the dual role of an ignition barrier and an R-21 thermal break . EPA (energystar) standards state that fiberous insulation be enclosed on all six sides to be effective and if not there is an airbarrier mis-alginment. There are several questions and please identify the authoritative source of you answers.
1) Should the batt be considered part of the thermal layer or be ignored completely.
2) If the exposed batt is part of the thermal layer, how much should the thermal value be degraded (if at all) for being exposed (i.e. no aligned airbarrier on the bottom?
3) Does the pitch of the roof system matter to your answer in #2?
4) If the batt needs to be covered, what would you use and what are the critical properties (air barrier, vapor barrier, etc.)?
5) Would you recommend this strategy and if not what is a better strategy at comparable cost.