Replies

1. Expert Member
   Dana Dorsett|2018年11月12日下午03:45|#1

   The short answer is "no" on the vapor barrier. I assume there is soffit to ridge venting in each rafter bay?

   Is it a full 12.0", or is it 11.25", like dimensional lumber?

   巴特的设计不是很好地贴合桁架。使用用于钢制框架作用的BATT的I型式式桁架，但适用于尺寸木材afters/托梁间距的Batts在宽度上太害羞了。如果是开放的织带，则需要用某些东西填充空隙，否则它将成为绝缘的热旁路。

   R38 may meet current NC code, but IRC 2012 and later code for US climate zone 4A would be R49, or U0.026 (= R38.5 "whole assembly, after thermal bridging, with credit for air films, wallboard & roof sheathing, etc.).

   Some low density R38s are only good for R37 when compressed to 11.25" in a 2 x 12 bay. Even if your trusses are a full 12.0" there would only be 11.0" after the mandatory 1" air space created by the baffles, so a low-density R38 just isn't going to cut it, even for meeting the letter of NC code.

   岩石羊毛R30为7.25英寸的阁楼，岩石羊毛R15为3.5英寸，当堆叠时，厚度高达R45 @ 10.75英寸。这更接近R49。

   With a continuous layer of 1" foil faced polyiso on the underside of the trusses, long-screwing/nailing the wallboard through the foam would then get you to R51, meeting IRC code on an R-value basis. If it's an 11.25" deep truss the R45 rock wool compresses against the baffle to about R43 ag, and with R6 polyiso it still meets code. If it's a true 12.0" it would work with 3/4" polyiso. Foil faced foam is easy to reliably air seal using foil tapes.

   Yes, foil facers are a true vapor barrier, but it's the ease of air sealing we're after not vapor retardency, and the assembly is relying on soffit to ridge ventilation to keep the roof deck's moisture under control.

2. GBA Editor
   马丁·霍拉迪（Martin Holladay）|2018年11月13日上午05:41|#2

   B. Weaver,
   Dana's answer assumed that you have some kind of trusses. But you described these "trusses" as 2x12 -- so I'm guessing they may be rafters, not trusses.

   In any case, you don't need an interior vapor barrier. As with all cathedral ceilings, you have to pay attention to airtightness. (Air leakage causes far more problems than vapor diffusion.)

   For more information, see"How to Build an Insulated Cathedral Ceiling."

3. Expert Member
   Dana Dorsett|Nov 13, 2018 07:54am|#3

   If it's just a 2x12, not a truss, a high density "cathedral ceiling" R38C fiberglass batt gets you to R38 in the cavity at 10.25", with the necessary inch of air space. But that's still R11 shy of the R49 prescribed IRC 2012 or later codes. A layer of 1.5" foil faced polyiso would be enough to get you there on a perfromance basis, despite adding up to "only" R47 in that stackup. That takes up anogther 3/4" or so out of the headroom from the rock wool solution in my original post here, and is a little bit harder to build, long nailing the ceiling gypsum to the rafters through 1.5" of foam rather than 3/4"- 1" of foam.

4. 
   乔恩·R|Nov 13, 2018 09:11am|#4

   Lstiburek：“在6或更高的气候区域以外的气候区域中，不需要或建议进行内部阁楼组件侧蒸气控制...”