Double Walls

Scope Images
Air seal and insulate double-walls that are half-height or full-height walls used as architectural features in homes.
Air seal and insulate double-walls that are half-height or full-height walls used as architectural features in homes.
Scope

Air seal and insulate double-walls that are half-height or full-height walls used as architectural features in homes.

  • Install a continuous air barrier on the exterior of the interior wall.
  • Seal all seams, gaps, and holes of the air barrier with caulk or foam.
  • Install insulation in the interior wall cavity equal in R-value to other exterior wall cavity in the home, without misalignments, compressions, gaps, or voids.

OR

  • Dense-pack the entire cavity of the double wall assembly with blown insulation, ensuring the insulation is in contact with the insulation to the top of the wall cavity with no misalignments, compressions, gaps, or voids in the insulation.
  • Ensure that the air barrier is continuous and in full contact with the insulation.
  • If spray foam insulation is used for the wall cavity insulation, the spray foam can serve as the air barrier if it is at least 5.5 inches thick if open-cell or at least 1.5 inches thick if closed-cell spray foam insulation.  
  • ENERGY STAR requires that an air barrier be installed at the exterior vertical surface of the wall insulation in all climate zones and also that an air barrier be installed at the interior vertical surface in IECC Climate Zones 4-8. The air barrier should be continuous and should be in full contact with the insulation. Taped, mudded drywall could serve as the air barrier if sealed at all seams. (ENERGY STAR 2015).

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

The double walls described here are half or full walls included in a home design as an architectural feature that provides a more dimensional appearance. This is not the same as the double-wall construction technique used to provide deeper wall cavities for more insulation on all of the exterior walls of a home. That technique is described in Double Wall Framing.

The important thing to keep in mind about a double wall used as an architectural feature is that if it is located on the exterior wall of a home, it is part of the home’s thermal envelope. So, the wall must be insulated to at least the level of the home’s other exterior walls, and the insulation in the wall must be aligned with and enclosed by air barriers on all six sides.

There are three ways to insulate a double wall and achieve a fully aligned air barrier:

  1. Fill the entire cavity with blown insulation.
  2. Separate the interior cavity from the exterior cavity and fill the interior cavity.
  3. Insulate the exterior cavity with spray foam.

These three ways are described in the steps below. The insulation and fully aligned air barrier for an accent double wall may be installed by insulators, framers, or drywallers. This task should be included in the contract for the appropriate trade, depending on the workflow at the specific job site.

How to Air Seal and Insulate a Double Wall Using Blown-In Insulation

  1. Frame the exterior wall using the same stud framing as the rest of the exterior wall, preferably 2x6, 24-inch on-center. See Minimum Wall Studs. Sheathe with the same exterior sheathing as is used on the rest of the exterior walls.
  2. Install a second section of framing to the inside of the exterior wall framing, where the thicker wall is desired, using 2x4 or 2x6, 24-inch on-center framing. (See Figure 1.)
  3. Tack netting to the interior face of the interior wall studs and blow in loose fiberglass or cellulose insulation when the rest of the walls are insulated. Ensure that the wall is completely filled to the proper density to minimize settling.
  4. Drywall when the rest of the walls are drywalled. Caulk the drywall to the framing at the top plate, bottom plate, and studs. Mud and tape seams and corners. The interior drywall and exterior sheathing serve as the interior and exterior air barriers.
Double wall with exterior air barrier
Figure 1 - Blown-in insulation fills the double-stud cavity. The mudded, taped, and caulked drywall and the taped rigid foam exterior sheathing serve as the interior and exterior air barriers. 

How to Air Seal and Insulate a Double Wall Using Batt Insulation

  1. Frame the exterior wall using the same stud framing as the rest of the exterior wall, preferably 2x6, 24-inch on-center. See Minimum Wall Studs. Sheathe with the same sheathing as is used on the rest of the exterior walls.
  2. Install an air barrier over the studs on the section of wall that will be double framed. (See Figure 2.)  This air barrier can consist of drywall, OSB, plywood, or rigid foam. The use of polyethylene or any other material that is a Class 1 vapor barrier is not recommended.
  3. Caulk along the interior facing side of the air barrier at all four edges.
  4. Install the second wall framing using 2x4 or 2x6, 24-inch on-center framing. Push the second framing members against the caulk to seal the framing to the air barrier along all four sides.
  5. Fill the interior wall cavity with batt insulation to the R-value required for exterior walls in your climate zone. 
  6. Drywall when the rest of the walls are drywalled. Caulk the drywall to the framing at the top plate, bottom plate, and studs. Mud and tape seams and corners. The interior drywall and exterior sheathing serve as the interior and exterior air barriers.
Double wall with interior air barrier
Figure 2 - OSB is sealed with caulk to the exterior side of interior wall studs to serve as an exterior barrier for the batt insulation which is installed to align with the drywall which serves as the wall’s interior air barrier. 

How to Air Seal and Insulate a Double Wall Using Spray Foam Insulation

  1. Frame the exterior wall using the same stud framing as the rest of the exterior wall, preferably 2x6, 24-inch on-center (see Minimum Wall Studs). Sheathe with the same sheathing as is used on the rest of the exterior walls.
  2. Install the second wall framing using 2x4 or 2x6, 24-inch on-center framing. (See Figure 3.)
  3. Fill the double-stud wall cavity with the desired depth of spray foam along the inside surface of the exterior sheathing. 
  4. Drywall when the rest of the walls are drywalled. The foam does not need to touch the interior drywall, because the foam serves as its own air barrier. The exterior sheathing serves as the exterior air barrier.
Double wall with spray foam
Figure 3 - Spray foam insulates and air seals this double wall. The spray foam serves as an air barrier as well so it does not need to be touching the interior drywall.  
Ensuring Success

The site supervisor should visually inspect the exterior wall prior to installation of the drywall or a center air barrier to confirm that insulation has been fully aligned with the air barrier. Blower door testing used in conjunction with an infrared camera may help indicate areas of thermal bypass or lack of alignment between insulation and air barrier layers in walls after drywall is installed.

Climate

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

National Rater Field Checklist

Thermal Enclosure System.
2. Fully-Aligned Air Barriers.6 At each insulated location below, a complete air barrier is provided that is fully aligned as follows:
Walls: At exterior vertical surface of wall insulation in all climate zones; also at interior vertical surface of wall insulation in Climate Zones 4-8.8
2.5 Double-walls and all other exterior walls. 

DOE Zero Energy Ready Home (Revision 07)

Exhibit 2 DOE Zero Energy Ready Home Target Home.
The U.S. Department of Energy’s Zero Energy Ready Home program allows builders to choose a prescriptive or performance path. The DOE Zero Energy Ready Home prescriptive path requires builders to meet or exceed the minimum HVAC efficiencies listed in Exhibit 2 of the National Program Requirements (Rev 07), as shown below. The DOE Zero Energy Ready Home performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled target home that meets the requirements of Exhibit 2 as well as the mandatory requirements of Zero Energy Ready Home Exhibit 1.

Exhibit 2, Insulation and Infiltration) Whole house leakage must be tested and meet the following infiltration limits:

  • Zones 1-2: ≤ 3 ACH50;
  • Zones 3-4: ≤ 2.5 ACH50;
  • Zones 5-7: ≤ 2 ACH50;
  • Zone 8: ≤ 1.5 ACH50;
  • Attached dwellings: ≤ 3 ACH50.

The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide. 

IECC climate zone map
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC).

Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC). 

 

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

National Rater Field Checklist

Thermal Enclosure System.
2. Fully-Aligned Air Barriers.6 At each insulated location below, a complete air barrier is provided that is fully aligned as follows:
Walls: At exterior vertical surface of wall insulation in all climate zones; also at interior vertical surface of wall insulation in Climate Zones 4-8.8
2.5 Double-walls and all other exterior walls. 

Footnote 6) For purposes of this Checklist, an air barrier is defined as any durable solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage. EPA recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 in. or 1.5 in., respectively, to qualify as an air barrier unless the manufacturer indicates otherwise. If flexible air barriers such as house wrap are used, they shall be fully sealed at all seams and edges and supported using fasteners with caps or heads ≥ 1 in. diameter unless otherwise indicated by the manufacturer. Flexible air barriers shall not be made of kraft paper, paperbased products, or other materials that are easily torn. If polyethylene is used, its thickness shall be ≥ 6 mil.

Footnote 8) All insulated vertical surfaces are considered walls (e.g., above- and below-grade exterior walls, knee walls) and must meet the air barrier requirements for walls. The following exceptions apply: air barriers recommended, but not required, in adiabatic walls in multifamily dwellings; and, in Climate Zones 4 through 8, an air barrier at the interior vertical surface of insulation is recommended but not required in basement walls or crawlspace walls. For the purpose of these exceptions, a basement or crawlspace is a space for which ≥ 40% of the total gross wall area is below-grade.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards. See the guide 2015 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

Exhibit 2 DOE Zero Energy Ready Home Target Home.
The U.S. Department of Energy’s Zero Energy Ready Home program allows builders to choose a prescriptive or performance path. The DOE Zero Energy Ready Home prescriptive path requires builders to meet or exceed the minimum HVAC efficiencies listed in Exhibit 2 of the National Program Requirements (Rev 07), as shown below. The DOE Zero Energy Ready Home performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled target home that meets the requirements of Exhibit 2 as well as the mandatory requirements of Zero Energy Ready Home Exhibit 1.

Exhibit 2, Insulation and Infiltration) Whole house leakage must be tested and meet the following infiltration limits:

  • Zones 1-2: ≤ 3 ACH50;
  • Zones 3-4: ≤ 2.5 ACH50;
  • Zones 5-7: ≤ 2 ACH50;
  • Zone 8: ≤ 1.5 ACH50;
  • Attached dwellings: ≤ 3 ACH50.

Footnote 12) Building envelope assemblies, including exterior walls and unvented attic assemblies (where used), shall comply with the relevant vapor retarder provisions of the 2015 International Residential Code (IRC).
Footnote 23) Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.

American Society for Testing Materials (ASTM) E1677-11

Standard Specification for Air Barrier (AB) Material or System for Low-Rise Framed Building Walls. This specification covers minimum performances and specification criteria for an air barrier material or system for framed, opaque walls of low-rise buildings. The provisions are intended to allow the user to design the wall performance criteria and increase air barrier specifications for a particular climate location, function, or design. 

2009 International Energy Conservation Code (IECC)

Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria, Air barrier and thermal barrier: Exterior wall insulation is installed in substantial contact and continuous alignment with the air barrier. Air permeable insulation is not used as a sealing material.

2012 IECC

Exterior insulation for framed walls is in substantial contact and continuous alignment with the air barrier. Table R402.4.1.1 Air Barrier and Insulation Installation, Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material.

2015, 2018, and 2021 IECC

Table R402.1.2 Insulation and Fenestration Requirements – meet or exceed the insulation levels listed in Table 1.

Table R402.4.1.1 Air Barrier and Insulation Installation. Walls: Insulation in exterior framed walls is in substantial contact and continuous alignment with the air barrier. General requirements: A continuous air barrier is installed in the building envelope; breaks and joints in the air barrier are sealed. Air-permeable insulation is not used as an air-sealing material.

Section R402.4.1.2 Testing.  The building should be tested for air leakage in accordance with ASTM E 779 or E 1827 (or RESNET/ICC 380 in 2018 IECC) and should have an air leakage rate of ≤ 5 in CZ 1 and 2 or ≤ 3 in CZ 3-8.

The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC can be found in Table 1.
Figure 1. The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC can be found in Table 1. (Source 2021 IECC).

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

2009 International Residential Code (IRC)

Table N1102.4.2 Air Barrier and Insulation Inspection Component Criteria, Air barrier and thermal barrier: Exterior wall insulation is installed in substantial contact and continuous alignment with the air barrier. Air permeable insulation is not used as a sealing material.

2012 IRC

Exterior insulation for framed walls is in substantial contact and continuous alignment with the air barrier. Table N1102.4.1.1 Air Barrier and Insulation Installation, Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed.  Air permeable insulation is not used as a sealing material.

2015, and 2018 IRC

N1102.4.1.1 Air Barrier and Insulation Installation. Walls: Insulation in exterior framed walls is in substantial contact and continuous alignment with the air barrier. General requirements: A continuous air barrier is installed in the building envelope; breaks and joints in the air barrier are sealed. Air-permeable insulation is not used as an air-sealing material.  

Retrofit: 

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

References and Resources*
Author(s)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Measures
Building Science-to-Sales Translator

Double-Wall Framing = Double-Wall Thermal Blanket

Image(s)
Technical Description

One way to achieve very high levels of insulation in walls is to build two stud walls separated by an air space. The inner wall provides framing for attaching gypsum board; the outer wall does the same for sheathing, a weather barrier, and siding. Two 2x4 framed walls spaced three inches apart will provide a wall cavity about 10 inches deep. This spacing eliminates thermal bridging. When fully insulated, this double-wall thermal blanket creates a quiet, efficient, and comfortable home.

Double-Wall Thermal Blanket
Sales Message

Double-Wall Thermal Blanket construction blocks excessive heat loss and gain though structural framing while providing much more insulation. What this means to you is less wasted energy along with enhanced comfort and quiet. Knowing there is one opportunity during construction to lock in quality construction, wouldn’t you agree advanced thermal protection is a great investment?

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