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 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.

  • 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 Reference

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 Reference

-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 Reference

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

Figure 4 - Cover insulated kneewall with rigid foam or other solid air barrier, caulked at edges. Add attic floor insulation Reference

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 
ENERGY STAR Certified Homes (Ver. 3/3.1 Ver 08) Rater Field Checklist.
Fully-Aligned Air Barriers. A complete air barrier shall be provided that is fully aligned with the insulation at exterior surface of walls in all climate zones; and also at interior surface of walls for Climate Zones 4-8.

DOE Zero Energy Ready Home
DOE Zero Energy Ready Home (Rev 05) Exhibit 2: DOE Zero Energy Ready Home Target Home. Infiltration:  Climate Zones 1-2: 3 ACH 50; Zones 3-4: 2.5 ACH50; Zones 5-7: 2 ACH50; Zone 8: 1.5 ACH50. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.

climate zone map

Training

Right and Wrong Images

Presentations

None Available

Videos

  1. Plates/Blocking at Top of Walls Adjoining Unconditioned Spaces
    Publication Date: July, 2015
    Courtesy Of: Risinger Homes

    Video describing how to insulate skylight shafts and knee walls correctly.

  2. Attic Knee Walls
    Publication Date: July, 2015
    Courtesy Of: Risinger Homes

    Video describing how to insulate and air seal attic knee walls.

CAD Images

Compliance

The Compliance tab contains both program and code information. Exact code language is copyrighted and 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

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist

Thermal Enclosure System: 

2. Fully-Aligned Air Barriers.5 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 4-8.7

2.3 Attic knee walls and skylight shaft walls.8 

Footnotes:

(5) 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, paper-based products, or other materials that are easily torn. If polyethylene is used, its thickness shall be ≥ 6 mil.

(7) 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.

(8) 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.

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

DOE Zero Energy Ready Home

DOE Zero Energy Ready Home (Rev 05) Exhibit 1: Mandatory Requirements: Item 1, Homes must be certified under ENERGY STAR Qualified Homes Version 3. Item 2, Ceiling, wall, floor, and slab insulation shall meet or exceed 2012 or 2015 IECC levels.

Exhibit 2: Design the home to meet the HERS index of the DOE Zero Energy Ready Home Target Home, for which insulation levels must meet the 2012 IECC and achieve Grade 1 installation, per RESNET standards.
Exhibit 2:  Infiltration: Climate Zones 1-2: 3 ACH 50; Zones 3-4: 2.5 ACH50; Zones 5-7: 2 ACH50; Zone 8: 1.5 ACH50. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol. Building envelope assemblies, including exterior walls and unvented attic assemblies (where used), shall comply with the relevant vapor retarder provisions of the 2012 International Residential Code.

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.

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 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.

2009 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 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.

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 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 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.*

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

2015 IRC

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.*

* Due to Copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.

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

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: April, 2012

    Case study about a production builder that achieves HERS scores of 60, compared to 85 for other builders that build new homes to Florida code.

References and Resources*

  1. Author(s): DOE
    Organization(s): DOE
    Publication Date: August, 2015
    Standard requirements for DOE's Zero Energy Ready Home national program certification.
  2. Author(s): EPA
    Organization(s): EPA
    Publication Date: September, 2015
    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).
  3. Author(s): Lstiburek
    Organization(s): BSC
    Publication Date: January, 2010
    Fact sheet providing detailed information about air sealing attics.
  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: October, 2011
    Guide describing details that serve as a visual reference for each of the line items in the Thermal Enclosure System Rater Checklist.

Contributors to this Guide

The following Building America Teams contributed to the content in this Guide.

Building Science-to-Sales Translator

Fully Aligned Air Barriers =
Whole-House Draft Barrier

Technical Description: 

A whole-house draft barrier is a continuously connected layer of solid or air-tight materials that block air leaks. This barrier can also function as part of a water barrier, thermal barrier, and vapor barrier, if the location and materials are compatible. For example, rigid foam insulation can provide a combined function. Rigid foam sheets can be used to block air flow when seams are sealed with tape, caulks or adhesives, or liquid applied sealants. An example of an interior air barrier may be the drywall on the home’s walls and ceilings, when the seams are taped and mudded, and caulk, spray foam, or gaskets are used to seal around wiring, plumbing, and other penetrations. Insulation should be in full contact with the air barrier layer.

Alternate Terms

Air Contaminant Barrier
Energy Saving Air Barrier
Advanced Air Barrier Technology
Professionally-Installed Draft Barrier
Moisture Control Air Barrier
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?
Last Updated: 03/14/2016

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