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Green Building News

And the Best Wall Is…

Christine Williamson discusses the "perfect wall" before moderating the debate and revealing the winner of this fun tournament of wall assemblies

Jess Chaloux

If you haven’t been following the Sweet Sixteen this year it’s probably because you thought basketball games were all cancelled due to the global pandemic. You’d be right about that, but you’d also be missing out on the fantastic educational opportunity afforded you by the architects, builders, designers, and tradesmen who submitted their wall sections for comparison in the #WallAssemblySweet16 competition on Instagram, and here on GBA.

Beginning in April, our BS* + Beer discussion group in Kansas City hosted a tournament of sixteen wall assemblies in head-to-head competition for the sake of fun and education. On April 28th Christine Williamson presented her thoughts on the “perfect wall” assembly and then moderated a discussion with finalists Mike Maines and Ben Bogie before awarding the championship belt to the winner. The discussion was fantastic and the results were surprising so grab a cold one, sit back and enjoy the Wall Assembly Sweet 16 Finale.

-Travis Brungardt is a partner at Catalyst Consctruction in Prairie Village, Kansas and a founder of the Kansas City BS* + Beer group.




  1. user-2069108 | | #1

    Please give us an executive summary. i would need more than a 6 pack of cold ones to listen to 1 and 3/4 hours of this rambling presentation.
    I am sure there is good information in there but I don't have the patience for 1 and 3/4 hours.

    1. KauaiBound | | #8

      This was a mostly-great discussion. Lots to learn even though most climates don't need a super-insulated wall assembly.

      That said, think this contest went off the rails because no boundaries were set - and of course the most expensive assembly won.

      Think it desperately needed a more-nuanced contest - what's the most cost-effective wall based on most important criterias that Christine noted: 1) fenestration 2) air sealing and 3) thermal.

      1) Fenestration is mostly a matter of cost - but sure would be nice to get recommendations for cost-effective windows because most of us are only exposed to the few brands that can afford lots of advertising and paid influencers. There has to be some small window manufacturers offering outsized value.

      2) Air Sealing - this is where the most bang-for-the-buck savings are to be had, yet there's limited discussion on details/products other than those brands that do the most advertising and are pushed the most by paid influencers.

      3) Thermal - this contest seemingly focussed on thermal which is last in the heirarchy of importance and suffers from fast-diminishing returns. Do we really need very expensive super-insulated homes when it is increasingly cheaper to build less thermally-efficient homes and instead add more solar panels? On the social sites I visit (almost all builder-focussed), it's almost never discussed - because it's money out of their pockets to admit a less-insulated, easier-to-construct home with an extra solar panel or two is much cheaper/faster/less complicated (and may be given credit from an appraiser whereas extra insulation/efficiency is often ignored).

      What pisses me off the most is the lack of discussion about costs - it's a fantasy to discuss pros/cons without including costs.

      1. Expert Member
        Michael Maines | | #9

        If you followed the weeks-long discussions on @BS_and_Beer_KC's Instagram page, you would likely agree that the lack of any rules was part of the appeal--submissions were of every type and shape, and cost was discussed frequently, as was vapor movement, constructability and everything else you would consider when building a wall. My submission (probably not the most expensive submission, but among the more expensive ones for sure) was simply the best wall I've used; others entered assemblies that were cost-effective for their locations, and many of them made it past the first round. Even the final round was neck-and-neck between a relatively low-cost double stud wall (the submission was 12" but the same wall can be thinner/cheaper) and Ecocor's outrigger system that I submitted, which was nearly identical in many ways to Steve Baczek's submission which made it to the second round.

        The whole point of the contest was not to win, it was to educate and share ideas. That's why announcing the winner is relatively pointless--it's the content that was important. If you don't think the Ecocor wall makes sense, that's fine, don't use it. But if you followed along during the contest, or listened to Christine's overview of what makes a good wall, I would be shocked if you didn't learn anything. I know I learned several things and it also helped me think more about climates and markets where I don't work and thus don't have to consider.

        Anyone who is "pissed off" about this contest or the summary needs to reassess their priorities. The organizers in particular should be thanked for their efforts in putting this unique contest together--it was no small undertaking. And thanks to Christine Williamson for creating and presenting an overview, and moderating the discussion.

        1. Expert Member
          MALCOLM TAYLOR | | #10

          I enjoyed it. Funny how the restrictions on public gathering ended up making the discussion more wide-ranging and relevant to more regions. At least there are some upsides to all this disruption.

          I do think that the incredible recent proliferation of wall assemblies is a temporary thing. over time I suspect two or three in each climate zone will dominate, and the rest will rarely be used. That's probably a very good thing. Given the skills necessary to be a successful builder, including economic literacy, ability to manage workforce, subs, and consultants, get approvals, etc, it's unrealistic to expect them to evaluate a new custom assembly for all the factors Christine listed each time a client comes through the doors.

          An boy - you guys sure drink some strange things!

  2. malady | | #2

    It is well worth the time, particularly the first 30 minutes or so.

  3. Jon_R | | #3

    tl;dr. I believe the winner is a cellulose filled double wall with Zip as the interior side air and vapor retarder.

    With 38 perms on the exterior and < 1 perm on the interior, I'd be interested in moisture measurements while AC is being used. 10 perms or less on the exterior may be wise in warmer climates. Or replace the Zip with plywood.

    1. Expert Member
      Michael Maines | | #4

      The two finalists were Ben Bogie's double stud wall with cellulose insulation and Ecocor's PassivWall, an outrigger system with a thick layer of cellulose on the exterior, which I submitted. Ecocor has done extensive modeling and is a Passive House certified opaque assembly. Both systems have several projects with data loggers to track real-world performance. They both work fine in heating-dominated climates but I don't know anyone who has used them in cooling-dominated climates. To find out who won, you'll have to watch the show.

      One note, when Zip gets damp it opens up to about 3 perms. Dry it's about 1.8 perms. (for 7/16" sheets.)

      1. maine_tyler | | #6

        Do you expect to see a large shift to exterior wood fiber insulation with Maine and regional builders if/when GO logic starts producing Maine-based rigid wood insulation?

        1. Expert Member
          Michael Maines | | #7

          Tyler, yes--especially if GO Lab can come in at a price that makes them competitive with rigid foam, as planned. I've talked with many high performance builders who will likely change to framed walls with exterior wood fiber, or at least offer it as an option.

    2. Expert Member
      RICHARD EVANS | | #5


      Your thoughts remind of a comment Katrin Klingenberg made on GBA a few years ago regarding a similar wall detail on her Smith House:

      "We teach people the recipe for superinsulated walls in all climates and tell them very clearly: don't replicate the Urbana wall in Louisiana - it'll fall apart."

      I also remember Thorsten Clupp talking about his Arctic Wall (very similar to Chris's and Katrin's wall) and how it works well in a dry, cold climate Fairbanks. He stated that he would never use that wall in a wet, humid place like Juneau.

      I believe Michael Maines when he says these walls have been tested and they work. But I disagree with Chris Corson that his walls would work anywhere "between the North Pole and the South Pole" :-)

  4. don_christensen | | #11

    I love Chris Corson's work, but...what's up with those stairs? No handrail in sight. Is that OK?

    RE: exterior wood fiberboard insulation - Would you protect that from insects the same as with rigid foam? Are the wood fiber products treated with something?

    I learned a lot from the presentation. Christine Williamson's tutorial at the beginning is worth watching twice.

    1. Expert Member
      Michael Maines | | #13

      Donald, I believe Chris was in his home office, in the basement of his house. Railings are always required when there are more than three risers but some situations are less of a concern than others.

      Wood fiber exterior insulation has a paraffin additive but nothing to prevent rot or insect damage. (The blown product has a borate additive.) As with most types of exterior insulation, separate protection from insect damage is advised.

  5. karlb_zone6a | | #12

    (arriving late to the party, but only by 2.5 months!)

    Christine does a great job at 01:15:30 of summing up the big concerns with cellulose-insulated double-stud walls (or similar):
    1) the interior air-control layer needs to be faithfully executed (to keep internal moisture out of the cellulose)
    2) external bulk water (and to a lesser extent, external vapor drive) need to be effectively and robustly managed, because there isn't going to be sufficient energy flow to dry-out wet cellulose

    Given these concerns, would these assemblies be improved by sandwiching the cellulose between inner and outer layers of mineral wool? That is:
    - would these layers provide a path of "lesser" resistance for water vapor to diffuse out? (i.e., the inner layer of mineral wool, between cellulose and the sheathing / air+vapor control layers)
    - would a outer layer of mineral wool, behind a rainscreen, help protect the cellulose from bulk water intrusion?

    1. AlexPoi | | #15

      The best way to make this assembly more robust would be to install water resistant rigid wood fiber boards like gutex or steigo boards on top of the cellulose cavity instead of the proclima membrane. This would also reduce the condensation potential as these panels have a R4 insulating value and are very vapor permable. Unfortunately, that kind of material isn't made in NA yet (you have to import it from Germany) and is therefore very expensive.

      Here in canada, we have the sonoclimat eco 4 panel which is a wood fiber panel but that is not water resistant. Therefore you need to install a wrb on top of it which takes more time. But at least the price isn't prohibitive like the other wood fiber boards.

      The wall I'm thinking of using when building my house next year is :
      Furring (service cavity)
      Taped sheathing
      Double stud wall filled with cellulose (the internal wall is the load bearing one)
      Wood fiber board (sonoclimat eco 4)
      Wrb (solitex mento)

      I'm in a seismic region so I'll need to strap the floors together since my sheating will be on the inside but this way I can close the house to the elements before insulating it and I can air seal it from the inside.

      I think we'll see these kind of walls more and more if we ever get fiber board produce here in NA at a decent price. And cellulose is probably going to be replaced by wood fiber insulation as we'll be running out of newpapers to recycle in the next decade.

      1. Expert Member
        MALCOLM TAYLOR | | #16


        Where are you building that a double stud wall with additional exterior insulation makes sense that's also a high seismic area?

        1. AlexPoi | | #17

          A bit north of Quebec city. It's not as bad as BC but we're still in a high risk zone. People don't seem to realize it though. I see plenty of houses being built with no sheathing on only metal braces. If we ever get a 7 magnitude earthquake, it's going to be ugly.

          1. Expert Member
            MALCOLM TAYLOR | | #18

            I was in one in pretty good shake when I lived in Ottawa. Cracked the drywall. Like many people I didn't know it was problem there.

            I wish someone would start manufacturing Gutex style sheathing out our way. Most of the mills making conventional products are in trouble or have shut down.

  6. Expert Member
    Michael Maines | | #14

    Karl, that's an interesting idea, a hybrid mineral wool/cellulose assembly in a double stud wall.

    The inner layer would raise the assembly R-value slightly, as it has about 15% higher R-value than cellulose, but not enough to change the basic characteristics of the wall. Mineral wool may be a bit more air- and vapor-permeable than cellulose, but the limiting factor to inward drying is the air- and vapor-permeance of the wall surface (usually painted drywall) and/or vapor retarder, so no real difference in practice.

    An outer layer of mineral wool would be detrimental, as some testing (by Building Science Corp and others) have shown that borate-treated cellulose in contact with damp wood very likely protects the wood from decay. Mineral wool is more durable than cellulose in some ways, but it leaves the framing lumber and sheathing as the most absorbent materials on the "wet" side of the wall, with no significant contribution performance.

    What could make sense if you want a double stud wall with more advanced performance is to install a variable permeance membrane on the exterior side of the inner wall, dense-pack the outer cavity with cellulose and create an interior service cavity, which can be filled with mineral wool batts. In this case any type of batt or blown insulation could be installed in the service cavity, because a perfect installation is less important when you have a thick exterior blanket. There are some order-of-operation issues with this assembly which is why most double stud builders do it like Ben Bogie.

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