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Air Sealing and Insulating Attic Knee Walls - Code Compliance Brief

Overview: 

The intent of this brief is to provide code-related information about attic knee walls  to help ensure that the measure will be accepted as being in compliance with the code.  Providing notes for code officials on how to plan review and conduct field inspections can help builders or remodelers with proposed designs and provide jurisdictional officials with information for acceptance.  Providing the same information to all interested parties (e.g., code officials, builders, designers, etc.) is expected to result in increased compliance and fewer innovations being questioned at the time of plan review and/or field inspection.

Attic knee walls are typically short, vertical walls approximately 2 to 3 feet high in an attic that separates attic space from conditioned space.  Most common attic knee walls are associated with the creation of a bonus room that can have many uses.  The walls screen out the unusable space where the rafters meet exterior wall top plates and/or the attic floor. 

Unfortunately, the space on the unconditioned side of the knee wall often is not well sealed or insulated and the wall structure by itself does not provide enough of a thermal barrier to adequately maintain the temperature of the conditioned side of the knee wall in a vented non-conditioned attic.  Furthermore, attic knee walls really need to thermally perform better than an exterior wall, especially in the summer months when the temperature difference between the unconditioned attic space and the conditioned space is likely to be much greater than to the temperatures that exterior walls are exposed to.  During the summer, the temperature in an unconditioned attic can exceed 130⁰F. 

Properly sealing and insulating attic knee walls can significantly reduce air leakage and heat loss between the conditioned space and the unfinished attic. It is important to choose an air-barrier and insulation material that will reduce the air leakage and heat loss as much as possible.  For example, rigid foam board can act as both a thermal barrier and an air barrier across the unconditioned side of the knee wall.  Some insulation options either do not provide air sealing or are difficult to install outside of the wall framing. 

While the energy code does not specify any one type of material or how to air seal and insulate attic knee walls, Building America recommends best practices as to what materials perform better in specific situations and how the materials should be properly installed.  Refer to the “description” tab of this guide (https://basc.pnnl.gov/resource-guides/attic-knee-walls) and the Technical Validation Section of this brief for additional resources on best practices on air sealing and installing insulation and recommended air-barrier and insulation materials. 

This brief provides an overview of the energy provisions of the 2009 through 2015 International Energy Conservation Code (IECC) and International Residential Code (IRC) code for attic knee walls.  While the IECC/IRC does not specifically define attic knee walls, they are included in the definition of the “building thermal envelope[1]” as other building elements that enclose conditioned space or provide a boundary between conditioned space and exempt or unconditioned space.

 


[1]Building Thermal Envelope” is defined as the basement walls, exterior walls, floor, roof, and any other building elements that enclose conditioned space or provide a boundary between conditioned space and exempt or unconditioned space.

Plan Review: 

Per the 2015 IECC/IRC, Section R103.3/R106.3, Examination of Documents. The code official/building official must examine, or cause to be examined, construction documents for code compliance.

This section lists the applicable code requirements followed by details helpful for plan review regarding the provisions to meet the requirements for "attic knee walls."

  • Construction Documentation. Review the construction documents for details describing attic knee wall insulation, installation, air sealing, and construction techniques.

    — 2015 IECC/IRC, Section R103.2/N1101.5 (2012 IECC/IRC, Section R103.2/N1101.8), Information on Construction Documents. Construction documents should include:

    • Insulation materials and their R-values
    • Air sealing details.

      — NEW language (section) in the 2015 IECC/IRC, Section R103.2.1/N11015.1, Thermal Envelope Depiction. The building’s thermal envelope should be represented on the construction drawings.

      — Insulation. All three versions of the IECC/IRC allow compliance to be demonstrated by the following three prescriptive approaches: 1) R-value computation, 2) U-factor alternative, or 3) total UA alternative. This brief provides the general prescriptive requirements per the R-value computation approach.

      — 2015 IECC/IRC, Section R402.1.2/N1102.1.2, Insulation and Fenestration Criteria. The building thermal envelope should meet the requirements of Table R402.1.2/N1102.1.2, based on the climate zone specified in Chapter 3 of the IECC and Section N1101.10 in the IRC.

      — 2015 IECC/IRC, Section R402.1.3/N1102.1.3, R-Value Computation. Insulation material used in layers, such as framing cavity insulation, or continuous insulation should be summed to compute the corresponding component R-value. The manufacturer’s settled R-value should be used for blown insulation. Computed R-values should not include an R-value for other building materials or air films.

      — Excerpt from the Insulation and Fenestration Requirements by Component

      2015 IECC/IRC Table R402.1.2/N1101.1.2, Table R402.1.1/N1102.1.1 (2012 IECC/IRC)
      (R-values are the same for both versions)

    • Insulation and Fenestration Requirements by Component Table

      Climate Zone 1 2 3 4 Except Marine 5 and Marine 4 6 7, 8
      Wood Frame Wall R-value 13 13 20 or 13+5a 20 or 13+5a 20 or 13+5a 20+5 or 13+10a 20+5 or 13+10a

      aThe first value is cavity insulation, the second value is continuous insulation, so "13+5" means R-13 cavity insulation plus R-5 continuous insulation.

      2009 IECC/IRC Insulation and Fenestration Requirements by Component Table 402.1.1/N1102.1

      Climate Zone 1 2 3 4 5 6 7-8
      Wood Frame Wall R-value R-13 R-13 R-13 R-13 R-20 or 13+5 R-20 or 13+5 R-21
    • Air Sealing/Air Leakage Control
    • 2015 IECC/IRC, R402.4./N1102.4, Air Leakage. The building thermal envelope should be constructed to limit air leakage.
    • R402.4.1/N1102.4.1, Building Thermal Envelope. The sealing methods between dissimilar materials should allow for differential expansion and contraction.
    • R402.4.1.1/N1102.4.1.1, Installation. The components listed in the Air Barrier and Insulation Installation Table[2] should be installed in accordance with the manufacturer’s instructions and the criteria listed as the applicable method of construction. Below are the General Requirement and components from the table that are applicable to sealing and insulating attic knee walls.
    • R402.4.1.1/N1102.4.1.1, Air Barrier and Insulation Installation Table
      • Air Barrier General Requirements. A continuous air barrier[3] should be installed in the building envelope. The exterior thermal envelope contains a continuous air barrier. Breaks or joints in the air barrier[4] should be sealed.
      • Air-Barrier Criteria:
        • Walls – The junction of the top plate and top of exterior walls should be sealed.
      • Insulation Installation:
        • Walls – Cavities within corners and headers of frame walls should be insulated by completely filling the cavity with a material having a thermal resistance of R-3 per inch minimum. Exterior thermal envelope insulation for framed walls should be installed in substantial contact and continuous alignment with the air barrier.
    • 2012 IECC/IRC, R402.4/N1102.4, Air Leakage. The building thermal envelope should be constructed to limit air leakage.
      • R402.4.1/N1102.4.1, Building Thermal Envelope. The sealing methods between dissimilar materials should allow for differential expansion and contraction.
      • R402.4.1.1/N1102.4.1.1, Installation. The components listed in the Air Barrier and Insulation Installation Table should be installed in accordance with the manufacturer’s instructions and the criteria listed as the applicable method of construction. Below are the components from the table that are applicable to sealing and insulating attic knee walls.
      • R402.4.1.1/N1102.4.1.1, Air Barrier and Insulation Installation Table
        • Walls – The junction of the top plate and top of exterior walls should be sealed. Exterior thermal envelope insulation for framed walls should be installed in substantial contact and continuous alignment with the air barrier.
    • 2009 IECC/IRC, 402.4.1/N1102.4.1, Air leakage, Building Thermal Envelope
      • The building thermal envelope should be constructed to limit air leakage. The sealing methods between dissimilar materials should allow for differential expansion and contraction. Sources of infiltration (see listing below) should be caulked, gasketed, weather-stripped, or otherwise sealed with an air-barrier material, suitable film, or solid material:
        • All joints, seams, and penetrations
        • Utility penetrations
        • Walls and ceilings separating a garage from conditioned spaces
        • Other sources of infiltration.

EXISTING BUILDINGS

Since the most recent edition of the 2015 IECC/IRC regarding existing buildings was restructured and includes new language regarding additions, alterations, and repairs, the following provisions are for the 2015 code versions only.

Review the construction documents and confirm whether compliance is required based on the scope of work proposed on the existing building:

  • Work proposed is exempt (not required) to meet the provisions of the code
  • Work proposed is not exempt and proper documentation has been submitted that specifies compliance will be met.

2015 IECC/IRC, Section R501.1.1/N1107.1.1 Additions, alterations, or repairs: General. Additions, alterations, or repairs to an existing building, building system or portion thereof should comply with Section R502/N1108, R503/N1109 or R504/N1110. Unaltered portions of the existing building or building supply system are not required to comply.

— ADDITIONS - Building additions that include attic knee walls are subject to the air-sealing and insulation requirements listed in the IECC/IRC.

  • R502.1/N1108.1 General. Additions to existing buildings should conform to code as they relate to new construction without requiring the unaltered portion of the existing building or building system to comply.
  • R502.1.1.1/N1108.1.1.1 Building Envelope. New building envelope assemblies that are part of the addition should comply with Sections R402l1/N1102.1, R402.2/N1102.2, R402.3.1/N1102.3.1 through R402.3.3.5/N1102.3.5 and R402.4/N1102.4
    • Exception: where non-conditioned space is changed to conditioned space, the building envelope of the addition must comply where the UA (U-factor x Area), as determined in Section R402.1.4/N1102.1.4 (U-factor Alternative[5]), of the existing building and the addition, and any alterations that are part of the project is less than or equal to UA generated for the existing building.

      Note: showing compliance with this exception is very difficult in that the builder must know what the existing insulation and fenestration ratings are and typically documentation for the fenestration is no longer available and assumptions would have to be made as to the insulation R-value installed in existing framed and sealed walls.

— ALTERATIONS

  • R503.1/N1109.1 General. Alterations to any building or structure must comply with the requirements of the code as new construction. Alterations should be such that the existing building or structure uses no more energy than the existing building or structure prior to the alteration.
  • R503.1.1/N1109.1.1 Building Envelope. Building envelope assemblies that are part of the alteration must comply with Sections R402.1.2/N1102.1.2 (Insulation and Fenestration Table) or R402.1.4/N1102.1.4 (U-factor Alternative), and Section R402.2.1/N1102.2.1 through R402.2.12/N1102.2.12, R402.3/1/N1102.3.1, R402.3.2/N1102.3.2, R402.4.3/N1102.4.3 and R402.4.4/N1102.4.4
  • Exception: The following alterations need not comply with the requirements for new construction provided the energy use of the building is not increased:
    • Existing wall cavities exposed during construction, provided that the cavities are filled with insulation
    • Construction where the existing wall cavity is not exposed.

 


[2]"U-factor Alternative" An assembly with a U-factor equal to or less than that specified in Table R402.1.4/N1102.1.4 should be permitted as an alternative to the R-value in Table R402.1.2/N1102.1.2.

[3]Table R402.4.1.1 of the IECC and Table N1102.4.1.1 of the IRC.

[4]"Continuous air barrier" is defined as a combination of materials and assemblies that restrict or prevent the passage of air through the building thermal envelope.

[5]"Air Barrier" is defined as material(s) assembled and joined together to provide a barrier to air leakage through the building thermal envelope. An air barrier may be a single material or a combination of materials.

Field Inspection: 

Per the 2015 IECC, Section R104, Inspections, construction or work for which a permit is required is subject to inspection.  Construction or work is to remain accessible and exposed for inspection purposes until approved.  Required inspections include footing and foundation, framing and rough-in work, plumbing rough-in, mechanical rough-in, and final inspection.

Per the 2015 IRC, Section R109, Inspections.  The wording is somewhat different in that for onsite construction, from time to time the building official, upon notification from the permit holder or his agent, can make or cause to be made any necessary inspections.  Further details are provided for inspections regarding foundation, plumbing, mechanical, gas and electrical, floodplain, frame and masonry, and final inspection.  Any additional inspections are at the discretion of the building official.

This section provides details for inspecting to the specific provisions for insulating and sealing attic knee walls where one or more specific type of inspection per the IECC or IRC may be necessary to confirm compliance.  Verifying code compliance for insulating and sealing attic knee walls would typically be at the framing and rough-in work inspection.

Inspections should provide verification in the following areas:

  • Cavity insulation, as indicated in approved construction documents, completely fills the knee wall cavity with no compression or gaps, the manufacturer’s R-value mark is readily available, and meets the approved R-value per construction documents.
  • Continuous insulation (if applicable) is installed in accordance with manufacturer’s installation instructions, the manufacturer’s R-value mark is readily available, and meets the approved R-value per construction documents.
  • Batt insulation is cut neatly around any wiring and plumbing, or insulation readily conforms to available space and extends behind piping and wiring.
  • Joints, seams, holes, and penetrations are caulked, gasketed, weather-stripped, or otherwise sealed.
  • Continuous air barrier is properly installed as indicated in approved construction documents.  Confirm the insulation is installed in substantial contact and continuous alignment with the air barrier.
  • The junction where the rafters meet the exterior wall top plates and/or the attic floor are sealed.
Technical Validation(s): 

This section provides additional information and helpful resources.

Guide to Attic Knee Walls:  https://basc.pnnl.gov/resource-guides/attic-knee-walls

Guide to Attic Air Sealing, Building Science Corp., J. Lstiburek, January 2010, http://buildingscience.com/documents/guides-and-manuals/gm-attic-air-sealing-guide/view?searchterm=guide%2520to%2520attic%2520air%2520sealing  

“Solving Comfort Problems Caused by Attic Knee Walls,” Allison A. Bailes III, PHD, Green Building Advisor, http://www.greenbuildingadvisor.com/blogs/dept/building-science/solving-comfort-problems-caused-attic-kneewalls#ixzz3mzitfmBW

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