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Bringing Attic into the Conditioned Space

Jay M | Posted in Building Code Questions on

I have a roof framed with raised heel trusses and the rafters are made of 2x4s and 2x6s.

If I wanted to bring the entire attic into the conditioned space, is it common to add on to the rafters by nailing another another 2×6 to build out the rafter so I could hold bigger batts in there to meet the R49 requirement?

Thanks

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Replies

  1. Walter Ahlgrim | | #1

    What you are thinking about is field modifying your trusses the only one who can really answer questions is the people that engineered and built your trusses.

    I have to ask why you want to condition your attic make it into living space?

    It seems very unlikely the current ceiling joists are anywhere close to making code as floor joists.

    Walta

  2. Expert Member
    Armando Cobo | | #2

    IMO, you have limited options. To scab 2x6s to top cords of raters is almost impossible to fit between webs, and you would create lots of voids if saddling them... too risky.
    Your best option I can think of, is to spray 4” ccSPF (in CZ5, see IRC Table 806.5) and the rest of your R49 in ocSPF, around 8"-9". That takes care of your condensation control and air sealing between trusses and webs. Of course you could install 4" R20 rigid foam on top of the roof decking, but I’m guessing it’s not an option now, unless this is a new build, and you already installed the roof, or you’re planning to reroof any time soon.

    1. Jay M | | #3

      Thanks Armando - This is actually a new build and my trusses are still be installed. If I were to go with the rigid foam on top, I still have time. Although I would need to find a product that makes the install easy because I imagine my framing team has never insulated a roof before. Does anyone have any recommendations on a easy nailbase to use?

    2. Expert Member
      Armando Cobo | | #4

      There are few things I would consider. If I’m installing shingled roof, you either install a second layer of sheathing as a nailing base, or you need to go the ccSPF and ocSPF under the roof decking route, because you can’t nail shingles on foam. I use the rigid foam on top of the sheathing when installing tile or metal roofs. You could use Zip roof sheathing system, but you need to make sure the framers roll the tape correctly. Just in case, some folks use underlayment on top of the Zip sheathing s a second layer of protection. As far as foams, polyiso is a good choice.
      There are several synthetic underlayment that have become popular, but I would check with your roof installer to what they’re comfortable with or what’s available in your market. Also, if you search on Google for “best rated synthetic roofing underlayment”, you’ll find several websites to guide you. I like to specify Protecto Wrap’s Jiffy Seal Butyl Ice & Water Guard HT and Grace Ice and Water Shield.

      1. Jay M | | #5

        To follow back up with you. I was leaning towards the Zip Roofing Product for my sheathing. I do have some valleys in my roof, so I was thinking of using Ice & Water there and maybe the eves (3') but not for the entire roof.

        Although on Ice and Water, is that even really required because I am using ccSPF on the underside of the roof? That along with the Zip Tape should reduce any air leakage so I shouldn't have to worry about Ice Dams? I should also be hitting my insulation requirements, so is that overkill/insurance at that point?

  3. Walter Ahlgrim | | #6

    I have to ask why you want to condition your attic?

    Why do you want spray foam it is the most expensive way to get an R of insulation on the market?

    Why do you want to move the insulation from the attic floor to the roof and double the surface area that will lose heat and need to be insulated?

    Please tell me you are smart enough to find a way to keep your HVAC equipment and ductwork inside the conditioned space.

    I never understood the appeal of Zip roofing product. What is the up side to justify the higher cost?

    Walta

    1. Jay M | | #7

      Hi Walta,
      Looking to condition the attic to allow for more flexibility in the future in case we need another bedroom for a growing family.

      My architect did not spec any space for the air handler on the main floor (sigh, I wish I caught that sooner)

      The reason why I'm looking at Zip is I have family helping me with this build so things are going very slow. I need something that can withstand the elements while its exposed.

      I don't want to use spray foam at all actually. Half of the attic is 2x10's the other half is 2x6' rafters. I'm only concerned with the 2x6's. I had thought about 4 inches of open cell then I needed a way to extend out the rafter with another 2x6 like in the example attachment. Would something like that work?

      1. Expert Member
        Armando Cobo | | #8

        Please read answer #2 again and the code reference I gave you above. You can NOT install permeable insulation against the roof decking!
        You could build 2" ventilated channels between the truss' top cords using sealed rigid foam or OSB/plwd, but that's a lot of labor and sealing work, and too many chances for mistakes with untrained personnel.

        1. Jay M | | #9

          Thanks Armando - I was under the impression that the open cell would serve as my air impermeable barrier, so only closed cell can serve that function?

          1. Expert Member

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