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Community and Q&A

The Perfect Wall

hunterchap | Posted in Green Building Techniques on

Hey folks,

Just wanted to hop on here with a quick question as I’m in the process of building my first home within the year. I was looking at Lstiburek’s “Perfect Wall” assembly and I noticed that he has two layers of sheathing. One directly behind the housewrap, which is standard building practice in my area of Massachusetts, but then he has one fastened to the interior 2×4 wall, inside the first layer of cellulose. My question is whether or not this is necessary and what purpose this serves?

For context, I’m building a PGH. I intend to use LP Smartside with a 1×3 furring vertical rainscreen behind it, Tyvek or equal housewrap, 1/2″ CDX sheathing, two 2×4 double stud walls with a 3-4″ gap, with dense pack cellulose insulation, and then drywalling the interior. My exterior 2×4 wall will be load-bearing, as opposed to Lstiburek’s interior wall being the load-bearing wall. I hadn’t anticipated that second layer of sheathing on the interior and it seems somewhat cost-prohibitive for me, a 27 year old building their first home. Therefore, I was wondering if you guys and girls could explain the thought process behind that detail and whether or not it is entirely necessary in the assembly to build a PGH? I’ve attached Lstiburek’s Perfect Wall for reference.

Thanks so much and have a great day!

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Replies

  1. Expert Member
    Michael Maines | | #1

    The outer layer is to provide solid backing for the WRB and to contain the dense-packed cellulose. I have used a very similar assembly but with only Pro Clima Mento on the exterior, tightly stretched, tightly stretched and airtight, and it works great. The membrane does pillow out a bit so you need to use thicker furring undre the siding.

    1. hunterchap | | #2

      Michael, I'm a huge fan of your work, thanks so much for the reply! So if I'm understanding correctly, you don't use two separate layers of sheathing, just the one on the exterior. And you're a Pro Clima Mento guy as well? I've researched Mento as well as Blueskin but will have to weigh the costs a bit because they seem rather expensive. However, extremely important so probably worth the investment.

      1. Josh_Dillingham | | #3

        Because the inner wall is load bearing in "the perfect wall" that inner sheathing would be structural for shear strength. Which would be the opposite for you if your exterior wall would be load bearing. From my understanding the advantages of having the interior wall be load bearing is that most or all (depending on how you built it) of the insulation would be outside the structure which would eliminate any issues of warm conditioned air or vapor cooling and condensing on your structural sheathing and framing. In the perfect wall that air and vapor will be trapped by the interior sheathing and what does escape would condense on a non structural surface. The other benefit of having an interior load bearing wall is that you would be eliminating all thermal bridging from floor joists.

        That being said I also choose the path you are planning with a double stud wall with the exterior wall being load bearing and depending on my air sealing of my drywall to prevent conditioned warm air from condensing on my structural sheathing and framing.

        I finished my house earlier this year in southern Vermont and my layers are as follows: vinyl siding>1/2 zip sheathing, taped>2x6 structural framing 24"oc>3 inch gap>2x4 non structural wall>1/2 drywall, air sealed with latex paint. And my wall cavity is filled with 12 inches of dense pack cellulose.

        The house is performing great in the first winter so far. 2000sqft And is maintaining a temperature of 72-73 degrees downatairs and 67-68 upstairs with a single 12000btu Mitsubishi cold climate heat pump using about 15 kwh per day when the outside times are 10-20 degrees. And used about 25 kwh per day with temps in the single digits all day while still maintaining the inside temperature. Only time will tell if my air sealing of the drywall was good enough to eliminate the condensation moisture issues, but I plan on opening up the walls in a few spots to check on that later this year.

        1. hunterchap | | #4

          No kidding, thanks for the info man I appreciate it. If you don't mind me asking, whereabouts in Sothern Vermont? My family used to have a ski house in Wilmington so I spent a majority of my childhood up there and at Mount Snow and Haystack. Glad your build went well. Sounds like were doing pretty much the same exact thing. I'm 1,800 SF but same assembly pretty much.

          1. Josh_Dillingham | | #5

            I'm in Newfane, which is about 15 miles from Wilmington. I can see Haystack from my neighbors deck. Feel free to email me if you have any questions. I'm no expert, but I'm sure I've already made all the mistakes you're trying to avoid, haha. And I love talking about this stuff more than most people around me like to listen to it. Dillinjl (at) Gmail (dot) com

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