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Best exterior wall assembly

  • 1.  Best exterior wall assembly

    Posted 08-25-2020 17:53
    I'm designing and building my own home in South Texas, and I've been reading about Building Science exterior wall assemblies where the insulation recommended for this climate is 2" min. rigid insulation installed on the outside of the 2x4 framing and sheathing.  Does anybody has any experience with this method of building? and if so, what details you use for windows, doors, slab to wall, wall to roof, etc.

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    Jorge Pena
    President
    JPA architects
    San Antonio TX
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  • 2.  RE: Best exterior wall assembly

    Posted 08-26-2020 17:43
      |   view attached
    I resctfully recommend that on the exterior of your sheathing, you provide a fluid-applied weather barrier, followed by a rainscreen mat to allow air movement channel the condensation out and keep mold away. Depending on the exterior material you are contemplating, you can have your rigid insulation screwed to the sheathing through the weather barrier (the good ones are self-sealing = reason to avoid sheet goods) or install a sandwich with a second layer of sheathing for a smooth substrate for your surface material.

    Attached is a stone veneer detail over a sandwich panel with closed cell spray foam in the exterior stud cavity.

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    William Eberhard AIA
    Managing Partner
    Eberhard Architects LLC
    Cleveland OH
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  • 3.  RE: Best exterior wall assembly

    Posted 08-26-2020 17:51
    Hi Jorge:  DORKEN systems offers alot of free training online and as an architect I can say that they have been very helpful in understanding the parts required to build these high-performance walls.   The other thing that is necessary is a whole lot of customized sheet metal for window frame outs, penetrations and so forth.  Good luck.

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    Maia Gendreau AIA
    San Jose CA
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  • 4.  RE: Best exterior wall assembly

    Posted 08-26-2020 18:07
    The system sounds like EIFS- Exterior insulation and finishing system. It's basically a polymer stucco material with sheathing, a scratch coat with embedded fiberglass mesh, 2" thick minimum aged styrofoam insulation and one or two coats of the finish stucco.
    Benefits are that you can form the stucco with grooves or projections, of have it formed into cornices and other details.
    But- Make sure it is back drained to let any water that gets in gets weeped out. There were a lot of problems in residential use- mold and such- so it is imperative that the system is installed by an experienced contractor that follows the manufacturer's instructions to the letter.
    The manufacturers include Dryvit, Sto,  and a few others.
    The IBC code requires Special Inspections in some types of construction but the Residential code might not.
    Be sure to follow the details around windows, doors, and penetrations. The fiberglass mesh and finish coats all have to be returned to the inside of the rough opening under the flashing.
    We mostly do commercial projects- not much wood framing. If you have questions on this let me know.

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    Peter Elkin AIA
    Peter Elkin Architect
    Commack NY
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  • 5.  RE: Best exterior wall assembly

    Posted 08-27-2020 19:20
    We have used Zip-System Insulated R-Sheathing by Huber in central Texas.  It is OSB laminated to rigid insulation with a weather barrier and comes in a variety of thicknesses.  The window fins are applied to the exterior of the sheathing and sealed with a proprietary tape, so the thickness is not an issue.  Contractors like it because installation is not difficult.  It's important to seal the bottom of the insulation board to prevent fire ant damage, but you can set the studs back so the sheathing butts into the foundation.

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    Jane Kittner AIA
    Kittner & Pate Design Associates
    Waco TX
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  • 6.  RE: Best exterior wall assembly

    Posted 08-27-2020 00:03
    I’d recommend 2x6 studs, which would allow R19 batts in the cavities. What is your exterior wall cladding? Thermally broken z-furring would probably work well. The window details would depend on where they are in the wall assembly. Are they set back at the weather barrier (plywood) or are they at the face of exterior rigid insulation? Are they nail fin? At the base of wall, the c.i. would ideally extend down below finish floor. If you have an exposed concrete base, then this could be done with creating a sandwich of conc. on each side of 2” rigid insulation. This will be more expensive and probably not practical with low skilled residential labor. Extending the c.i. below finish floor is probably not super critical in your climate. It’s more important in the blizzard belt. I’m in Norman, OK, and most people in this region don’t understand basic building envelope details. Just using c.i. will make your house way better that most. Good luck.

    Aaron


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  • 7.  RE: Best exterior wall assembly

    Posted 08-27-2020 06:40
    I'd recommend that you check "ZIP system R-Sheathing" by Huber Engineered Woods. This product includes the continuous rigid insulation adhered to the OSB sheathing and may have details similar to your application. (The rigid insulation is positioned between the OSB and the studs.)

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    John Dickerson AIA
    Dickerson Architecture
    Monroe NC
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  • 8.  RE: Best exterior wall assembly

    Posted 08-27-2020 19:34
    Having lived & practiced in Texas for many decades, including much light & residential construction, and being familiar with the labor force you will probably be dealing with, I recommend you follow the KISS principal with your design. I second the recommendation of the use of the ZIP System R-Sheathing for your MRB, continuous insulation & air barrier. Follow the manufacturer's recommendations for detailing around the rough openings. Google "ZIP System 2.0"; consider contacting and consulting with Matt Risinger- he's just up the road from you in Austin. Get the highest-performance windows & doors your budget can stand. Consider carefully how your high-performance walls terminate- top & bottom and how they tie into the rest of your thermal envelope. I recommend focusing on a gap-less continuous insulation wall envelope with a TESTED air barrier resistance for the structure of not more than 1.0 ACH50 rather than the Code maximum value of 3.0 ACH50 with testing optional. Put this requirement in your contract with the builder. Consider having bonus $xxx in the contract for every 0.1 ACH50 below the contracted maximum ACH50 that you can verify by third-party testing. This will REALLY get the contractor excited & on your side.
    In your location, you have a serious termite issue (Formosa termites in addition to domestic termites) starting where the foundation/slab meets grade. Termimesh is an environmentally-friendly, non-toxic & permanent alternative to rodenticide. Your exterior walls will need a permanent, effective termite barrier where it meets the slab.
    Sooner or later you will have to replace your heat pump unit and it's lineset. Unless you plan ahead, the replacement work will thoroughly butcher your wall air sealing efforts. 2x4 framing is fine. Consider using Advanced Framing if your AHJ is OK with it. This will reduce your lumber cost, reduce waste & inefficiencies and reduce thermal bridging.
    Best of luck on your project!


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    James White AIA
    Depew NY
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