Flash and Batt in Alaska: Risks and Concerns

This video discusses the principles of the “flash and batt” insulation method and demonstrates the challenges of installing this type of insulation in Arctic and subarctic environments.
Current U.S. building codes provide guidance as to the minimum amount of closed cell (~2 pounds/cubic foot) spray foam insulation that must be applied to the underside of the roof deck to avoid the condensation issues detailed in this video. (See 2018 IRC R806.5, Unvented attic and unvented enclosed rafter assemblies: codes.iccsafe....) This section states that in climate zone 7 a minimum of R-30 air impermeable insulation must be applied directly to the underside of the roof deck, a number that increases to R-35 in climate zone 8. This translates to a minimum of approximately 5 inches of spray foam insulation in climate zone 7 and 6 inches in zone 8.
Also, the code does not directly address all of the potential air leakage pathways presented by framing-to-framing connections in an assembly of this type. Best practices dictate that all joints in framing members in this roof assembly be air sealed with durable, long-lasting adhesive caulking, although this effort seems to be rarely made. It also remains questionable whether consistent air sealing of the framing members can be achieved in roofs under production-oriented jobsite conditions. Insulating a roof system in this manner also does not address heat losses through thermal bridging at the framing members.
While the flash and batt method enables considerable cost savings in the Lower 48, the more stringent requirements for zones 7 and 8 make it a much less cost-effective method, and increases the appeal of other types of roof systems. As detailed at the end of the video, one such alternative is a roof design that moves both the insulation and vapor control layers above the roof deck.
For further reading we recommend the following article by Allison Bailes on the Green Building Advisor forum: www.greenbuild...
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