May 23, 2016

The Double-Stud Wall Simplified: Low Cost, High Performance

double_studSimplify.

We know pushing standard code-minimum construction toward high performance is complicated. So we’re always looking for ways to simplify – to simultaneously reduce cost while optimizing efficiency and occupant comfort. [See our recent post on Breaking The “Upfront Cost Barrier”.] The double-stud wall is a well established method for creating a very economical, durable and high R-value assembly  in new construction. This type of wall can be built quickly with standard methods and keeps the material pallet very simple when compared to other thick wall assemblies (studs, one type of insulation, membranes + tapes).  Integrated with Pro Clima airsealing and moisture control components, the double-stud wall provides unmatched economic value, safety from moisture damage, and long-lasting performance.

Design Elements

With Pro Clima component integration, we’re taking it one step further. Below we illustrate a sheathing-less double-stud wall (that’s right: no structural sheathing) that provides the following characteristics:

  • Minimized material costs
  • Maximized moisture drying potential
  • Removal of formaldehydes, VOC’s and other toxic chemicals readily found in SPF, rigid foams, OSB and plywood
  • Easily adjustable wall thickness to meet your design R-value.
  • Space between walls for continuous insulation
  • Utilization of dense-pack insulation (cellulose, fiberglass, mineral wool or sheepswool) – or smartly stack batts (see this project).
  • No special materials or connections needed for the framing component
  • Fits with the typical platform framing method

You can still frame your walls on the deck and raise them into place as you’ve been doing for decades – but without all that sheathing they’ll be a little lighter. With this system, you build a house out of 2×4’s, fibrous insulation, SOLITEX MENTO Plus weather resistive barrier, INTELLO Plus smart vapor retarder, and not much else. Pro Clima components and double-stud construction provide optimized insulating and moisture-control conditions that effectively eliminate formaldehyde based glues/sheathing while enhancing sustainability and comfort.

The elements of a cost efficient double stud wall assembly: Interior perspective


Down To The Basics

The wall consists of an outer load-bearing wall and an inner finishing wall. The floor and roof loads are stacked on top of the outer wall studs. This advanced framing method can allow in certain cases use 24″ o.c. assembly if your floor and roof loads meet the design criteria. The outer wall is framed like any other stick-built wall, with the exception that the shear load is carried by 2x lumber nailed diagonally to the back side (in the insulation cavity). It’s important to note that each structure will have very different shear and uplift retention requirements due to variables in building height, # of windows, local codes, shape of building, seismic requirements, etc.

The inner wall is connected to the deck and the underside of the trusses or joists. It requires minimal framing material and opening headers as it need only retain the insulation and carry the finishing surface (drywall). In taller walls, it’s important to install a connect the inner and outer studs for additional strength as well as partition the bays every second bay – to make densepacking of the double stud cavities easier to quality control (reach proper density).

The elements of a cost efficient double stud wall assembly: Exterior perspective

The inner wall has one additional framing element: the service cavity.  We consider the service cavity to be essential in this construction, for 5 main reasons:

  1. It provides room for pipes and wires to travel without multiple penetrations of the airtight layers
  2. It offers simple installation of utility boxes and light fixtures
  3. It protects the Intello Plus membrane from possible puncture once the wall is closed, as the drywall is a sacrificial layer
  4. Together with  INTELLO Plus it supports the dense-packed installation with horizontal ledges – which helps distrubute the weight/pressure of the insulation at the INTELLO plus and provides best results for insulation density in walls.
  5. It provides an additional thermal break when the service cavity space is filled with insulation

To optimize the insulating value of the dense-pack insulation – airtight membranes are placed on both sides of the cellulose, thereby preventing thermal bypass,  as well as optimize the drying reserves of this highly insulated wall. At the interior side is the INTELLO Plus membrane, airtight with intelligent vapor control, making it vapor open in the summer to facilitate inward drying and vapor retarding in the winter to prevent vapor accumulation into the insulation. The INTELLO Plus is reinforced so that it substitutes for the typical mesh used in a dense-pack installation. At the exterior side is SOLITEX MENTO Plus:  airtight, waterproof, reinforced and vapor open, allowing for maximum drying potential to the outside without being restricted by an exterior sheathing (plywood/OSB are class II/low class III vapor retarders).

Window Installation

Double-Stud_Optimized

The window is installed into a plywood box that ties together the inner and outer walls. We offer a wide selection of Window Air Sealing Tapes, but to keep it simple you need only Tescon Profil (or the even faster Tescon Profect) for the airtight connections at the interior and exterior of the window (don’t forget to pre-make your window corners), and Extoseal Encors for a heavy-duty weatherproof sill.

There are multiple ways to create a thermal bridge free window installation – there are many variables dependingon on the window type and brand. The most important thing is to make sure that the window is precisely connected to your interior and exterior airtight and moisture control layers. This will ensure that your installation will not have condensation due to air movement at this thermally weak intersection. By using SOLITEX MENTO Plus as your exterior weather tight layer and attaching it directly to your installed window, you can provide the needed continuous water intrusion barrier while providing the correct vapor drying characteristics and assembly air tightness.

The same goes for the window connection to the interior INTELLO Plus airtight membrane. Small air leakage at this connection will allow the  interior winter humidity to enter the insulated cavity.  Making an air tight connection at all openings is the best way to prevent future structural damages. However additional safety in this highly insulated wall is provided by having a vapor open exterior without vapor retarding sheating. This wall make up allows small amounts of unforseen moisture (introduced by small airleakages or exterme wind driven rain events), to quickly and safely dry out of this assembly.

Further Details

For more details and variations on this concept, download our free CAD details for Foam Free Double-Stud Assemblies. Also see our blog post on the Integrated Service Cavity approach, in which INTELLO Plus is placed at the exterior of the interior stud wall to create a full, built-in 3.5″ service cavity.

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27 Responses to The Double-Stud Wall Simplified: Low Cost, High Performance

  1. Tom Bassett-Dilley June 10, 2016 at 4:44 pm #

    Good ideas for the assembly–. Couple of thoughts:
    1. On the exterior view your first layer of furring (on top of WRB) is horizontal, which would potentially drive moisture draining down WRB into the assembly–seems like you could have simply the vertical furring for horizontal siding attachment and avoid the water damming problem.
    2. I would prefer to have the interior wall be the load-bearing one, to avoid thermal bridging of the joists into the cold part of the wall (plywood floor diaphragm could run through to brace outside wall). I think that can be a trick at foundations, but do-able.
    3. Do you know how much “pillowing” of the fabric is expected if you fill cellulose to 3.5pcf minimum in the walls? Will the 1.5″ furring space be sufficient?
    4. Any idea if there’s a 1-hour fire rating for this assembly?
    Thanks for putting good info out there for us!
    Tom

    • Ken June 18, 2016 at 7:01 am #

      Hi Tom,
      Thank you for the comments.
      1. Yes, the furring could go any number of ways – we typically don’t see issues with the way that is illustrated but depending on the stud spacing different sequencing can be preferred. Based on your comment though, we’ll likely clarify this condition. Thanks.
      2. Yes, the load bearing could go either way and the primary reason for showing it outboard is continuity at foundation.
      3. Yes, 2x 3 battens provide sufficient clearance for dense packing cellulose with 16″ o.c. studs. If you have 24″ o.c. studs you may require “flying battens”. See your detail books on wood framing for more information. We should have a double-stud book out later this summer….
      4. We have not investigated what it would take to get a fire rating on such an assembly – but it’s a good questions. We’ll try to look into that.
      Again, appreciate your comments. And sorry for the delayed response.
      All best,

  2. Mark June 17, 2016 at 8:25 pm #

    what material could be the zig-zag line on the interior side of the window, between the drywall and window buck?

    • Ken June 18, 2016 at 6:51 am #

      Mark,
      It could be any number of insulation boards, wood fiberboard, mineralwool, fiberglass, even foam. Depending on the bearing capacity you might introduce blocking for support.

  3. Rick Evans August 18, 2016 at 3:18 pm #

    Hi Ken, Great information, thank you!

    I couldn’t find any information on “flying battens”. Any chance you could post a link to a description?

    Thank you.

  4. A.K. Harrison November 21, 2016 at 9:26 am #

    Can this same configuration be used in a hot humid climate in the US southern Gulf Coast? I plan to use 12 inches of cellulose.

    • Ken November 21, 2016 at 9:54 am #

      Hi A.K.,

      Yes it can BUT you should install a vapor retarding WRB outboard to prevent moisture loading of the enclosure over the summer months. Two Pro Clima membranes are possibilities: DA membrane at 1.43 perms or Fronta Humida membrane at 6.5 perms. You can see them here:
      https://foursevenfive.com/product/da/
      https://foursevenfive.com/product/solitex-fronta-humida/

      We are happy to discuss.
      – Ken

      • A.K. November 22, 2016 at 5:59 pm #

        Thanks Ken,
        If I understand correctly for the DA, (Caution: DA should be primarily used in combination with vapor open materials and membranes).
        Can the Solitex Fronta Humida also be added to the outside of the Solitex Mento Plus. I prefer SFH for the the higher perm rating.

        • Ken November 25, 2016 at 7:05 am #

          Hi A.K,

          I’m not sure I understand the question. Would be good to review a sketch of assembly – even a very rough sketch – that we can discuss on the phone. My email is ken@foursevenfive.com

          • A.K. November 25, 2016 at 4:14 pm #

            Where the Solatex Mento Plus is on the outside for Northern Climates, here in the south do I install the Sola Mento Plus along with the “DA” or the Fronta Humida or do I replace the Sola Mento Plus with one the two choices above that you recommended. I wanted to confirm if it was “in addition to” or it was a “replacement”. Thanks

          • Ken November 26, 2016 at 5:51 am #

            I see. The DA or Fronta Humida would replace the SOLITEX Mento Plus – as they are all waterproof and airtight. HOWEVER the Mento Plus is reinforced to hold back dense pack insulation. If you plan to use dense pack insulation you can use the typical insulweb containment fabric and then cover that with DA or Fronta Humida. If you plan to use batt insulation then the DA or Fronta Humida are fine. You can also install them over sheathing.

          • A.K. November 28, 2016 at 2:44 pm #

            Ken,
            Thanks for the help and clarification.

          • Ken November 28, 2016 at 3:05 pm #

            Pleasure. Let us know if there is anything else we can do. Cheers.

  5. Dale A. Burdin November 22, 2016 at 2:30 pm #

    Please help my family and I build our double stud house!
    I am a former house remodeler and have framed a few houses but never what I want to build for our family. I am looking for any publications that would make the process, both learning and actual building, smoother.
    If anyone can help, we surely appreciate it.
    Dale

    • Ken November 22, 2016 at 2:38 pm #

      Dale,
      Where are you located?

  6. Scott January 12, 2017 at 7:26 am #

    Any changes to this design using American made casement windows(with flanges) and installing them as outies..? I am in zone 5 ,Ct.

    • Ken January 12, 2017 at 9:49 am #

      Hi Scott,
      No, not really – it’s just a matter of the taping of the window being different. It can work very easily.
      Let us know if you need anything else.
      Best,
      Ken

  7. Seamus Sheehy February 5, 2017 at 6:23 pm #

    How are the internal 600mm or 400mm internal battens for the GWB spaced to fix to the internal diagonal cross bracing? Are the internal diagonal cross bracing pieces also spaced at 600 or 400 centres? Thanks.

    • Ken February 9, 2017 at 2:15 pm #

      Seamus,
      Thank you for your question. The cross-bracing only occurs at the exterior structural studs – but within the double stud cavity. The service cavity battens are attached to the vertical interior wood studs – so there should be no need to coordinate between them. Hope this makes sense.

  8. Derik March 13, 2017 at 12:32 am #

    Any progress on the double stud book?

    Thanks.

    • John March 13, 2017 at 9:03 am #

      Definitely progress…. stay tuned to the monthly newsletter, we’ll announce there when the time comes.

  9. Derik March 14, 2017 at 12:58 pm #

    How tested is this method of sheathing-less double stud? It seems like a great solution to the condensation on sheathing issue but is it too early to really know if this method is sound? Or do you have some real world results? We are getting ready to build double stud in zone 6. We are in a fairly dry portion of Montana but we get plenty of snow and cold weather so the condensation has been a concern. I’m very intrigued with this method though, just wondering if it has any shortcomings to be aware of. Do you have any projects using this method you could share? Thanks!

    • John March 14, 2017 at 3:19 pm #

      Hi Derik,

      We definitely do have some testing we can share. First off, Pro Clima products have been tested thoroughly and used in Germany for many years – which is why we can so easily stand behind them, as a supplier. But we know that we need examples from the US to really drive the point home. We have a few examples on the blog here. First there’s the Whitchurch Project. We have three blog posts describing the assembly and the results of monitoring, which was also covered by JLC. Find those posts here. Another example is our video with Northern Timbers, wherein Alex Carver cuts open the Solitex on the finished home to address that exact concern. Very interesting. Watch that here. We have more examples would could discuss. Drop us a line to hear more, and enjoy those posts in the meantime.

  10. Jim Faust March 27, 2017 at 7:52 pm #

    We’re going to build a home this spring. It will have a 30′ by 36′ footprint. We’ll use a 9/12 attic truss in order to accommodate some living space above.. What would be a ballpark figure to use your system for a double stud wall?
    I’m very nervous about not having exterior sheeting. I hate OSB so would use plywood. How would plywood sheeting change the configuration of your proposed double stud wall?

    • John March 28, 2017 at 9:17 am #

      Hello Jim,
      If you’re looking for total cost of construction we don’t have a ballpark figure, but as for our materials side of it – we need to do a proper take-off for you – but average order is around $1,500. We know from the projects that we’ve worked with, that this type of structure is the most affordable available for doing high performance, foam-free, low-energy. I’m wondering what part of the sheathing-less assembly makes you nervous? Is it the structural aspect?
      One of our best, real-world example project I would want to direct you to is this Net-Zero On A Budget project — https://foursevenfive.com/475-on-site-net-zero-upstate-ny-passive-esque-house-on-a-budget/
      They used a structure based on a traditional barn: post-and-beam. Very cost effective and highly efficient.

  11. Floris March 29, 2017 at 6:23 pm #

    Hi Jim,
    Adding to John’s comment and your question regarding plywood sheathing in double stud wall. Yes, the double stud wall will work with plywood on the exterior and INTELLO on the interior. Do need to have robust airtightness on the inside as usual and vent the rainscreen. We recently did a blogpost about this and did a WUFI study – which would perform better with plywood than OSB as it is a bit more permeable than OSB.

    https://foursevenfive.com/new-mold-growth-index-and-safe-double-stud-walls-wufi-pro-shows-intello-plus-is-key/

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