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Community and Q&A

Dew point concerns with exterior rockwool in Maine

[email protected] | Posted in General Questions on

I’m putting together plans for a home I’ll be building in Central Maine where the average temperature from December-February is 19.4*F (

I wanted to go with above-code insulation levels and use exterior insulation, but I’d like to keep my use of foam above grade to a minimum, so I’m looking to use Roxul Comfortboard outside my sheathing.

At first, I was planning on this as a wall assembly:

(interior to exterior)
2×8, 16″ o.c. wall with dense-packed cellulose (actually 2×6 studs with 1″ strips of foam and then 1×3 strapping on the interior side to form a 2×8 cavity) – R29 or so
7/16″ ZIP sheathing
2″ Roxul Comfortboard – R8
Reverse board and batten pine siding (1×3 battens doubling as a rain screen, and 1×10 boards)

Then I read “Robust Walls” by Ted Cushman ( and realized this assembly wouldn’t keep the sheathing above the dew point at the average winter temperature in my location. 

Now I’m planning to use a 2×6 stud cavity (so R 21 with the dp cellulose) to reduce the ratio of interior: total insulation, but following the rough calculation Joe Lstiburek describes for finding sheathing temperature at the average winter temperature, I’m finding that even if I use 3″ sheets of Comfortboard (R 12) on the exterior, my sheathing will still be below the dew point. 

Here’s my math for that assembly:

Average monthly temperature = 19.4*F
Percentage of total insulation that is on the interior = R 21 / (R 21 + R 12) = 63.63%

So sheathing temperature on average in winter going with the same interior temperature (70*F) and RH (35%) used in the article = 70*F – [(70*F – 19.4* F) x 63.63%) = 37.8*F

The dew point for that interior temperature and RH is 40*F, so even with 3″ of rockwool on the exterior, my sheathing would still go below the dew point at the average winter temperature. I really don’t want to use/buy a 2 sheet layer of rockwool on the exterior…

Apologies for the long build-up here, but here are my questions:

– Given that I’m pretty sure this assembly can dry to the interior and exterior, and that it uses cellulose for cavity insulation, do I need to be as concerned about moisture collecting in the walls? Would an assembly in my location with R 21 walls and R 12 exterior insulation be safe in that circumstance? Do I even need to worry about the R value of my exterior insulation if the assembly is vapor-open on both sides?

-I’ve read that in climate zone 6, where I live, R 11.25 external insulation should prevent moisture from accumulating to dangerous levels in a 2×6 wall. Using Lstiburek’s formula, it looks like I need R 15 external insulation over my 2×6 walls to prevent the inside of my sheathing from getting wet at the average winter temperature — that’s the same amount of exterior insulation recommended for climate zones 7 and 8! Which of these guidelines should I follow to ensure my walls won’t rot? 


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  1. Expert Member
    Michael Maines | | #1

    Evan, this topic is debated here regularly, and both sides make good points. I say that Rockwool is vapor-open so it's ok if it's not as thick as it should be to keep the sheathing above the dewpoint; every house built without exterior insulation sees more wet sheathing, everything else being equal. But it's important to include a deep rainscreen vented at the top and bottom so you don't limit the drying potential. I also recommend adding an interior variable permeance membrane.

  2. [email protected] | | #2

    Thanks for your reply, Michael! I really appreciate you weighing in. Just to make sure I'm understanding you correctly, you're saying you wouldn't worry about using 3" of Rockwool outside my 2x6 wall? Do you think that the 1x3 batten should provide a big enough rainscreen or should I consider going with a 2x instead? Lastly, do you think that painted drywall would suffice for an interior vapor barrier, or should I be looking into a "smart" vapor barrier of some kind?

    1. Expert Member
      Michael Maines | | #4

      Evan, to be clear, it's always best to include enough exterior insulation to keep the condensing surface above the dewpoint, typically 45°F, which around here (I'm also in central Maine) means at least 1/3 of the total R-value is on the exterior, preferably more. But if you need to skimp for various reasons, you are correct--I would not worry about having not-quite-enough exterior insulation, as long as the exterior insulation is vapor-open so it can dry readily to the exterior. 1x3 battens that are open at the top and bottom (with bug screens of some sort) should provide plenty of air flow.

      Maine's building code now follows the 2015 IRC (for the most part) and says that if you have at least R-11.25 insulation on the exterior of an insulated 2x6 wall, you only need a class 3 interior vapor retarder--i.e., painted drywall. As long as your assembly is reasonably airtight it should work fine. You can add a measure of safety by including a variable permeance membrane but it's not required, or likely necessary.

  3. Jon_R | | #3

    > this assembly wouldn’t keep the sheathing above the dew point

    This isn't necessary for a good wall. Read Side bar 2 here carefully. Exterior perms matter.

    You can also build a good wall with the options provided in the 2021 IRC:

    Make sure your walls are airtight! Preferably with air sealing on both sides.

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