Attic Knee Walls

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Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.
Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.
Scope

Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.

  • Install a top and bottom plate or blocking at the top and bottom of all knee wall cavities.
  • Install insulation without misalignments, compressions, gaps, or voids in all knee wall cavities.
  • Install a continuous air barrier on the exterior side of the attic knee wall framing with a rigid air barrier or other supporting material to prevent the knee wall cavity insulation from sagging and to create a continuous thermal barrier. Rigid air barrier material could include rigid foam insulation, drywall, plywood, or OSB, among others.
  • Seal all seams, gaps, and holes in the air barrier with caulk or foam. 
  • 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.  
  • Install blocking in the joist bays below the knee walls to prevent air flow under the knee walls. 

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

Knee walls, the walls that separate conditioned from unconditioned space in an attic, can be a source of significant air leakage if a continuous air barrier is not provided to prevent unconditioned air from flowing under the knee wall and under the floor boards of the attic room. There are two ways to block off this air flow: 1) a continuous air barrier can be installed on the exterior of the kneewall framing from the top of the knee wall down to the attic floor, including the spaces between the attic floor joists from the bottom of the knee wall to the ceiling deck below, or 2) a continuous air barrier can be installed along the underside of the attic roofline from the top of the knee wall to the top plate of the home’s exterior wall. With either method, the air barrier should be installed before installing attic floor insulation in the unconditioned portion of the attic.

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 (ENERGY STAR 2015). Air barrier material can include thin sheet goods such as rigid insulation, dry wall, OSB, plywood, or rolled batt insulation that is covered with spray foam. These materials 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.

Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the 2009 IECC require that builders meet specified infiltration rates at the whole-house level. See the “compliance” tab for these specified infiltration rates.

How to Air Seal Knee Walls along the Roofline

  1. Insulate and air seal the ceiling of the attic room.
  2. Continue the insulation along the roofline to the roof edge (Figure 1).
  3. Cover the insulation with a sheet material (drywall or rigid foam insulation) that is caulked where it meets the plywood floor sheathing, which is extended to the outside wall.
    one way to air seal and insulate kneewalls – add insulation and a rigid air barrier along roof line of unconditioned attic space outside kneewall
    Figure 1 - One way to air seal and insulate kneewalls – add insulation and a rigid air barrier along roof line of unconditioned attic space outside kneewall.

How to Insulate and Air Seal Floor Joist Cavities under Knee Walls

Step 1a: Insert solid wood blocking or rigid foam board in the floor joist cavity openings under the kneewall (Figure 2). Seal the edges with a continuous bead of caulk or foam sealant.

Air seal floor joist cavities under kneewall with rigid foam, plywood or OSB caulked in place
Figure 2 - Air seal floor joist cavities under kneewalls with rigid foam, plywood, or OSB caulked in place.

-OR-

Step 1b: Stuff Floor joist cavities with rolls of fiberglass batt and cover them with spray foam to the edges (Figure 3).

Stuff cavities under kneewalls with rolls of fiberglass batt and spray foam in place

Figure 3 - Stuff cavities under kneewalls with rolls of fiberglass batt and spray foam in place.

Step 2: Apply caulk to the exterior face of the framing of the top plate, bottom plate, and framing at each side of the kneewall. Install rigid foam or another solid air barrier over the knee wall framing (Figure 4). Seal any seams in the rigid barrier with tape or caulk.

Step 3: Fill the attic floor joist bays with insulation (batt, blown, or spray foam) to meet or exceed the code minimum R-value (Figure 4).

Cover insulated kneewall with rigid foam, caulked at edges. Add attic floor insulation
Caption

 

Ensuring Success

Blower door testing, conducted as part of whole-house energy performance testing, may help indicate whether air leakage at knee walls has been successfully sealed. An infrared camera and/or visual inspection may also be used to determine locations of air leakage at the knee wall.

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
2.3 Attic knee walls and skylight shaft walls.9

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 9) Exterior air barriers are not required for attic knee walls that are ≤ 24 in. in height if an interior air barrier is provided and insulation extends in all directions from the top of this interior air barrier into unconditioned space at the following levels: CZ 1-5: ≥ R-21; CZ 6-8: ≥ R-30.

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.

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.

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
2.3 Attic knee walls and skylight shaft walls.9

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 9) Exterior air barriers are not required for attic knee walls that are ≤ 24 in. in height if an interior air barrier is provided and insulation extends in all directions from the top of this interior air barrier into unconditioned space at the following levels: CZ 1-5: ≥ R-21; CZ 6-8: ≥ R-30.

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.

Air Barrier Association of America (ABAA) 07261

Self-Adhered Sheet Air Barrier. 2006. Air Barrier Association of America, Walpole, MA. This specification for self-adhered sheet air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.

ABAA 07262

Fluid-Applied Air and Vapor Barrier. 2012. Air Barrier Association of America, Walpole, MA. This specification for air barriers that are fluid-applied and also act as vapor barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.

ABAA 07263

Closed Cell, Medium-Density Spray Polyurethane Foam Air Barrier. 2011. Air Barrier Association of America, Walpole, MA. This specification for closed cell, medium-density spray polyurethane foam air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.

ABAA 07265

Fluid-Applied Vapor Permeable Air Barrier. 2012. Air Barrier Association of America, Walpole, MA. This specification for fluid-applied vapor permeable air barriers is developed by a professional association, the Air Barrier Association of America, to provide guidance to the design professional.

2009 - 2021 International Energy Conservation Code (IECC) and International Residential Code (IRC) Minimum Insulation Requirements: 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 2009, 2012, 2015, 2018 and 2021 IRC, can be found in this table.

2009 International Energy Conservation Code (IECC)

Table R402.1.1 Insulation and Fenestration Requirements – meet or exceed the insulation levels listed in this table.

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

Section 402.4.1, Building thermal envelope. Knee walls to be sealed. Section R402.4.2 Air sealing and insulation is demonstrated by testing or visual inspection. Testing. The building should be tested for air leakage should have an air leakage rate of ≤ 7 at rough-in.

2012 IECC

Table R402.1.1 Insulation and Fenestration Requirements – meet or exceed the insulation levels listed in this table.

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. Table R402.4.1.1, Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope. The exterior thermal envelope contains a continuous air barrier. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material.

Section R402.4.1.2 Testing.  The building should be tested for air leakage and should have an air leakage rate of ≤ 5 in CZ 1 and 2 or ≤ 3 in CZ 3-8.

2015 and 2018 IECC

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

Table R402.4.1.1 Air Barrier and Insulation Installation, Ceiling/attic: Access openings, drop down stairs, and knee wall doors to unconditioned attic spaces are sealed. Walls: Insulation in exterior framed walls is in substantial contact and continuous alignment with the air barrier. Knee walls shall be sealed. 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 for 2018 IECC) and should have an air leakage rate of ≤ 5 in CZ 1 and 2 or ≤ 3 in CZ 3-8.

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)

Section N1102.4.1, Building thermal envelope. Knee walls to be sealed. Table N1102.4.2 Air Barrier and Insulation Inspection Component Criteria, Walls: Insulation in exterior framed walls is in substantial contact and continuous alignment with the air barrier. Table N1102.4.2, Air barrier and thermal barrier: Air-permeable insulation is not used as a sealing material.

2012 IRC

Table N1102.4.1.1 Air Barrier and Insulation Installation, Walls: Knee walls to be sealed. Table N1102.4.1.1, 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 IRC and 2018 IRC

Table N1102.4.1.1 Air Barrier and Insulation Installation, Ceiling/attic: Air barrier in any dropped ceiling/soffit is substantially aligned with insulation and any gaps are sealed. Access openings, drop down stairs, and knee wall doors to uncodniditoned attic spaces are sealed. Walls: Insulation in exterior framed walls is in substantial contact and continuous alignment with the air barrier. Knee walls shall be sealed. 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.

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Case Studies
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.
Author(s)
Lstiburek
Organization(s)
Building Science Corporation
Publication Date
Description
Fact sheet providing detailed information about air sealing attics.
*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

Fully Aligned Air Barriers = Whole-House Draft Barrier

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Technical Description

A whole-house draft barrier is a continuous layer of air-tight materials that block air leaks. This barrier can be integrated with other materials to also function as a water barrier, thermal barrier, and vapor barrier. For example, rigid foam insulation can be used to block thermal flow as well as air flow when seams are sealed with tape, caulk, adhesives, or liquid-applied sealants. Some rigid foams have an integrated water control layer as well. Additionally, drywall can serve as an interior air barrier when the seams are taped and spackled, and caulk, spray foam, or gaskets are used to seal around wiring, plumbing, and other penetrations. It also serves as the vapor barrier when finished with paint. Insulation should be in full contact with the air barrier layer.

Whole-House Draft Barrier
Sales Message

Whole-house draft barriers block air flow that can undermine the thermal protection with a complete high-performance insulation system. What this means to you is less wasted energy along with enhanced comfort, quiet, and durability. Wouldn’t you agree it would be a shame to only get a partial return on your investment in advanced insulation?

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