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Best way to insulate my conditioned unvented attic roof rafters/wall

Lostnmyhome | Posted in General Questions on
My name is Tyler a DIY’r, I have a question that I’m hoping you can answer for me. 

I want your recommendation on how I should economically insulate my 3rd floor unvented attic space ceiling that I’m converting to livable space located in the Cincinnati, Ohio area. 

The house is a 130-year old Italianate two-story brick with the entire 3rd floor as attic & has box gutters. Ceiling is roughly 1000sqft. As it sits now the front of the house is a 9:12 pitch vaulted ceiling with 2×6 rafters, 20” to 24” O.C. With 1” x 12”, then 7/16 OSB and titanium 30 UDL synthetic felt. The right rear side of the house is a 9:12 with 2×6 rafters, 16. O.C. then 7/16 OSB and the titanium felt. On the left rear side of the house, I added a dormer with a 1.5:12 pitch with 2×6 rafters, 16” O.C., 7/16 OSB and Carlisle Ice&Water Shield. The dormer walls are 2×6, tyvek, and hardie siding.
The 9:12 roof will be getting the Image 2 S.S. concealed fastener metal roof. The 1.5:12 roof will be getting  1.5” V.S. Concealed fastener metal panels. 

It will have its own HVAC system(mini-split) up there.

I have read at least 30-40 articles and thousands of comments on this topic from GBA, Building Science, Fine Home Building, 475 and others. It all has my head turning. Any suggestions would be greatly appreciated, happy to send pictures if needed. 

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  1. Laura62584 | | #1

    Good freaking luck! Lol Im doing my unvented ceilings as we speak. Ive done months of research, read every article and thought I had a good plan and theres always someone who thinks it will fail. Im just going for it and hoping it works out. Because I already have a new metal roof on the house and diffusion vents and exterior foam board arent an option for me at this point. Im using 2” iso foam board foil faced against my Shiplap with one-part can foam around the edges to seal it up. Ill use Rockwool batts to fill the rest of the cavity, 3.5”, And do a 1” foam board (unfaced) along the underside of the rafters for thermal bridging. On the inside of that a vapor retarder will allow any moisture thats gets into the cavity dry toward the interior. Im debating how to do the inside of the ridge since thats the area thats most likely to rot. Im debating doing just Rockwool and the unfaced foam board with the vapor retarder to allow the area to dry inward... its a total quess tho cuz i have no other options right now.. lol Good luck! Im following this thread..

    1. Lostnmyhome | | #2

      I hear you laura62584!
      Luckily I haven’t put my roof on yet. I’m leaning towards putting 3” XPS foam panels(R-15) over my existing exterior underlayment , then5/8 OSB, new underlayment then my metal roof. Next doing 5.5”rockwool (R-23)in the interior rafter bays then fire retardant drywall & paint. In all I should have roughly an R-38 in total & drying towards the interior if needed.

      Can someone comment on if this is a good/correct procedure!!
      Thanks Tyler

      1. Laura62584 | | #6

        Things keep changing as I go along and I read more and more info on techniques people have used.. I like the idea of the thermal bridging foam on the underside of the rafters only being as wide as the rafters to keep the pocket open for drying potential. But for me that would bring my R value down to 28 (30 is code here). Im also now debating the vapor retarder. Alot of articles say you dont need it... Im wondering if I do or not.. Ugh. If I only put 1” strips of foam under the rafters rather than full sheets and use the R-15 rockwool i have I would have a 1/2” gap between the rockwool and the vapor retarder. Im not sure if that would invite moisture or not. I wish I had put foam on my roof deck but sadly I was talked out of it. I am pretty confident that my air sealing is going to be good enough on mine to keep it from getting damaged. Just trying to dial in the details now.

  2. GBA Editor
    Brian Pontolilo | | #3

    Hi Tyler.

    Two common and safe approaches to an unvented roof assembly are to install rigid foam insulation above the roof deck and then fill the cavities with fibrous insulation, or working from the inside, to install a layer of closed cell spray foam against the roof sheathing followed by fibrous insulation.

    In the latter case, you'll likely need to find a way to deepen your rafter bays to meet R-49, which is the current IRC minimum in your area, I believe, and a good target R-value for a retrofit. If you can deepen the rafter bays in a way that mitigates thermal bridging, perhaps with strips of rigid foam, even better.

    An interior air barrier is important with these assemblies, but they need to be able to dry inward, so a vapor retarder is not necessary.

    You can use closed-cell spray foam alone, but it is economically and environmentally costly.

    In both of these approaches, the layer of impermeable insulation must have enough R-value to provide condensation control, which would be R-20 in a roof insulated to IRC minimums in your area, I believe.

    These are pretty simple assemblies and are all explained well here: How to Build an Insulated Cathedral Ceiling.

    Where it gets tricky is making it work in an existing roof with limited space, and with the exterior rigid foam option, finding a contractor willing to do the work for a reasonable price (it's just not common enough yet).

  3. Lostnmyhome | | #4

    Thanks for the reply Brian. I have read that article before & many others. It is why I stalled putting on my metal roof. Ccspf is not an option, so this is the method I’ve chosen.
    Couple reasons I’m thinking R-15 on the exterior:
    -zone is in between 4(R10)& 5(R20)
    - single 3” layer of XPS panel instead of two 2” panels stacked( home is in a historic district so I can’t do to much altering to the Facade appearance)
    -read an article where at least 27% of the total R value should be on the exterior In my zone
    -building inspector approved my plan

    R-23 on the interior:
    -highest R-value I can get in a limited space
    -highest R-value that allows drying inward
    - ccspf is not an option
    - read an article that stopping air infiltration (barrier) is more important than R-value(????).
    - inspector approved my plan

    I feel like my plan will work My concern is:
    - will it work?
    -do I have to remove the existing synthetic felt & ice & water shield that is already installed on my 7/16 OSB? (Seems like I shouldn’t- extra layer of protection)
    -Thermal Bridging?
    - I could add 1/4” XPS, but would that
    help? & would that prohibit inward
    -Ridge rot?
    I will have a small triangular opening at
    the ridge where I could fit a little extra
    wool batting, if that would help?

    1. Lostnmyhome | | #5

      Oops Brian, reading into things more I am wrong in my Zone & R-15 on the exterior. Looks like I do have to go with at least an R-20. I’m going to go with the two 2” EXP Owens Corning panels stacked to get my R20, then sheathing, shielding, & Metal. Historical people will have to deal with the subtle change. Ccspf is still not an option on the interior so the best R I can get is still 23. A total R-43 looks like the best I can achieve. My concern questions above(previous reply) are still questions?

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