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Sealed attic with vented roof?

user-7528760 | Posted in Energy Efficiency and Durability on


I am currently finishing of an unfinished space above our garage and master bed room. I want to get the areas and efficient as possible and am doing all the air sealing and insulation myself. We are using Mitsubishi mini splits as well. I am also using all stone wool insulation. My question is, would adding foam board above the insulation and sealing all seams be beneficial or more work than it’s worth? In other words a sealed attic space, unconditioned, with a ventilated roofline. A sealed unconditioned space above ceiling insulation I have attached pics to show what I am referring to. Any advice would be appreciated.

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  1. user-2310254 | | #1

    I will give this post a bump to take it back to the top of the Q&A list.

  2. GBA Editor
    Deleted | | #2


  3. GBA Editor
    Brian Pontolilo | | #3

    Hi user-7528760.

    It would be good to get your real name. It makes the forum a much friendlier feeling place.

    Without knowing what climate zone you are in, I can't comment on how much insulation you need, but in general, with those materials, your radiant barrier sheathing, and planning for a vented roof, one good approach would be this: Install strips of approximately 1 inch thick blocking below the sheathing running from the top plate to the ridge. Then attach the rigid foam to those strips and seal it in place with canned spray foam. Now you have created an insulated vent chanel. You can fill the remainder of the bays with mineral wool, or any other insulation that you would like. Make sure to air seal the rafter cavities at the top plate and to do a good job air sealing the drywall, particularly those can lights, which are not necessarily a good idea in a cathedral ceiling.

    Of course, that's not the only way to go. I recommend that you read this article: How to Build an Insulated Cathedral Ceiling

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