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Roof insulation

PMWatson18 | Posted in Energy Efficiency and Durability on

Hello,

I’m looking to buy a 1963 house in zone 5 Indianapolis, IN, that has inadequate insulation in the ceilings. Some cathedral ceilings with fiberglass insulation in the 2 X 6 joist and normal 8′ tall ceiling with fiberglass on top of the ceilings. What is the best way to add more insulation to this roof? I think I want to add 4″ of nail base across the entire roof on the outside and tear out the cathedral ceilings plaster and install more insulation there, maybe with some closed cell foam and batts or Roxul. If I add the nail base, do I need to also add in insulation on the underside everywhere even in the non cathedral areas? I want to make the roof non -vented at the same time.  Thanks, Matt

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Replies

  1. user-2310254 | | #1

    I think you are on the right track, but be sure to check out this article. It provides a lot of advice on how to properly insulate a cathedral ceiling:

    https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

  2. GBA Editor
    Brian Pontolilo | | #2

    Hey Matt,

    I also recommend that you read the article Steve shared. If you put all of the insulation above the roof, you next step is to figure out how you are going to air seal. This article will also be helpful: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing

  3. Expert Member
    Dana Dorsett | | #3

    In zone 5 as long as as more than 40% of the total R is the nailbase you can use fiber insulation between the rafters. A typical 4" polyiso nailbase is R20+ , so you'd be good for up to R30 between the rafters. A rock wool R23 designed for 2x6 framing would have plenty of dew point margin, so there is no need for any spray foam on the interior side.

    Even though the center-cavity R would still be a few shy of R49, it would still meet IRC 2018 code minimum on a U-factor basis due to the R20 thermal break over the rafters provided by the nailbase.

  4. PMWatson18 | | #4

    Thanks all for the responses! The 4" nail base and interior rock wool sounds like the easiest combination. A couple of questions that I have after reading the articles, can I still leave the insulation that is there on top of the non-cathedral parts? Based upon the required membrane that needs to go on top of the plywood below the nail base, it looks like I have to tear off the existing shingles to add the membrane, correct? There was one pictures showing adding the nail base directly on top of the shingles, is this an OK process? If so, I think that I would need to check the weight limits with the extra nail base material and additional shingles. Thanks again.

    1. Expert Member
      Dana Dorsett | | #5

      >"... can I still leave the insulation that is there on top of the non-cathedral parts? "

      If there is insulation on the attic floor and you're adding nailbase + 5.5" rock wool to the roof the attic then becomes conditioned space. While the insulation on the attic floor no longer counts toward your code-compliance R, if it's very thick it could affect your dew point control strategy at the roof deck. Using an interior side "smart" vapor retarder such as Certainteed MemBrain or Intello Plus on the interior side of the roof insulation may be needed to protect the roof deck in those situations.

      If just installing the nailbase at the roof deck (and NOT the rock wool between the rafters) in those non-cathedralized ceiling areas it shouldn't be a problem from a roof-deck moisture point of view.

      >"Based upon the required membrane that needs to go on top of the plywood below the nail base, it looks like I have to tear off the existing shingles to add the membrane, correct? There was one pictures showing adding the nail base directly on top of the shingles, is this an OK process?"

      Consult the nailbase manufacturer on the particulars. I'd personally at least strip the shingles no matter what. Whether leaving #15-#30 felt is allowable depends on manufacturer recommendations. If the roof is plank-sheathed some sort of fully adhered membrane makes sense for air-tightness. With plywood roof decks taping the seams before applying the underlayment may be enough.

  5. PMWatson18 | | #6

    I found some "off grade 3.2" nailboard" with 5/8" OSB, would it be best to add 1" under that to get 4.2"? Or add 1" closed cell phone on the inside? Obviously, I will be much better then what is on the house now, but still trying to get code requirements. Thanks, Matt

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