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Cathedral ceiling insulation

5grams | Posted in General Questions on

Good Afternoon,

I had posted a previous question regarding insulating a new construction vaulted ceiling (2×12 raftered) in Michigan (zone 5).  Exterior roof is standing seam metal roofing over osb covered with ice & water shield.  There is no exterior insulation, due to our poor planning.

From piecing together your and Dana’s previous recommendations we have decided to make this roof “hot” using 5.5″ closed cell foam sprayed directly to underside of osb sheathing.  We will fill the remaining ceiling cavity with  R-23 rockwool (5.5″) and covered with 1/2″ drywall.

I have deduced from reading many of your recommendations that we should NOT install a vapor barrier below the rockwool.  Is this correct?  Do we need any kind of vapor retarder, either a film or in the paint?  Do you see any problems with our plan?

Thank you for your help!

Katie G.

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  1. GBA Editor
    Martin Holladay | | #1

    Your plan will work. You don't need an interior vapor retarder or vapor barrier. However, you should strive to make your ceiling airtight.

    For more information, see "Flash-and-Batt Insulation."

  2. 5grams | | #2

    Thank you, Martin!

  3. Expert Member
    Peter Yost | | #3

    Hi Katie -

    And try to keep your interior relative humidity as low as you can for your comfort and the the winter (30% would be best but no higher than 40%).


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