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This building technique seems to be suffering for lack of a name.

EthanT | Posted in Green Building Techniques on

Having read almost every article I can about ways to put together a passive/prettygood house… having dived deep into AAC and CLT… I finally have come upon a wall system which seems to combine the simplicity of AAC/CLT with external insulation, the effectiveness of a PERSIST wall, and the cost effectiveness of conventional/smart framing. It just seems that this wall doesn’t have a name (perhaps it is a modified PERSIST?)

BSC titles the article (https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-advanced-frame-mineral-fiber-board) “2×6 Advanced Frame Wall Construction with Mineral Fiber Insulation Board” which does not roll off the tongue.

Am I wrong in thinking that this seems like the easiest to build, easiest to detail, and most cost effective wall assembly out there? While it is, in a way, listed in the article How to Design a Wall(https://www.greenbuildingadvisor.com/articles/dept/musings/how-design-wall). But it seems to me that this wall type is missing a name.

Could it be called PERSIST+ (as in +cavity insulation?) Am I missing something – perhaps this isn’t just PERSIST with cavity insulation?

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Replies

  1. EthanT | | #1

    Sorry to piggyback on my own post... but I guess I'm wondering a few things... One is, is it really worth doing the cavity insulation that is shown the the BSC article, when this introduces the problem of potential moisture problems in the wall (cold sheathing?) and also adds a second insulation type? Why not just add more rigid insulation to the exterior? Lastly, am I crazy to think that I could combine rockwool insulation with cork, to create the following assembly:

    smart frame wall --> taped zip --> rockwool --> cork

    That's it. No strapping or siding needed. I guess an open question is how to fasten everything back to the ZIP. Also, while I've got your attention, is there any problem with using finish plywood as the interior sheathing/finish on the smart frame wall (as opposed to gyp. board?)

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Ethan,
    I'll just deal with the "while I've got your attention" question.

    The chief benefit to gypsum, as opposed to finished panel products, is that it gets a coating of mud on joints, corners, penetrations and fasteners. No matter how precise the framing, there is no way you will be able to just cut and install plywood without scribing and fitting each piece. Holes for electrical boxes, which in gypsum are easily routed out after the board is tacked, become exercises in fine carpentry. Fasteners now become flaws you need to hide with filler. The inevitable scratches and dings gypsum surfaces take on during construction are now permanent features of the finished house.

    Can you do it? Sure, but it's worth stopping to think why gypsum walls are ubiquitous in all types of houses built in North America. You are better off drywalling the house, then using other finishes like wood or tile where they will most effect the architecture.

  3. GBA Editor
    Martin Holladay | | #3

    Ethan,
    Call it a "framed wall with continuous exterior mineral wool." GBA has lots of articles about this type of wall:

    Installing Mineral Wool Insulation Over Exterior Wall Sheathin

    Installing Roxul Mineral Wool on Exterior Walls

    Wrapping an Older House with Rock Wool Insulation

    Windwashing in Exterior Mineral Wool

    The main advantage of this approach is that the wall sheathing can dry outward.

    The main disadvantages of this approach:

    1. Mineral wool board is almost always more expensive than rigid foam -- and is significantly more expensive than recycled (reclaimed) rigid foam.

    2. Mineral wool is a little squishy, so it can be challenging to get the furring strips co-planar.

    -- Martin Holladay

  4. EthanT | | #4

    Martin. Thank you. I have concerns about foam, even recycled. That being said, What about using as layer of cork as the final insulation layer and siding... avoiding furring strips altogether!

  5. GBA Editor
    Martin Holladay | | #5

    Ethan,
    The furring strips serve two purposes: they provide a substrate that can be used to fasten the siding, and they provide an air gap to encourage fast drying. Cork would do neither.

    -- Martin Holladay

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