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R-13 batts in a 2×6 wall?

up_state_ny | Posted in General Questions on

I’m building in zone 5. I have already framed the 2×6 walls, sheathed with OSB, and the windows furred 1.5”. Was going to place R-7.5 EPS foam on the outside sheathing and R-19 batts in the cavity.  However, to prevent moisture from forming on the inside face of the sheathing, a conservative approach is to place R-13 in the cavity.  My question is: can one insulate a 2×6 wall with R-13?


I’m building in zone 5. I have already framed the 2×6 walls, sheeted w/ osb, Tyvek house wrap, and the windows furred 1.5”. The plan is R7.5 eps foam on the outside sheeting and R19 in the cavity.  A local manufacturer produces 1.5” – R7.5 eps foam @ 40F (25psi density).  The sheeting comes w/ or w/o a porous face.  I’m a little concern about moisture from forming on the inside face of the sheeting.  Please review and comment.

In addition to the exterior 1.5” – R7.5 eps, is there any advantage to the cut-and-cobble method of adding additional eps foam to the 2×6 cavity?

Considering there is already Tyvek house wrap applied to the osb, should I get the 1.5” – R7.5 eps w/ or w/o the porous facing?

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  1. Expert Member
    Dana Dorsett | | #1

    You can, but why would you?

    The empty 2" channel is a fire path and potential convective thermal bypass path around the batts. In some locations code would require a fire-stop mid-way up the wall if the cavity isn't completely filled.

    R19 batts only perform at R18 when compressed to 5.5" in a 2x6 cavity. R20 is code minimum.

    If you are installing R7.5 foam on the exterior, there is nearly zero chance of moisture "...forming on the inside face of the sheathing..." even with R20 (or R18) in the cavities.

    Even if the average temperature of the sheathing was cold enough to accumulate moisture (which it won't be with R7,5 on th exterior), it doesn't form on the inside surface- it enters the wood in the form of adsorb. Moisture doesn't appear on the inside face until the wood's moisture content is insanely high, nearly saturated.

    1. up_state_ny | | #8

      I get your point, the Dew-Point calculation only yields an extreme moisture event and is not a true model of the vapor / liquid phase change. So you feel that the continuous R7.5 insulation on the exterior and R19 or R21 batts is an adequate assembly that will perform well over time. (where does the code say R20).

      Could you please comment on the following two (2) questions:

      In addition to the exterior 1.5” - R7.5 eps, is there any advantage to the cut-and-cobble method of adding additional eps foam to the osb in the 2x6 cavity?

      Considering there is already Tyvek house wrap applied to the osb, should I get the 1.5” - R7.5 eps w/ or w/o the porous facing.

      1. Expert Member
        Dana Dorsett | | #11

        At 1.5" EPS or XPS can only be counted on for R6.3, long-term, unless it's graphite-loaded EPS. It's better to either use 1.5" of polyisocyanurate, or 2" of polystyrene (EPS/XPS). If you're going to cheat the lifecycle R value with 1.5" polystyrene, unfaced EPS, at 1.5" is still more vapor permeable than the sheathing, whereas unfaced 1.5" XPS is less then 1-perm- a Class-II vapor retarder. The vapor permeance of Tyvek is north of 20, and will not affect the drying rate of the assembly.

        There is no advantage to cut'n'cobble in the wall cavity from either a thermal more moisture transport point of view. There is no way to air-seal the cut'n'cobble sufficiently perfectly to avoid convection transported moisture around it (if not the first year, eventually.) It's safer/better to put the foam on the exterior.

        By "dew point control" we're not talking about extreme events or phase change, but rather the average winter temperature of the sheathing relative to the average wintertime dew point of the interior conditioned space air. If the average temp at the sheathing is cooler than the dew point of the indoor air there is a vapor pressure gradient moving moisture from indoors into the wall cavity. The dew point of the entrained air in the insulation and wall cavity more or less tracks the temperature of the sheathing during the colder months. There is no phase change of the water as it enters & leaves the wood. The moisture in the wood is in the form of adsorb, not liquid.

  2. GBA Editor
    Martin Holladay | | #2

    For more information on this issue, see "Rethinking the Rules on Minimum Foam Thickness."

    1. up_state_ny | | #7

      Thank you for the suggestions. Ok, R13 in the 2x6 wall by itself is a bad idea. Also, I’m not comfortable w/ interior vapor barriers mentioned in the suggested article. However, you mention cut-and-cobble method in your, “Calculating the Minimum Thickness of Rigid Foam Sheathing,” article, 10.15.2018. Are you advocating I can use the 1.5” – R7.5 on the exterior and 2” of eps on the other side of the osb, w/ R13 batts adjacent to the drywall?

  3. jaccen | | #3

    Dana, I understand and agree with your fire path concerns. Any chance you could elaborate on your thermal bypass concerns? If the exterior foam is taped and/or the exterior OSB is taped (along with all the other standard to GBA air/water sealing techniques), I'm not sure if the thermal bypass will be more than just a hypothetical concern. I would love to hear your thoughts on it, however.

  4. Expert Member
    Dana Dorsett | | #4

    A batt that happens to be imperfectly installed, maybe a half-inch too short, not tucked perfectly in at the corners and edges would have a fairly low impedance path around the batt when there's a 2" deep full-width channel into which the air could flow. The oversized channel makes it too easy, putting much bigger demand on perfection on the installation. While there is good, better, and best execution in construction, there is no such thing as "perfect" yet.

    This is not merely hypothetical- the Building Science Corp people have measured it, and produced these handy images.

    If for some reason I can't quite fathom you insist on installing only 3.5" of fiber insulation in that cavity, use only high-density R15s (fiberglass or rock wool), which are more expensive than R20 mid-density batts, and will underperform the full cavity fill. High density batts are sufficiently air retardent that they at least don't have appreciable convective losses WITHIN the batt.

  5. user-6863358 | | #5

    Entire fiberglass industry has high density batts available for 2 x 6 construction (R21 and R23) as well as medium density R20 as mentioned by Dana. These batts are 5 1/2" thick designed to fill the entire 5 1/2" deep cavity. Dana's points about installing a product that does not fill the cavity are spot on.

  6. jaccen | | #6

    Many thanks for the link and your further explanation. It's greatly appreciated.

  7. Jon_R | | #9

    So you will barely meet code for external foam - this will cause some sheathing moisture accumulation (but typically not enough to be a problem). More conservative than code minimum is always a good idea. A good way to do that is to allow > 1 perm of drying to the exterior. You can do that by selecting unfaced external EPS.

    This is speculative, but I also believe that your design would be more resilient if you applied something like Bora-Care with Mold Care to the OSB (creating a similar effect to borates in cellulose).

  8. walta100 | | #10

    You may want to consider damp spray cellulose you are likely to get a better fill and it resists air flow much better, it is likely cost slightly more.

    If you choose fiberglass read about insulation grading. This work in not seen as important to most contractors. “It’s hard to do a perfect job”


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