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Better choice of exterior wall construction for new home?

silasjames | Posted in Green Building Techniques on

We’re building a new home in central Iowa. We’re in climate zone 5. We thought we had a final above grade wall spec. Now, we’re entertaining a different one. I think the first one is more robust, but I’d like an opinion on the two walls:

Wall 1 from inside to out:
1/2″ drywall with latex paint
MemBrain “smart vapor retarder”
2×6 wood stud, 16″ o.c.
R-23 HD fiberglass batt, Knauf EcoBatt (my dad is an insulation contractor, this will be a legitimate “Grade 1” fiberglass installation – he is the most meticulous batt installer I’ve ever seen, and I’ve been on literally hundreds of pre-dyrwall thermal bypass inspections for code / Energy Star, etc.)
7/16″ OSB with EcoSeal air sealing detail (picture frame all stud cavities / seal all edges/seams,etc.)
Tyvek (we’re doing “Innie” windows/doors)
2″ foil-faced polyiso, Johns Manville – R-13 total
1×3 or 1×4 strapping
Field-painted fiber cement lap siding, factory back-primed – Maxitile
The rain screen will be vented at the bottom, and will vent into the soffit at the top. There will be screen material covering the gap at the bottom of the wall, attached to the OSB before Tyvek. The screen will wrap around the bottom of the strapping and attach to the face of the strapping before attaching the siding.

Comments about wall 1:
Using average daily temps for December, January & February in Des Moines, the interior face of the OSB calculates to ~ 41.9 degrees. At 70 deg F and 35% RH, Dewpoint is ~41.2 deg F. The insulating value of the polyiso keeps the inside face of the OSB warm enough to greatly reduce the potential for condensation much of the winter. Unfortunately, the OSB and/or the foil-faced polyiso greatly reduce or eliminate drying potential to the exterior. The MemBrain should reduce vapor drive from in to out in winter, but should also allow inward drying if conditions require.

Wall 2 inside to out:
The same wall system as above up to and including the 2×6 wood studs and FG insulation
Pre-made panel consisting of 2″ EPS laminated to 7/16″ OSB, R-value ~9.51 (think SIPS panel, but with OSB on only the exterior side) picture frame all stud cavities, seams, gaps, edges, etc.
Tyvek (innie windows/doors)
1×3 or 1×4 strapping
Field-painted fiber cement lap siding, factory back-primed – Maxitile

Comments about wall 2:
Using average daily temps for December, January & February in Des Moines, the interior face of the EPS calculates to ~ 38.4 degrees. At 70 deg F and 30% RH, Dewpoint is ~37.2 deg F.

Due to some situations with supplier relationships and other factors, wall 2 would cost less to build. We would need to maintain a lower RH in the house, which should be manageable with only 4 occupants, balanced ventilation system and stand-alone dehumidifier (if needed).

My question:
In wall 2, since the OSB, the most moisture-sensitive material in the assembly, is outside the thermal boundary (i.e. cold), and the EPS insulation is vapor semi-permeable (not sure of the actual perm rating yet, but confident 2 inches is still over 1 perm), am I going to experience moisture condensation somewhere in the foam and/or OSB? Is there going to be enough wetting to be cause for alarm? The construction savings of wall 2 are significant, but wall 1 is affordable. Is it worth it to switch to wall 2? Any other comments on wall 1?

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  1. iLikeDirt | | #1

    "Best" is a pretty subjective term. Personally, I don't think I'd label anything covered in OSB as "best." Cheapest or most cost-effective, maybe, but certainly not best! :) Anywhere as rainy and humid as the midwest and I personally would feel more comfortable with a lot less wood in my house.

  2. silasjames | | #2

    Response to Nathaniel G
    There should be a question mark behind the heading. I'm curious which system is the best between the two mentioned above. Perhaps I should have asked which system is better :)

    What would you recommend as a wood-free alternative?

  3. ohioandy | | #3

    If I understand the stackup, wall 2 has the OSB separated from the studs by 2". It's been addressed here before: do engineers object to this compromise of racking resistance in the wall system? You don't have seismic issues in Iowa, but you do have wind. So maybe you cut in some steel T-bracing. In that case, couldn't you eliminate the OSB altogether, tape the foam as your air barrier, and apply vertical strapping over the studs? Wondering how code addresses these questions.

  4. silasjames | | #4

    Response to Andy Chappell-Dick

    The first iterations of my wall spec looked like this:
    1/2" drywall with latex paint
    2x6 wood studs 24" o.c. with R-24 HD "Canadian" fiberglass batts
    Either angle bracing or plywood corners
    2" foil-faced polyiso detailed to double as WRB (presumably taped seams)
    Vertical strapping
    Fiber cement cladding

    I started reading about the unknowns regarding medium- to long-term structural integrity of foams as WRBs. I'm nervous about even the highest quality tapes' longevity. If the foam shrinks or moves and the tape delaminates, what will happen to the poor house in 1, 5, 10, 25, 50, 100, 500 years?

    Every time I change a spec, I also wonder if I'm overthinking it. Perhaps my spec change increased the wall performance by one point out of a possible ten million. That's why I love this site - those with lots of experience both digesting the data and putting it to use can weigh in on it and offer an opinion. Then I can stop obsessing and make a choice. :)

  5. GBA Editor
    Martin Holladay | | #5

    Either wall would be robust from a moisture perspective. In Climate Zone 5, a 2x6 wall with exterior rigid foam only needs rigid foam with a minimum R-value of R-7.5 to stay out of trouble. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

    If you are seriously considering Option 2, however, you need to consult an engineer to be sure that the 2-inch nailbase with foam facing the studs provides enough bracing for your walls.

  6. srenia | | #6

    In the Ames area. Many long days figuring out the best solution. I do retro fits mainly. Around R20 with a 1.5 arch 50 blower is the sweet spot for cost and energy savings here. U value around 90 to 95 percent effecient.

    The ideal would be roxul solution with a modiefied Swedish wall system for new construction. I would put the sheathing on the outside though. Use either Huber Zip system or rain and ice shield wall. Use roxul Comfortboard outside of it. ISO does not perform as well in cold weather. The roxul comfortboard does not have the shrinkage of foam. Eliminate the house wrap and membrain if you want. Its not required in this system. Use as many picture windows as code allows.

    Fiberglass r23 would be around the same price as the roxul at retail. The Rockwool is a better product for fires, water and sound. The comfortboard on the outside gets rid of most of the dew point worries since the rockwool can breath to ice shield or Zip. (Not all, but a lot) The air seal on sheathing is key. You'll save more money hitting a blower door test at 1.5 arch 50 than 5 arch 50 with more insulation.

    On the retro fits I have been using R5 on the interior - mastic sealed, caulked and wrap the interior of the windows with foam as well. Interior wood jambs 3/8" larger on the doors. Adustible electrical outlets. Roxul in the wall and floor cavities. R100 cellous in the attics. Ideally I would used roxul comfortboard but it not in the budget and the foam is acting like a interior air barrier. Elimantes replacing siding.


  7. Expert Member
    Dana Dorsett | | #7

    A couple of comments:

    The 2" EPS (not including the OSB) runs about R8.4 when the average temp through the layer is 75F, but is about R9 when the average temp through the layer is 40F, and rises to about R9.4 when the average temp through the foam is 25F. It's average mid-winter performance will then be north of R9, which give you even more margin at the EPS/fiberglass boundary than you had counted on.

    The vapor permeance of Type-II EPS at 2" will run about 1.5 perms, which is not a big problem for OSB facing a vented rainscreen cavity. If they are building it with Type IX EPS (possible, but not likely), it would be tighter still at about 1 perm, which is also fine. The permeance of 7/16" OSB is about 2 perms when it's dry (higher when damp) , and can pass moisture more readily than 2" of the most likely grades of EPS.

    Polyiso hand falls off a performance cliff when the average temp through the foam is below freezing (which is about what the mid-winter average will be), and it may only average R8-8.5 in January. That is still sufficient dew point control for zone 5 per IRC 2012 chapter 7, but it's average wintertime performance won't beat EPS when you have R23 of fiber on the interior side- it's just too cold.

    The crossover temp where EPS beats polyiso is when the average temp through the layer is 40-45F according to this overlaid set of thermal conductivity curves:

    The brownish non-linear curve is polyiso, the dark black is high density EPS. Type-II EPS would have about the same slope, but would be slightly above than that black line. So call the crossover point where polyiso becomes more conductive than EPS something like +5C/ 41F, which is roughly where you will be when it's 25F outdoors with your stackup, which is about the mean January temp for Des Moines. When temps are in the teens EPS will be clearly outperforming the polyiso, when it's above 30F the polyiso will be outperforming the EPS, but not by nearly as much as the labeled R values would indicate.

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