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Double Wall Framing - Code Compliance Brief

Double stud wall wood framing is not specifically addressed in the International Residential Code (IRC) or International Energy Conservation Code (IECC); it is neither encouraged nor discouraged.  Recommendations of where the vapor retarder and air barrier should be placed, based on research and testing are further defined in this Code Compliance Brief.

Double stud wall wood framing is not specifically addressed in the International Residential Code (IRC) or International Energy Conservation Code (IECC); it is neither encouraged nor discouraged.  Minimum code requirements for insulation, moisture, and air leakage are based on a typical 2x4 or 2x6 (16” or 24” on center) single stud wall.  Minimum insulation R-values specified in the IRC and IECC are based on assuming a single stud wall (e.g., R-19 cavity insulation will completely fill a 2x6 single stud wall).  Double stud wall wood framing can obtain much higher R-values.  Moisture and air leakage are addressed in the codes for a single stud wall but not for a double wall.   Recommendations of where the vapor retarder and air barrier should be placed, based on research and testing, are described below.

Plan Review

  • Insulation:  Since these walls will have a much higher R-value of insulation than minimum code, review the proposed installed R-value and how the overall building is meeting compliance (prescriptively, trade-offs, or performance based).   If the latter of the two are proposed, confirm that the credit given for these higher R-value walls was calculated correctly (correct credit was given).   For example, per the 2012 IECC in Climate Zone 5, the minimum insulation required in a wood-frame wall is R-20 cavity or R-13 cavity plus R-5 continuous insulation or insulated siding.  Double wall framing can obtain values around R-30 or higher for cavity insulation.
  • Moisture:  In a double stud wall, the recommendation, based on testing and studies performed by Building Science Corporation, is to install the vapor retarders on the exterior side of the interior wall, dependent upon climate zone.

In the 2012 IRC, vapor retarders are required dependent upon Climate Zone and vapor retarder class.   Vapor retarders are required in colder climates.   If installed, it should be located on the exterior side of the interior wall (see Description tab) and care should be taken that the insulation on both sides of the vapor retarder is fully aligned with the barrier the entire length of the wall.   Note: the IRC (Section 702.7) states Class I or II vapor retarders are required on the interior side of the frame walls in Climate Zones 5, 6, 7, 8, and Marine 4 with some exceptions for basement walls, below grade portion of any wall, or construction where moisture or freezing will not damage the material.  The IRC defines material vapor retarders into three classes:

  • Class I: Sheet polyethylene, unperforated aluminum foil.
  • Class II: Kraft-faced fiberglass batts.
  • Class III: Latex or enamel paint.

Class III vapor retarders are permitted where any of the following conditions are met per Table R702.7.1 in the 2012 IRC.

Climate Zone Class III Vapor Retarders Permitted for
4 Vented cladding over wood structural panels.
Vented cladding over fiberboard.
Vented cladding over gypsum.
Insulated sheathing with R-value ≥ 2.5 over 2 x 4 wall.
Insulated sheathing with R-value ≥ 3.75 over 2 x 6 wall.
5 Vented cladding over wood structural panels.
Vented cladding over fiberboard.
Vented cladding over gypsum.
Insulated sheathing with R-value ≥ 5 over 2 x 4 wall.
Insulated sheathing with R-value ≥ 7.5 over 2 x 6 wall.
6 Vented cladding over fiberboard.
Vented cladding over gypsum.
Insulated sheathing with R-value ≥ 7.55 over 2 x 4 wall.
Insulated sheathing with R-value ≥ 11.25 over 2 x 6 wall.
7 and 8 Insulated sheathing with R-value ≥ 10 over 2 x 4 wall.
Insulated sheathing with R-value ≥ 15 over 2 x 6 wall.
  • Air Leakage: Confirm on the plans that a continuous air barrier is specified on the interior side of the interior wall.

Field Inspection

  • The insulation completely fills the cavity with no compression or gaps.
  • Vapor retarder (if applicable) is installed on the exterior of the interior wall properly.
  • Air barrier is installed on the interior of the interior wall properly.  All seams, gaps, and holes are sealed properly.  The 2012 IRC, Table N1102.4.1.1 and IECC, Table R402.4.1.1 require a continuous air barrier be installed.  Confirm corners and headers are insulated and the junction of the foundation and sill plate sealed.  The junction of the top plate and top of exterior walls are sealed.  Confirm the exterior thermal envelope insulation for framed walls is installed in substantial contact and continuous alignment with the air barrier. 

 

Technical Validation(s)

Building America Top Innovation Hall of Fame Profile

Vapor Barrier

Disclaimer

This content is a work created with funding provided by the United States Department of Energy under Contract no DE-AC05-076RL01830 for the operation of Pacific Northwest National Laboratory. The information and guidance provided by Pacific Northwest National Laboratory (PNNL) in the content are intended solely for educational purposes only and do not constitute formal training or certification. It is provided with the explicit understanding that neither the United States Government nor the United States Department of Energy, nor the Contractor, nor any or their employees, nor any jurisdiction or organization that has cooperated in the development of these materials, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness or any information, apparatus, product, software, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Viewers assumes full responsibility for all actions that they may take from information provided in this content including ensuring the safety, code compliance, and proper functionality of any products they choose to install. Installation and use of such products should be performed in accordance with local regulations and manufacturer instructions.