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Air sealing wall to attic connections

timot1 | Posted in Energy, Heating & Insulation on June 13, 2022 12:37am

What are my available options for air sealing wall to attic connections?  I’m building my house now so everything is at a rough framing stage.  I’m installing “room in attic” trusses spanning 32′.

I’m doing stegowrap for floor.  Then taping edges to walls using Siga’s Rissan tape from floor to wall.  Then using Prosoco Cat 5 liquid flashing on walls.  In attic I was thinking of doing “cut and clobber” rigid insulation under roof leaving a 1″ to 1 1/2″ gap for air flow and topping of with comfortbatt insulation.  If I need additional insulation I may add comfortboard 80 and 1×3 furring strips on the inside of the ceiling joists.   

Alternatively, I was also considering using closed cell spray foam but I don’t like the idea of an unvented attic.

I’m a plywood guy so I’m looking for options other than zip system.

Please let me know if you need any additional info.

Thanks,

Tim

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  1. User avater
    unclemike42 | Jun 13, 2022 07:35am | #1

    The critical data you have left out is location and heating/cooling zone of the house.

    This informs the need for and location of the vapor retarder.

    Are you coating the outside of your wall sheathing with the waterproofing?

    Best plan is to complete the design of attic insulation and ventilation before you start, with attention to R-value, air sealing, and vapor control.

    1. timot1 | Jun 14, 2022 01:42am | #2

      Hi,

      I'm in the PNW (Whidbey Island) so I believe the 4c marine.

      I plan to coat the exterior of the wall sheathing with the waterproofing. I was also considering adding the waterproofing to the exterior of the roof deck if I skipped the closed cell spray foam underneath.

      Thanks

  2. User avater
    unclemike42 | Jun 14, 2022 07:40am | #3

    https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjb5Kq86Kz4AhWPg-AKHSGxCjUQFnoECAYQAQ&url=https%3A%2F%2Fwww.carlisleccw.com%2Fdownload.aspx%3FfileID%3D39&usg=AOvVaw0PqNqQ5tZL6ZBkMC2_fM9J

    Above should link to a report which applies.

    Discussion includes the tricky point that you need insulation on both sides of your vapor barrier. (because you have enough need for heating to have to worry about condensation on the inside of the barrier)

    For the roof, The same concerns apply.

    Placing the air seal on the outside of the sheathing(roof and wall) would enable condensation on the inside surface of the sealant during heating season, which is not where you want any moisture.

    1. timot1 | Jun 14, 2022 11:19am | #4

      Thanks for the article.

      Verifying my wall assembly:
      Okay, so since Prosoco has a perm rating of .063 it's considered a vapor barrier. As a result, I must have an minimum of R7.5 exterior insulation in an R13 2x4 wall assembly.
      What about a 2x6 wall with R19 or R21? Am I still okay with 2" Roxul Comfortboard with an R8 outside my 2x6 walls? Also, because I'm Marine do I still need an interior vapor barrier with 1 perm or less (bottom of page 7 of article).

      Back to Roof:
      So Prosoco on roof deck is out for now since my roof trusses won't be able to handle the additional weight of exterior insulation and an additional layer of plywood with shingles (verified with truss company).

      That means from my Prosoco covered walls I will need to bring the air barrier under the roof deck. I believe the top cord will be a 2x12 so I have about 11 1/4" of cavity to work with.

      Option 1: 19/32 plywood deck, 1.5" air space from eave to ridge, cut an clobbered 2.5" of XPS insulation with "Big Gap" spray foamed edges (This becomes my air barrier), Roxul comfort batt R30

      Option 2: 19/32 plywood deck, 1.5" air space from eave to ridge, EITHER unfaced R38 insulation batt OR Roxul R15 + Roxul R23 comfort batt, 2" of XPS inside ceiling joists with "Big Gap" spray foamed edges effectively becoming my air barrier.

      Please let me know your thoughts. Do you see any issues? Is there a better way?

      Thanks again

      1. greg_e | Jun 15, 2022 09:01am | #6

        I have seen too many sip disasters to trust spray foam as an air barrier. Personally I would be much more comfortable with siga majpell or a similar barrier for the ceiling assembly. I would just omit the foam from option 2 and install the air barrier in its place. This would save money as well.

  3. User avater
    unclemike42 | Jun 14, 2022 01:14pm | #5

    My read of the paper is that you would need a more permeable coating on the outside of the wall to go with a lower permeability interior vapor barrier. (to let vapor escape the assembly to the outside.)

    This would be consistent with your option 2 for the attic. You can consider a sheet membrane instead of the foam board. (I know the truss interior supports complicates this somewhat. tape is your friend)

    You could run the insulation all the way down the roof slope and seal the vapor barrier directly to your wall interior vapor barrier. (leaving the knee-wall area within the conditioned space)

    I also could be missing something.....

    1. timot1 | Jun 15, 2022 11:57am | #7

      So this is what I have so far:
      Still in Zone 4 Marine-

      Wall assembly from exterior:
      Hardieplank, rain screen [non-PT 3/4" utility lumber], 2" Roxul Comfortboard 80, Prosoco Cat 5 (my air barrier), 15/32 CDX plywood, 2x6 stud walls, cavity filled with Roxul R23 Comfortbatt, Drywall, latex paint (my class iii vapor retarder)

      Roof assembly:
      Shingle, 30# felt, 19/32 CDX plywood, roof trusses in cavity [1.5" air space from eave to ridge, unfaced R38 insulation batt, 2.5" of XPS (staggered) inside ceiling joists with "Big Gap" spray foamed edges (I'm trying to reach an R49 in the ceiling), 475 intelloplus (not sure if this is needed- maybe I decide after installing the XPS), 3/4 furring strips, drywall, latex paint.

      Wall to Roof assembly connection:
      Tape prosoco covered wall to EITHER XPS OR 475 intello plus, continue 475 intello plus into room in attic.

      Please note: Prosoco actually has a vapor transmission rating of 14 perm NOT .063 perm (not sure where I got the other number). As a result, each assembly is vapor open. I could switch from xps on the inside of the ceiling to comfortboard and use intello plus.

      Am I posting this discussion to the right forum?

      Thanks Mike and Greg

  4. User avater
    unclemike42 | Jun 15, 2022 02:24pm | #8

    This is as good as any forum. Who shows up when, is interested, and has time to comment is variable.

    If you are looking for a qualified, informed, source to validate your design, you might want to look for someone to pay.

    If you plan to rely on latex paint as your interior vapor retarder, you have to make sure it (the paint/primer) is designed for this outcome. It also is important to properly treat any penetrations in the surface, including switches, outlets and HVAC penetrations.

    We may also be talking past each other as to the details of your design and how you will complete the HVAC envelope based on the details of your truss design. (How you will insulate and air seal attic room walls, and if the floor from there to the outside walls will need special treatment)

    You may want to extend the comfortboard on the outside of your wall sheathing (coated as you describe) up into the attic space and run the rock wool batts over the wall top plates to meet (maintaining the gap below roof decking). This assumes some overhang is in the design, which you will provide with air intakes to feed venting provisions at the ridge.

    1. timot1 | Jun 16, 2022 10:08am | #9

      What's the name of the professional that I could hire? Would it be a heating and cooling company, HVAC company, or insulation company? Do you have a guess as to what something like this would cost (verifying my assemblies)? There's a local insulation company that provides blower door testing. Can I trust that company to give the okay on the assemblies? Or could contacting Prosoco work?

      I'd like as much interior space as possible so I like to insulate and create an air barrier next to roof deck. The bottom roof (in the attached picture).

      I'll have 2' overhangs. Yes I will extend wall insulation over wall top plates (maintaining roof deck gap).

      Thanks,
      Tim

  5. User avater
    unclemike42 | Jun 16, 2022 11:17am | #10

    I like your second option. You can use this space for cubbies behind the wall, or for utility running. (air conditioning ducts.....)

    Also seems like it will be easier to achieve continuity between your wall and roof/ceiling vapor retarder installations.

    You will have to engage in a local search for professional help.

    I would start with folks who do energy efficiency analysis, architects who claim familiarity with energy efficiency, and perhaps engineering firms.

    Local folks would seem a good bet to find someone familiar with the local environment

    You also might find someone specifically focused on the energy and vapor control aspects. https://barrierenergy.com/how-to/architects-using-energy-efficient-design/ I found this person with a little google. seems a little south from you, but she may have some contacts in the PNW.

    Vendors such as 475 and Prosoco may offer advice/review, or may be able to provide a direct reference.

    You can also search FH for energy efficient design and try to contact authors of applicable content you think applies.

    Do you have a builder in mind, or is this going to be a DIY?

    Local builders may have suggestions as to architectural/engineering assistance for your house.

  6. User avater
    unclemike42 | Jun 16, 2022 11:20am | #11

    https://www.energy.gov/sites/default/files/2014/10/f18/BA_ZeroEnergyReady_CliftonView_062314.pdf

    describes a job by a local builder. Might be worth a call.

    Would be interesting if this builder is still a fan of SIP panels, or has had a change of heart since the article was prepared.

  7. Codedocent | Oct 21, 2022 11:29am | #12

    Establishing your state of Washington has helped a lot, narrow down weblinks for you to read:

    For general building envelope requirements please see Section:
    https://up.codes/viewer/washington/wa-energy-code-commercial-provisions-2018/chapter/CE_4/ce-commercial-energy-efficiency#C402.1

    For Air Leakage please see Section:
    https://up.codes/viewer/washington/wa-energy-code-commercial-provisions-2018/chapter/CE_4/ce-commercial-energy-efficiency#C402.5

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