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Interior T&G pine with rigid foam exterior insulation

Adam Emter - Zone 7a | Posted in Energy Efficiency and Durability on

Hi all,

I’m planning to insulate my new construction walls with 6″ of EPS foam. I live in Zone 7a. 2×4 walls will be insulated with r-11 unfaced fiberglass batts. My question is whether the T&G pine paneling that I’m planning to use on the interior walls would be too vapor impermeable. In other words, will my wall be able to dry to the inside? I know that using drywall is an acceptable practice, but don’t know much about the perm rating of this pine paneling. It is 3/4″ thick and has a coat of polyurethane. I’m planning to use either 6-mil poly or housewrap on the outside of my OSB sheathing. Any help would be appreciated. Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Adam,
    Tongue-and-groove pine boards, even when finished with polyurethane, are certainly vapor-permeable enough for you to stop worrying about inward drying.

    However, there is another problem that you didn't mention -- one that you should be worrying about. Tongue-and-groove boards are not an air barrier. In fact, they leak air like a sieve. If you are insulating between your studs with unfaced fiberglass batts, you need an interior air barrier. The usual solution in this situation is to install gypsum drywall first. Pay attention to airtightness when installing the drywall, especially at penetrations and electrical boxes, and make sure that you tape the drywall seams.

    Your tongue-and-groove boards can then be installed on the interior side of the drywall.

  2. Adam Emter - Zone 7a | | #2

    Thanks for the reply, Martin. Wouldn't the 6-mil poly or Tyvek on the exterior of the sheathing be sufficient air barrier? I have also considered peel-stick membrane to cover the sheathing, but it's about 5x the cost of poly or housewrap.

  3. GBA Editor
    Martin Holladay | | #3

    Adam,
    1. You don't want polyethylene anywhere in this assembly. Don't do it! For more information, see Do I Need a Vapor Retarder?

    2. Tyvek makes a pretty lousy air barrier, due to fastener holes and difficulty of keeping it from getting ripped by siding installers. For more information, see Questions and Answers About Air Barriers.

    3. It's always a good idea to have an air barrier on all six sides of an air-permeable insulation like fiberglass. The air barrier reduces the chance of convection currents.

    1. tannerc | | #8

      Martin, in the event that you are around and willing to respond to a “zombie” thread, I am curious about what I perceive as a change in your position here on the need for two air barriers and the effect of convection currents. This is relative to 2010, specifically comment #19 on your article: https://www.greenbuildingadvisor.com/article/one-air-barrier-or-two

      This is applicable to my construction, where I have detailed my exterior sheathing as an air barrier, have rock wool insulation, and plan to install t & g boards directly over the studs as an interior finish. I am in climate zone 4. I do still have the opportunity to install a membrane type interior air barrier if it is a worthwhile investment.

      Thanks!

      -Tanner

  4. Adam Emter - Zone 7a | | #4

    Martin, I really appreciate the feedback. Just trying to understand the different methods and the reasons/science behind it all. I have been researching the REMOTE method and was hoping to adopt that for my wall system. They suggest using a peel-stick membrane on the outside of OSB, but also say that 6-mil poly or Tyvek will work. Is their method misleading or am I understanding something wrong? Would you recommend using a peel and stick membrane as air/vapor barrier?

  5. GBA Editor
    Martin Holladay | | #5

    Adam,
    If you are going to follow the REMOTE or PERSIST approach, you can install peel-and-stick membrane on the exterior side of your wall sheathing. My usual advice to builders who do this is to keep the stud bays empty of insulation, so that all of the insulation is on the exterior side of the peel-and-stick. That way you will avoid any possibility that you have too much insulation on the interior side of your wall sheathing. (The colder the sheathing is, the more likely the chance of moisture issues.)

    For more information on PERSIST and REMOTE, see Getting Insulation Out of Your Walls and Ceilings.

    When Alaskan builders started developing variations on the PERSIST method, they suggested the use of polyethylene on the exterior side of the sheathing. This works, especially in Alaska, where polyethylene is used on most houses. But if I were designing a PERSIST wall, I would either use the traditional peel-and-stick approach (an admittedly expensive way to go) or just use taped Zip sheathing. (I just don't like polyethylene.)

  6. Adam Emter - Zone 7a | | #6

    Hi Martin, again I really appreciate the feedback. I have decided to go with a peel-and-stick membrane to cover the exterior walls. I have read the "Calculating the minimum foam thickness" article here on GBA and other publications that reference the need to have enough insulation on the exterior of the vapor barrier to prevent condensation. With my 6" of EPS on the exterior, I will have ~r-24. By my calculations for my climate and 40% indoor RH, my sheathing should almost never get below the dewpoint using a 2/3 exterior and 1/3 interior insulation ratio. My thinking is that r-11 batt insulation would be cheap insulation compared to the exterior EPS. I can add r-15 mineral wool batts in my 2x4 walls for about $1,100. Question for you is this: if it were your home, would you add the batt insulation or just leave the exterior EPS at r-24 and be safe at that. I live in zone 7a and plan to live in this house for decades. Appreciate the feedback!

  7. GBA Editor
    Martin Holladay | | #7

    Adam,
    In Climate Zone 7, according to the 2012 IRC, you need a minimum of 20+5 or 13+10 wall insulation. So R-24 would be close to code minimum -- not particularly impressive. I would aim for a higher R-value if I were you.

    On any wall with insulation on both sides of the wall sheathing, I don't recommend the use of an vapor-impermeable peel-and-stick product on the exterior side of the wall sheathing. If it were my house, I would simply tape the seams of the wall sheathing as an air barrier, and I would choose a vapor-permeable WRB like housewrap.

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