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Attic Knee Walls

    Scope
    Scope Images
    Image
    Install an air barrier on the exterior of attic knee wall insulation.
    Scope

    Ensure proper air sealing and insulation of 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 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.  
    • 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 Single-Family New Homes, and ENERGY STAR Multifamily New Construction.

    Description
    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 this air flow:

    1) Install a continuous air barrier on the exterior of the knee wall 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) Install a continuous air barrier 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 the 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. It should include the air sealing necessary to stop air flow at the edges and seams and adequate support to resist positive and negative pressures without displacement or damage (ENERGY STAR). 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. When the kneewall is part of the exterior wall separating conditioned from unconditioned space, it should be insulated to code. This can be accomplished by installing rigid insulation over the wall cavity insulation, or installing a rigid barrier over the wall cavity, air sealing the rigid barrier then applying additional spray foam or batt insulation over the rigid air barrier up to code. 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 2021 IECC require that builders meet specified infiltration rates at the whole-house level. See the Compliance tab for more details.

    How to Air Seal Knee Walls along the Roofline

    Step 1: Insulate and air seal the ceiling of the attic room.

    Step 2: Continue the insulation along the roofline to the roof edge (Figure 1).

    Step 3: Cover the insulation with a rigid air barrier that is caulked where it meets the plywood floor sheathing, which is extended to the outside wall.

    one way to air seal and insulate knee walls – add insulation and a rigid air barrier along the roof line of unconditioned attic space outside the knee wall
    Figure 1. One way to air seal and insulate knee walls – add insulation and a rigid air barrier along the roof line of unconditioned attic space outside the knee wall (Source: Technology Fact Sheet - Air Sealing 1999). 
    How to Insulate and Air Seal Floor Joist Cavities under Knee Walls

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

    Air seal floor joist cavities under knee walls with a rigid air barrier caulked in place
    Figure 2. Air seal floor joist cavities under knee walls with a rigid air barrier caulked in place (Source: Courtesy of Building Science Corporation). 

    -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 knee walls with rolls of fiberglass batt and spray foam in place
    Figure 3. Stuff cavities under knee walls with rolls of fiberglass batt and spray foam in place (Source: Courtesy of Building Science Corporation). 

    Step 2: Apply caulk to the exterior face of the top plate, bottom plate, and framing at each side of the knee wall. Install a rigid 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 to meet or exceed the code minimum R-value (Figure 4).

     

    Cover insulated knee wall with a rigid air barrier, caulked at edges; add attic floor insulation
    Figure 4. Cover insulated knee wall with a rigid air barrier, caulked at edges; add attic floor insulation (Source: Courtesy of Building Science Corporation). 

     

    Success
    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
    Climate

    Colder climates will increase the potential for and impacts of heat loss and air leakage if the attic knee walls are not properly insulated and air sealed.  Heat loss into the attic can warm the underside of the roof deck which can contribute to snow melt and ice dam formation. Significant air leakage from the house into the attic increases the potential for condensation and frost formation in the attic, if warm, humid conditioned air is allowed to escape into a wintertime attic with cold surfaces.

    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-specific air leakage requirements 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: 2021 IECC).

     

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

     

    Training
    Right and Wrong Images
    Image
    Wrong – No top plate was installed.
    Wrong – No top plate was installed.
    Image
    Right – Top and bottom plates were installed prior to backing and insulation.
    Right – Top and bottom plates were installed prior to backing and insulation.
    Image
    Wrong – No rigid backing was installed on the attic knee wall.
    Wrong – No rigid backing was installed on the attic knee wall.
    Image
    Right – Rigid backing was installed prior to insulating the attic knee walls.
    Right – Rigid backing was installed prior to insulating the attic knee walls.
    Image
    Wrong – The backing on this knee wall was not air sealed prior to adding insulation.
    Wrong – The backing on this knee wall was not air sealed prior to adding insulation.
    Image
    Right – The backing on this knee wall was air sealed prior to insulation.
    Right – The backing on this knee wall was air sealed prior to insulation.
    Image
    Wrong – The batt insulation on this knee wall is not properly supported and there is no air sealed rigid backing to provide a solid air barrier.
    Wrong – The batt insulation on this knee wall is not properly supported and there is no air sealed rigid backing to provide a solid air barrier.
    Image
    Right – The knee wall is properly insulated and air sealed.
    Right – The knee wall is properly insulated and air sealed.
    Image
    Right - Blocking has been installed between the ceiling joists and air sealed in place with canned spray foam to stop air flow beneath the attic knee wall.
    Right - Blocking has been installed between the ceiling joists and air sealed in place with canned spray foam to stop air flow beneath the attic knee wall.
    Image
    Right - These attic knee walls are insulated with rigid foam and sealed with spray foam to form a continuous air barrier at the gable end of this cathedral ceiling.
    Right - These attic knee walls are insulated with rigid foam and sealed with spray foam to form a continuous air barrier at the gable end of this cathedral ceiling.
    Image
    Right – This attic knee wall and the floor joist cavity openings beneath it are being sealed and insulated with spray foam.
    Right – This attic knee wall and the floor joist cavity openings beneath it are being sealed and insulated with spray foam.
    Image
    Right – Open-cell spray foam fills the attic floor joists.
    Right – Open-cell spray foam fills the attic floor joists.
    Videos
    CAD
    CAD Files
    Conceptual insulation at cape-style roof - ceiling and knee wall cavity insulation
    Conceptual insulation at cape-style roof - ceiling and knee wall cavity insulation
    Download: DWG PDF
    Conceptual insulation at cape-style roof - roof cavity insulation with rigid insulation closure
    Conceptual insulation at cape-style roof - roof cavity insulation with rigid insulation closure
    Download: DWG PDF
    Compliance

    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 Single-Family New Homes, Version 3/3.1/3.2 (Rev. 13)

    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 4-8.8,9
    2.3 Attic knee walls and skylight shaft walls.10

    Please see the ENERGY STAR Single-Family New Homes National Rater Field Checklist for applicable footnotes. 

    Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.

     

    ENERGY STAR Multifamily New Construction, Version 1/1.1/1.2 (Rev. 04)

    National Rater Field Checklist

    Thermal Enclosure System

    2. Fully-Aligned Air Barriers13 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. 10,15

    Please see the ENERGY STAR Multifamily New Construction National Rater Field Checklist for applicable footnotes. 

    Please see the ENERGY STAR Multifamily New Construction Implementation Timeline for the program version and revision currently applicable in your state. 

    DOE Zero Energy Ready Home

    Version 1, Rev. 07, Rev. 08, Rev. 09

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 1) (Item 2) in Rev. 09) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program. 
    Exhibit 1, Item 2) (Item 3) in Rev. 09) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels. 

    Version 2, Single Family

    Exhibit 1 Mandatory Requirements

    Exhibit 1, Item 2) Certified under the ENERGY STAR Single Family New Homes Version 3.2. 
    Exhibit 1, Item 3) Ceiling, wall, floor, and slab insulation shall meet or exceed 2021 IECC UA. 

    Version 2, Multifamily

    Exhibit 1 Mandatory Requirements

    Exhibit 1, Item 2) Certified under the ENERGY STAR Multifamily New Construction Program Version 1.2. 
    Exhibit 1, Item 3) ERI and ASHRAE paths: Ceiling, wall, floor and slab insulation for the building meets specified efficiency levels from the 2021 IECC. Prescriptive path: Ceiling, wall, floor, and slab insulation for the building meets or exceeds specified ZERH MF V2 Target Dwelling Design insulation levels in dwelling units, and specified efficiency levels from the 2021 IECC in common spaces.

    For Version 1, Rev. 09 and Version 2, Multifamily, please see the DOE Zero Energy Ready Home National Rater Field Checklist for mandatory infiltration requirements. 

    For all versions and revisions, see Exhibit 2 in the DOE Zero Energy Ready Home National Program Requirements for prescriptive path insulation and infiltration requirements. 

     

    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 performance 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) 072713

    Self-Adhered Sheet Air Barrier. 2019. Air Barrier Association of America, Walpole, MA. This specification for self-adhered sheet air barriers is developed by the Air Barrier Association of America to provide guidance to design professionals.

    ABAA 072703

    Closed Cell, Medium-Density Spray Polyurethane Foam Air Barrier. 2019. Air Barrier Association of America, Walpole, MA. This specification for closed cell, medium-density spray polyurethane foam air barriers is developed by the Air Barrier Association of America to provide guidance to design professionals.

    ABAA 072726

    Fluid-Applied Vapor Permeable Air Barrier. 2020. Air Barrier Association of America, Walpole, MA. This specification for fluid-applied vapor permeable air barriers is developed by the Air Barrier Association of America to provide guidance to design professionals.

     

    2009-2021 IECC and IRC Insulation Requirements Table

    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 this table

     

    2009 International Energy Conservation Code (IECC)

    Residential Requirements

    See Table 402.1.1 for prescriptive insulation and fenestration requirements. 

    See Sections 402.4.1 and 402.4.2 for mandatory air sealing and insulation requirements. 

    Commercial Requirements

    See Table 502.2(1) for prescriptive insulation and fenestration requirements. 

    See Section 502.4.3 for mandatory air sealing requirements. 

    201220152018, and 2021 IECC

    Residential Requirements

    See Table R402.1.2 (R402.1.1 in 2012) for prescriptive insulation and fenestration requirements. 

    See Table R402.4.1.1 for mandatory air barrier and insulation installation requirements. 

    See Section R402.4.1.2 for mandatory air leakage testing requirements. 

    Commercial Requirements

    See Table C402.1.3 (C402.2 in 2012) for prescriptive insulation requirements. 

    See Section C402.5 (C402.4 in 2012) for mandatory air leakage requirements. 

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IECC

    Section 101.4.3 (in 2009 and 2012). 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.)

    Chapter 5 (in 2015, 2018, 2020). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.

     

    ASHRAE 90.1 - 2010, 2013, 2016, and 2019

    See Tables 5.5-1 through 5.5-8 for prescriptive insulation requirements.  

    See Section 5.4.3 for mandatory air leakage requirements.

     

    2009 International Residential Code (IRC)

    See Table N1102.4.2 for air barrier, air sealing and insulation installation requirements. 

    Section R806.4 discusses vapor retarder and insulation requirements for unvented attics. 

    201220152018 IRC, and 2021 IRC

    See Table N1102.4.1.1 for air barrier, air sealing and insulation installation requirements. 

    Section R806.5 discusses vapor retarder and insulation requirements for unvented attics and unvented enclosed rafter assemblies.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IRC

    Section R102.7.1 Additions, alterations, or repairs. 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 the requirements of this code, unless otherwise stated. (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

    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.

    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.
    *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.

    Sales
    Building Science Measures
    Building Science-to-Sales Translator

    Fully Aligned Air Barriers = Whole-House Draft Barrier

    Image(s)
    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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