Thursday, May 20, 2010

Walls for New Construction, State of the Art from Betsy Pettit

I've built two homes with SIPs. Things didn't go quite as well as I envisioned, so I've been planning to try a double stud wall (not staggered stud) on the next project. My personal lessons learned to this point follow the same arc that Postgreen Homes has documented really well.

There were still a couple of fussy details about double stud design that were bothering me:
1. In order to cover the exterior insulation around the foundation/basement wall, the outer stud has to be cantilevered out too far.
2. Building a double stud wall is too much like framing two walls, which implies double the framing cost.

Building Science Corp and Betsy Pettit, AIA, have come to the rescue in the latest issue of the Journal of Light Construction (JLC):

They're promoting, and have tested, an Advanced Framing wall sheathed with foam. The innovation is the method to support the siding, 1x3 furring strips are attached to the stud wall with 6" screws. This furring is easier, cheaper and better than another stud wall.


  1. I'm with you on this - proposed a very similar arrangement in my Lagom House:

    However my approach was 2x8 walls, with one layer of foam. I'm not sure that the extra layer of foam would not pay for the deeper studs and wall insulation and make for a faster install.

    I'm not on board with the hold downs - seems like a difficult installation while there are so many simple clips and straps from Simpson.

  2. I agree about the hold downs,bolting down the sill plate has always been good enough in the past...

  3. Yes - well, there is a lot going on in their proposal. The tie rods are related to the decision to rely on the foam as sheathing, and reduce the plywood to limited shear panels. Typically the plywood contributes to tying the top down to the sill plate - without that they need to do something more substantial.

    And although its not mentioned specifically, I suspect they are reversing the conventional cold climate placement of the vapor barrier from the inside to the outside via the foam insulation. They are showing the joints sealed and taped which implies the vapor tight seal is the foam layer. In this case the two layers of foam are needed to move the dew point inside the foam where moisture can not condense. I'm wary of this and would want to study out of range temperatures to see what kind of cold spell might cause condensation in the wall cavity.

    Anyway, in the end I think this proposal is very smart, but at the same time strays too far from convention to keep costs low. Frankly I think my design is more familiar to the average builder, the work more predictable.

  4. Thanks for the link. Two comments:

    1. 6" screws are not cheap. Not cheap at all. Neither is all that PolyIso.

    2. Framing and extra stud wall is almost free as long as it's standard construction. Framers price on per square foot and 2x4's are cheap. 2x3's even cheaper.

  5. Chad,

    As I keep studying wall systems, I'm back to considering the double stud. For several reasons: cellulose is the cheapest per R value, I've figured out a way to overhang the foundation insulation adequately, the 100k method of eliminating the wiring in the outside walls, and I've decided to use an FPSF monolithic slab foundation.

  6. Kevin, I am in the process of redesigning my wall system right now, so this is timely. I've jettisoned the idea of exterior foam completely. The foam is expensive, and it forces this exterior vapor barrier which I just can't recommend. Following the Swedish models I've been studying:

    I am proposing a 2x6 system, and a 2x8 system. In both cases stud space filled with inexpensive unfaced fiberglass batts, R21 high density for the x6 walls, and R25 unfaced for the x8 (has to be a compressed 8" batt - its time for Certainteed to step up with a high density 7.25 wall batt), covered by a continuous vapor barrier on the interior side, followed by a 2x furred layer for wiring and plumbing filled with R6-6.5 fiberglass. The furring layer makes an effective thermal break albeit not as thorough as exterior foam, but there is nothing going on in this wall that is not strictly off the shelf product, same supply stream, same subs, and the same process every builder has been doing since they learned to crawl. Its market friendly.

    On the outside a builder can use anything they usually do, be it an expensive rain screen, or cheap vinyl over that damned taped Z stuff. And inside is your usual sheet rock, but no moratorium on switches or outlets or hanging pictures even..

    I'm on a different mission that you however - I want something that sacrifices a little efficiency for easy adoption. These should end up with an R24-5 and R30-32 walls that are simple, and robust rather than fussy and easily undermined. I think that is what the market needs. I hope we do a project together at some point to apply this.

    On your foundation - I want to know more detail about what you are doing there. Are you getting foam forms made, or are you buying from the CA vendor?

  7. Greg,

    We plan to just have a simple FPSF monolithic slab that is 100% thermally broken by R10 high density foam all around and underneath, including under the footing area. Since the updated FPSF manual doesn't have a drawing for a heated building showing that, we'll have stamped drawings.

    Do you plan to have 2x2 horizontal strapping a la the Mooney Wall?

    I like robust, for sure. And I think blown cellulose is more robust than fiberglass. Here's my thinking - staple up the mesh before nailing up the strapping. The strapping holds the mesh well without excessive stapling, and accomodates any bulging easily.

    Now the blower door test may be done before the sheetrock goes in, and corrections made. You can also install your air/vapor barrier on the same plane as the mesh.

  8. On the foundation insulation foam, I'd encourage you to make a thermal break between the grade beam edge, and the slab - bridge it with a CMU ladder tie so they can't move - and build up 6-8" of foam under the slab.

    Yes - 2x2 furring space, but not like a Mooney wall. My understanding of the Mooney is its just like a regular wall except the furring decreases the thermal bridging. Electrical boxes still put holes in your vapor barrier. I've posted my description of the wall on my blog:

  9. How did it go - I'm planning on building a hunting cabin, and wanted to try the latest and greenest method.

  10. I built my mountain cabin this way, and it went well, but I'm still not satisfied. See