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Wall detail

Jim Marty | Posted in Green Building Techniques on

I am a builder in Northwest Montana.  Climate zone five.. The area is relatively humid and it is known to rain in the winter. I am beginning to get a better grasp on the details of green building but  I spent 20 years as a home inspector and have seen a lot of things go bad when people attempted to do things and were not diligent about the details.
I am building a new home and have some questions. It seems like Joe’s ideal wall with a non-permeable membrane on the outside would trap moisture on the inside and allow for any humidity in the home to do damage to framing and sheathing from the interior.  I’m not clear on why a system that prevents any vapor migration from the outside to the inside is good.  I understand that it is best not to trap any building materials between two impermeable layers.  In this area it is common to frame 2×6 24″OC and spray 2 inches of foam against the sheathing and then fill with fiberglass.  Supposedly this stops any condensation on the  inside of the sheathing. A vapor  permeable membrane is placed on the outside  to allow vapor to escape but unfortunately this can allow exterior bulk moisture to contact the sheathing causing problems.   In addition there is significant conduction  of heat through the solid studs from the outside to the inside. I would like to prevent that on my next home. I am considering  2×6 24″OC with 2″of spray foam in the stud bays and then wrapping the entire house with 3/4″ foam board to prevent this conduction and then using house wrap or tarpaper  on top of that and then siding with  textured plywood.  I am trapping the sheeting between two vapor impermeable layers. Will this create a problem?  Any suggestions?

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Replies

  1. Brendan Albano | | #1

    Continuous exterior insulation protects your sheathing from moisture damage by keeping it warm. The amount of exterior insulation required to keep your sheathing warm enough depends on your climate zone.

    There are some good articles on Green Building Advisor (https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation) and BSC (https://buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies) that go into the details on this issue, but the basics are also covered by table R702.7.1 in the IRC. In order to be able to skip the class I or II interior vapor retarder (in your case the spray foam), you need to have R-7.5 of exterior insulation with a 2x6 wall. 1 1/2" of Polyiso would cover that. This would get you a wall along the lines of:

    - Siding
    - Rainscreen gap
    - WRB (you can also do the wrb inside the continuous insulation if you prefer, see https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go)
    - 1 1/2" polyiso (or other rigid insulation > R7.5)
    - Sheathing, seams taped (this is your air barrier)
    - 2x6 with fiberglass batt (or cellulose or whatever)
    - Gypsum board

    This is a very robust wall assembly with a proven track record. It lets you skip the expensive spray foam, and avoids the potential problems of having your sheathing sandwiched between two vapor retarders (the spray foam on the inside and the foam on the outside)

  2. Expert Member
    Peter Engle | | #2

    There are a lot of questions there, but I'll take a stab at it.

    First, you must differentiate between water vapor and bulk water. They behave differently and are controlled differently. A "vapor permeable membrane" used as a water resistant barrier does both functions as described. It allows vapor to pass through in both directions. it should never allow bulk water to contact the sheathing if it is installed and detailed properly.

    I'm not sure which iteration of Joe L.'s perfect wall you are talking about. The "perfect residential wall" that I am most familiar with has the WRB (which also serves as the air barrier and vapor retarder) on the outside of the sheathing, with 50% of the insulation outside of that , in the form of rigid foam or rockwool. The 50/50 rule works well in most climates. If you want to cheat that a bit, there are articles here on how to figure out just how much rigid foam you need on the outside to prevent condensation on the sheathing. The exterior insulation keeps the sheathing warm in winter, cool in summer, and stops the thermal bridging of the wood studs. With this wall, you don't need any spray foam in the stud bays - you can use fiberglass, cellulose, rockwoll or whatever other fluffy insulation you prefer.

    Note that you don't need to add housewrap to the outside of the foam insulation because it's already there on the inside of the foam. If you want a bulletproof solution, use one of the trowel-on, spray-on or roll-on fluid applied membrane materials instead of housewrap.

  3. Expert Member
    Peter Yost | | #3

    Hi Jim -

    Brendan has done a good job responding to your question, using key GBA resources readily available.

    In answer to your question about Joe's (Joseph Lstiburek, PE, Phd, and principal at Building Science Corp in Westford MA) perfect wall: I am pretty sure that Joe would agree that if any building assembly can be designed and configured to dry in BOTH directions, terrific. But any assembly should be designed and specced to dry in at least one direction. So, if your exterior membrane (and many to most are) is vapor impermeable (Class I vapor retarder), then you can design and spec the assembly to dry to the interior and combine management of interior moisture and elevating the first condensing surface so that vapor does not condense, you are good to go.

    Best - Peter

  4. Aedi | | #4

    >It seems like Joe’s ideal wall with a non-permeable membrane on the outside would trap moisture on the inside and allow for any humidity in the home to do damage to framing and sheathing from the interior.

    Joe's "perfect wall" puts all of the insulation outside the structure (i.e. the framing and sheathing), and all of his suggested assemblies in his "perfect wall" article puts at least some of the insulation on the outside of structure:
    https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall
    This is to keep the structure at approximately the same temperature as the interior of the building: that way, moisture does not condense on the structure and damage it. The IRC has the minimum required exterior insulation necessary to prevent condensation built right in, and you can review it on GBA here:
    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    >I am considering 2×6 24″OC with 2″of spray foam in the stud bays and then wrapping the entire house with 3/4″ foam board to prevent this conduction and then using house wrap or tarpaper on top of that and then siding with textured plywood. I am trapping the sheeting between two vapor impermeable layers. Will this create a problem? Any suggestions?

    As you have noted, it is usually not good practice to trap the sheathing like that. Though your construction reduces the risk of bulk water reaching your sheathing, if a hole does develop in the housewrap and water reaches sheathing, it would be unable to dry and fail very quickly.

    My suggestion is to skip the spray foam, as it is expensive and adds little to the assembly. As you are putting rigid foam insulation on the outside anyway, it is a good idea to thicken it and rely on the rigid foam as your air, vapor, and water barrier. This can be accomplished by a few different methods:
    - For some rigid foam boards, you can tape the seams and make the foam itself your air and water barrier.
    - You can place Grace's Ice and water shield or a similar product over the foam.
    - You can use a structural sheathing product with integrated foam, like Huber's Zip R system, and follow the manufacturers directions.

    If you are dead set on using spray foam in your assembly, consider using a more vapor permeable outside insulation layer, like rigid mineral wool board or maybe EPS. Alternatively, you can put spray foam on the exterior of your sheathing:
    https://buildingscience.com/documents/insights/bsi-048-exterior-spray-foam

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