Cavity insulation with exterior rigid foam
I’m looking for advice on stud cavity insulation. Walls will be (from outside in) LP smartside lap siding, 3/4″ rainscreen gap, 6″ EPS foam (R-24), self-adhering vapor/water/air barrier, OSB, stud cavity, then interior wall covering which will be either T&G pine or drywall with latex paint. My main floor walls are 2×4 and the basement walls are 4′ below grade ICF and 4′ above grade 2×6. I live in zone 7a – eastern North Dakota.
My questions are: 1) Is there any condensation risk if I install Roxul batts in my wall cavities? I have researched this to the point of insanity and everything I have read says that I should be OK with an insulation ratio of approximately 62% exterior/38% interior, but I am a pessimist by nature. The house will have an ERV installed and indoor winter humidity shouldn’t ever be more than 35-40%.
2) If you believe it is OK to install mineral wool in the cavities, would it also be OK to install r-15 batts designed for 2×4 wall into the 2×6 basement walls? Is there any problem with having a 2″ gap on the interior side of the batts? I know that installing r-23 batts would be a risky ratio of exterior to interior insulation.
The big disclaimer on this is that this is my own home. I am doing most of the construction myself, with focus on airtight construction and long-term energy efficiency and durability. I’m 34 and I plan to live in this home until I die or get pushed into a nursing home.
Thank you so much for the assistance!
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I should also mention that I will be installing my European tilt/turn windows as "innies," so I am concerned that not having any cavity insulation will create a bit of a thermal bridge between the window frame and wall framing.
Adam,
Here is a link to an article that explains the concepts that will put your mind at ease: Calculating the Minimum Thickness of Rigid Foam Sheathing.
In your climate zone, if you are designing an insulated 2x4 wall with exterior rigid foam, the rigid foam layer need a minimum R-value of R-10. You've got R-24, which is much more than the minimum. So relax.
Concerning your questions about the above-grade portion of your basement wall: You told us that it is framed with 2x6s, but you didn't tell us whether that wall has exterior rigid foam.
If it does have exterior rigid foam, the minimum R-value of that foam (for a 2x6 wall in your climate zone) is R-15.
For for information on dew-point calculations, see Are Dew-Point Calculations Really Necessary?
Martin, thank you for the response. The basement walls will also be covered with the same 6" of EPS. Would it be OK to install r-15 mineral wool batts down there, leaving a 2" air gap in the cavity? Also, I am just assuming that the mineral wool batts are a better choice than fiberglass batts. Is that assumption correct with this wall buildup?
Any thoughts?
Adam,
In your original question, you wrote, "I know that installing R-23 batts would be a risky ratio of exterior to interior insulation."
Since you have 6 inches of exterior rigid foam, however, there is no risk. I tried to explain that in my previous answer. The 6 inches of EPS makes everything warm and dry. So relax.
Go ahead and fill your 5.5 inch stud cavities will fluffy insulation. There is no need to only fill them halfway.
Dense fiberglass batts can perform about as well as mineral wool, if the fiberglass is installed very carefully by someone who cares about the quality of the work. You can use either product -- fiberglass or mineral wool. It's your choice.
There's no moisture accumulation down-side to filling the R24 foam clad 2x6 studwal, and that's even more true for the 2x4 framed wall. The IRC would give you a pass with as little as R15 on the exterior of a 2x6 / R23 wall:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
With R24 foam on the exterior you'd have HUGE margin, as long as the finish wall is reasonably air-tight, but vapor semi-open. Wallboard painted with standard latex paints would be fine, but t & g pine on it's own would be too air-leaky, risking moisture transport to the structural sheathing via convection through the rock wool. A layer of wallboard painted with a standard latex primer or a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus detailed as an air barrier between the t & g and Roxul would be necessary to block that convective moisture transfer to the structural sheathing.