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30% of insulation external in climate zone 5?

dpilot83 | Posted in Green Building Techniques on

I’m working on building a house in climate zone 5 (Northwest Kansas).

If I understand my builders preferences correctly, he would like to run 2″ of closed cell spray foam insulation in the wall cavity and then R21 (for 2×6 walls) over the closed cell insulation in traditional 2×6 framing.

I could be putting words in his mouth but I think he said when he does it the batts are just barely compressed and I think he feels he is getting close to R35 in the walls with the two combined.

My preference would be to run comfortbatt in the walls for R24 and then run either 2 layers of 1.5″ externally for R 12.6 or so or two layers of 1.25″ for R10.5 or so.

3″ total external would get me 34.4% externally and two layers of 2.5″ total would get me 30.4% external.

How important is that external percentage? I don’t know of anyone building with external insulation at all in our area and I don’t hear of people with condensation issues in their walls. I wonder if it’s because it’s probably fairly uncommon to build to 3.0 ACH 50 around here so we leak enough to let it dry?

We’re shooting for under 1 ACH 50 and I’d really like to keep humidity in the 35 to 45% range in the wintertime so I’m a bit concerned about condensation issues.

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  1. jollygreenshortguy | | #1

    The benefits of external insulation are obvious. So I won't restate them. Here are some other things to consider.

    If you use a vented rainscreen siding you'll reduce risk by allowing your wall to dry out both externally and internally. So the precise ratio of insulations won't be quite so critical.

    External insulation complicates the waterproofing and installation of doors and windows considerably. More complexity usually means more expense and more possibilities for mistakes. Your builder may be thinking of that. Flash & batt, like he's proposing, avoids that.

    IRC 2021, R806. provides a method for calculating the amount of external insulation needed to avoid sheathing condensation problems. This paragraph is for roofs but works for walls as well. Alternatively you can go with the ratio of 20/60 in the table R806.5. Yes, 30% should be fine. The use of a "smart" vapor retarder on the inside wall is probably also a good idea. Table R702.7(2) and (3). With a smart vapor retarder, a rainscreen and good controls for humidity inside the house you could probably get away with a bit less than 30%. So don't sweat it if the figure turns out to be a bit less.

    Though continuous external insulation is definitely an advantage, it's the total wall R value that matters and if you can achieve that without continuous insulation you've achieved the same result.

    The closed cell spray foams are among the worst for greenhouse gas emissions as well as occasionally being problematic for indoor air quality.

    One interesting option is a 2x6 wall with what I call "ladder framing", 2x2 framing applied horizontally to the inside face. R20 cellulose batts go between the studs and the "ladder" gets an additional 1 1/2" batt, R6. The effect is basically continuous insulation because the 2x6s are interrupted by the R6 batt. Use a smart vapor retarder on the inside face. You can install your doors and windows without the complications of exterior insulation and you avoid foam insulation products entirely. Instead of using 2 layers of batts you can also blow in dense packed cellulose. Cellulose is among the best materials as far as greenhouse gas emissions are concerned. This wall section gives you a whole wall R value of around R26.

  2. jollygreenshortguy | | #2

    R21 batts compressed by 2" will be more like R15

    1. dpilot83 | | #4

      @jollygreenshortguy, I was under that impression as well but I wasn’t sure. I need to find a resource that shows evidence of this.

      1. Expert Member
        Michael Maines | | #5

        This is from Owens Corning. I'm not sure if they still share it publicly; my copy is several years old, but it matches what JGSG says.

        1. dpilot83 | | #6

          Good info, thanks.

  3. dpilot83 | | #3

    Thanks @jollygreenshortguy.

    “Though continuous external insulation is definitely an advantage, it's the total wall R value that matters and if you can achieve that without continuous insulation you've achieved the same result.”

    You’re saying the location of the insulation does not matter when it comes to condensation in the wall?

    My biggest concern in building a house is spending a ton of money and doing a lot of work to make it airtight and due to the house not leaking like a sieve (which is the way a lot of old houses dry out) I end up with condensation issues that result in major repairs needed just a few years into ownership.

    The scenario I’m most concerned with is humid warm in the house during unusually cold days in the winter. This can create places in the wall cavity that are cold enough to condense the water out of the air. With a tight building envelope it can take a long time for that to dry out.

    My studying has suggested that if all of your insulation is inside the vapor barrier, it’s much more likely to have condensation inside the wall cavity. Having exterior insulation greatly reduces the odds of condensation within the wall cavity.

    Do I have that wrong?

  4. andyfrog | | #7

    "You’re saying the location of the insulation does not matter when it comes to condensation in the wall?"

    It seems like they're saying, yes it does matter, but it doesn't matter as much if you use a drained and ventilated rainscreen cavity and an interior vapor throttle.

  5. Peter_SISPanels | | #8

    Hi dpilot,

    I'm with an insulated sheathing manufacturer and I think we can help you out. Rather then installing 2 layers of 1.5″ or two layers of 1.25″ exterior insulation, we can laminate 2.5"-3" of your rigid insulation of choice to a OSB/Plywood so these wall layers can be installed simultaneously. Let me know if that is of any interest to you: 587-586-9201.

    Wishing you luck with your build!

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