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Interior and exterior rigid foam

hipwader | Posted in Energy Efficiency and Durability on

Hello, Wade here from Fairbanks, Alaska (Zone 8) with a rigid foam insulation question.

Currently working on an addition to a current single bedroom house. The current structure will remain the heart of the new home, and I will extend a wing off each end of the house.

Current wall profile of the existing structure from inside to outside: drywall, horizontal 2×4 furring strips, 1.5″ rigid XPS, 6 mil vapor barrier, 2×6 wall with fiberglass batt, tyvek, fiber cement siding. I was meticulous when insulating the original structure 10 years ago and it has proven to be quite a good envelope.

For the new wings I plan to recreate the same wall profile (sprayed open cell foam in 2×6 wall cavities instead of fiberglass batts). However recently I have been toying with the idea of trying make things even more efficient. The new structure is much more complicated, has structural steel columns, lots of engineered LVL’s stacked in it, and I am worried that I am going to have places where the interior XPS is going to be my only defense against the cold. Its a large house (4900SQFT) and I plan to be living in it for the next 30 years, so I am willing to spend the extra money on insulation now because I believe it will pay me back over the life of the house.

So my thought is to put 3 inches of rigid XPS insulation on the exterior of the house. That should give me R-15 and provide a warm OSB surface to prevent frost build up, right?

Siding will be a combination of fiber cement, T&G cedar, and EFIS. Fiber cement and cedar will be applied to vertical furring strips to get a little bit of rain screen effect.

Should I move forward with this plan or am I asking for trouble? I appreciate all the help.


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  1. GBA Editor
    Martin Holladay | | #1

    It makes a lot more sense to install rigid foam on the exterior of your wall than on the interior, so your plan is a good one. Exterior foam is better than interior foam because (a) it does a better job of insulating rim joists and partition intersections, (b) it keeps your sheathing warm and dry, and (c) there won't be as many penetrations through the foam.

    Remember, if you install exterior rigid foam, you should never install interior polyethylene.

    Here are links to two articles that you may find helpful:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

  2. hipwader | | #2

    Thank you for the quick response Martin.

    How would one deal with the interior poly on the portion of the house that is already constructed? Leave it be and hope for the best?

  3. GBA Editor
    Martin Holladay | | #3

    On re-reading your post, I noticed that the title is "Interior and exterior rigid foam." I hope that doesn't mean that you want to install interior and exterior rigid foam on the same walls.

    You already have interior rigid foam and interior polyethylene on your existing walls. Those walls can stay the way they are. Don't install any exterior rigid foam on those walls.

    Your new additions can be built differently, with exterior rigid foam instead of interior rigid foam. Don't try to put rigid foam on both sides of your studs, though. Exterior foam only is better than exterior foam plus interior foam.

    Q. "How would one deal with the interior poly on the portion of the house that is already constructed? Leave it be?"

    A. Yes.

  4. Dana1 | | #4

    In a dual-foam (interior + exterior foam) situation it's better to use foil faced polyiso on the interior(any thickness), which will deliver a higher R /inch and provide a vapor retarder, and unfaced EPS on the exterior (up to 4"), which is twice as vapor permeable than XPS, thus avoiding a moisture trap, providing at least some capacity to toward the exterior. Better still would be to use rigid rock wool on the exterior, which has orders of magnitude higher drying capacity.

    Exterior XPS is a lousy choice for a couple of reasons. The higher vapor retardency reduces drying rates to the exterior and demands absolute perfection of the interior vapor & air barriers. And it's blown with HFC134a, which gives it an initially higher R-value than EPS, but in a few decades it's performance drops to that of EPS as the blowing agent dissipates. And that blowing agent is itself a power greenhouse gas (~1400x CO2). The additional benefit of that ~19% higher initial R value isn't necessarily going to be made up in lowered greenhouse gas emissions (though it might in Fairbanks, at only 3"). At 4" 1.5lb density Type-II EPS has an initially higher R value than 3" of XPS, and in 50 years it'll be a 25-30% higher R value. And it has a higher vapor permeance than 2" XPS.

    In central AK it's nearly impossible to put enough exterior foam for sufficient dew point control at the interior with fiber insulation between the studs without an interior vapor barrier, due to the brevity and shallowness of the summer "drying season", and the total annual hours that the sheathing would be below the dew point of the conditioned interior air. It's safer to put ALL the insulation on the exterior in that climate if you're not going to set it up with good drying capacity to the exterior and an interior vapor barrier.

  5. hipwader | | #5

    Thanks for the info here guys. I am going to mull over the answers for a day. I am sure I will have some follow up questions.

  6. Expert Member
    Dana Dorsett | | #6

    FWIW: IRC code spells out R15 minimum on the exterior of fiber-insulated 2x6 framing to be able to skip the interior vapor retarder for a zone 8 climate, but in Fairbanks I doubt that's really enough:

    Assuming the paradigm they were working with was R20 cavity fill, that's R35 total center cavity, with R15 on the exterior, or 43% of the total R being outside the sheathing. That probably works for most of zone 8, but it seem sketchy for Fairbanks. If you had R10 foam on the interior and R20 cavity fill it would take at least R23 on the exterior to even hit the IRC code prescriptive ratio, so for anything less than that the drying toward the exterior needs to be enhanced.

    And with an interior side vapor barrier the assembly needs to be able to dry toward the exterior. At 3" the vapor permeance of most XPS is no more than 0.3 perms- maybe that's enough if the wall never leaks air from the interior or water from the exterior, but it's not very resilient. The 4" of unfaced Type-II EPS would run about 0.7 perms, which is usually fine, but still not super great. With rigid rock wool (any thickness) you'd be north of 25 perms, which offers huge drying capacity for the sheathing, and would be totally fine with a poly vapor barrier &/or foil faced foam on the interior.

  7. hipwader | | #7

    Thanks again for the information and advice. After some time to think and talk things over with people in the community I think I have developed a plan.

    For existing structure: Leave current wall profile in place and add two, 2" layers of ROXUL board on the exterior. That would put me at R10 inside the Poly VB, R20 in the wall cavity and then R16 on the exterior. Total R46

    For the new structure: I would leave out the interior Rigid foam, but still run horizontal 2x4's to fur out interior walls to run wire and put low profile J boxes. I would either leave out interior Poly all together, or use a MemBrain product. Spray in open cell foam in the wall cavities. Then 4" of Roxul on the exterior Total R38.5

    Am I missing anything here? Anything that I have described that will lead to major problems?

  8. GBA Editor
    Martin Holladay | | #8

    Your plan will work. I would leave out the MemBrain; in your case, it would be a waste of money.

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