May 25, 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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