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Using foam board on slant ceiling/roof vs using thicker beams and rockwool

littlebearinthewoods | Posted in Energy Efficiency and Durability on

I am building a 12:12 metal roof on a house with a livable loft. The loft has a bit of a gamble shape and the slant part are framed by (currently in the plan) 2×10 roof rafters. Up about 10ft the ceiling is flat (maybe a 3 foot gap between that and the roof ridge with visible collar ties.   I am using rockwool throughout however I need to achieve R38 (am in zone 5) for ceiling. I am not sure what they consider a slant wall but I assume it will be ceiling and I cannot do that with rockwool on that area. I have two options I am considering:

1. Change the 2×10 to 2×12 and use rockwool everywhere 
2. Use XPS or something similar for the slant area  then continue w rockwool on the actual flat ceiling etc.,,

I am adding an air gap as well.

I am a bit torn on the pros and cons of this and was wondering if anyone had some input on this.

Thanks a bunch

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  1. Expert Member
    PETER Engle | | #1

    If you are under the current (2021) International Residential Code (IRC), your roof should be R60 and your walls R30 in zone 5. Even if your local jurisdiction doesn't enforce these rules yet, you should still shoot for those levels, as they are considered by the current industry as the worst house you should build. Most jurisdictions consider sloped ceiling with roof above to be "ceilings", not "walls," so the R60 would apply.

    There is an allowable reduction in insulation required (to R30) for cathedral ceilings when the rafter spaces are not deep enough for R60, but there are limits to using this approach: The total area of R30 insulation must be less than 500 sf and also less than 20% of the total insulated ceiling area (of the whole house). From your description, your house may not meet these exceptions.

  2. littlebearinthewoods | | #2

    Hmm thank you.I am under IRC 2018 but I will give this some thought as I should indeed increase. Its def less than 500sqft but more than 20% of ceiling. One thought I also had was to add board insulation on the inside against the rafters as well though that sounds like a bit of a pain to deal with from what I read but doable. What would you recommend to achieve R60?

    I've also thought about unvented attic as its a pretty small space - I could then add insulation under roof metal and scrap the air space inside... But even then not sure how I would do rockwool in there. Hmmm

  3. plumb_bob | | #3

    You could use Ijoists as rafters to increase depth and allow more insulation, the Ijoist manufacturer will have a detail sheet on how to use this product as a rafter. And the flange allows for easily installed ventilation baffles.
    Usually a roof is not considered a wall until it is 60 degrees or steeper, a 12/12 is only 45 degrees.

  4. littlebearinthewoods | | #4

    Ah Ive been trying to find that 60/45 info in the IRC 2018. Do you know if it's in there? I know that lately there has been discussion in code on how to deal with curved roofs - i.e. where does wall end roof start.

    1. Expert Member
      PETER Engle | | #5

      It's in Chapter 702.7, specifically Table 702.7.1. The language is odd, as is typical for code-speak. Using a Class 1 or 2 vapor retarder is mandatory in heating climates. Table 702.7.1 allows Class III vapor retarders in certain situations, including when there is an appropriate ratio of continuous insulation on the exterior of the wall. The table gives allowable walls with 2x4 and 2x6 framing, which assumes that these walls are filled with fiberglass batt insulation and also that the walls meet the total R-value specifications in Chapter 11. It is not specifically stated here, but you can safely use the same ratios of interior and exterior insulation with pretty much any wall thickness.

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