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Insulated Interior Exterior Wall Intersections – Code Compliance Brief

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The intent of this brief is to provide code-related information about insulated interior/exterior wall intersections to help ensure that the feature will be accepted as being in compliance with the code. Providing notes for code officials on how to plan reviews 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.

The junction of interior and exterior walls can be tricky to insulate with all the necessary framing, but this junction must be insulated in a way that avoids thermal bridges in the exterior wall. All interior/exterior wall intersections should be insulated to the same R-value as the rest of the exterior wall. Install insulation should run continuously behind interior/exterior wall intersections. Use one of the methods listed below or an equivalent assembly:

  • Ladder blocking
  • Full-length 2 x 6 or 1 x 6 nailer behind the first partition stud.
  • UA Alternative
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 requirement for insulating interior/exterior wall intersections.

Construction Documentation. Review the construction documents to identify that the insulation of interior/exterior wall intersections is shown.

  • 2015 IECC/IRC, Section R103.2/N1101.5 Information on construction documents. Construction documents should include:

    – Details of the wall compliance approach taken
    – Insulation materials and their R-values or U-factor
    – Window and skylight U-factor (if using the UA tradeoff approach)
    – Details of the insulation at the intersection of interior and exterior walls indicating use of advanced framing techniques.

The 2009, 2012, and 2015 IECC/IRC do not specifically address advanced framing. However, all three versions of the IECC allow wall compliance to be demonstrated by three prescriptive approaches: 1) R-value computation, 2) U-factor alternative, or 3) total UA alternative. The implication is that the interior/exterior wall intersection must be addressed in one way or another.

There is a way to demonstrate compliance for walls that does not use advanced framing. This is the UA alternative. To use the UA alternative, the area of wall that is not insulated to the full R-value must be calculated and treated as a separate wall with its own area and an increased U-factor. The impact of the increased U-factor for this intersection (and any other thermal bridge in the structure such as a standard corner) must be made up by reducing the U-factor or one or more other building envelope components. For example, the builder might choose to use windows with a lower U-factor or a ceiling with a lower U-factor than required to tradeoff against the areas of the wall that are not fully insulated.

Note that only the UA alternative specifically mentions "thermal bridging."1 This is because both the R-value and U-factor assume that the minimum R-value or maximum U-factor is achieved for the entire wall assembly. To achieve that minimum R-value or maximum U-factor for the wall in the vicinity of the interior/exterior wall intersection, advanced framing techniques must be used. Standard T-post framing does not achieve the minimum R-value or maximum U-factor. Two techniques that can help achieve the minimum R-value or maximum U-factor are ladder blocking and the use of a full-length 2 x 6 or 1 x 6 nailer behind the first partition stud. In both cases, the structural support needed at the intersection is maintained, while the cavity available for insulation installation is greatly enlarged over the stud.

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.

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. Where insulated siding is used for the purpose of complying with the continuous insulation requirements of Table R402.1.2/N1102.1.2, the manufacturer’s labeled R-value for insulated siding should be reduced by R-0.6.

Excerpt from the Insulation and Fenestration Requirements by Component
Table R402.1.2/N1101.1.2 (2015 IECC/IRC)

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
a The first value is cavity insulation, the second value is continuous insulation, so 13+5 means R-13 cavity insulation plus r-5 continuous insulation.

2015 IECC/IRC, Section R402.1.4/N1102.1.4 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.

Excerpt from the Insulation and Fenestration Requirements by Component
Table R402.1.4/N1101.1.4 (2015 IECC/IRC)

Climate Zone 1 2 3 4 except
Marine		 5 and
Marine 4		 6 7, 8
Wall Frame U-factor 0.084 0.084 0.060 0.060 0.060 0.045 0.045

2015 IECC/IRC, Section R402.1.5/N1102.1.5 Total UA Alternative. If the total building thermal envelope2 UA (sum of U-factor times assembly area) is less than or equal to the total UA resulting from using the U-factors in Table R402.1.4/N1102.1.4 (multiplied by the same assembly area as in the proposed building), the building should be considered in compliance with Table R402.1.2/N1102.1.2. The UA calculation should be done using a method consistent with the ASHRAE3 Handbook of Fundamentals and should include the thermal bridging effects of framing materials. The SHGC requirements must be met in addition to UA compliance.

The general equation for calculating heat flow through building envelope components is:

Equation A.1

 

where the subscripts identify different series of materials that present a different path of heat transfer; e.g., Area1 is the area between the framing and Area1 is the area of the framing. The U-factor is the inverse of the sum of all the material R values for each path of heat transfer and includes the insulating value of surface air films. Equation (A.1) is sufficiently accurate unless any of the construction material is highly conductive (e.g., steel framing).

As an example, for envelope components with wood frame construction, Equation (A.1) becomes (A.2)

Equation A.2

 

Per ANSI/ASHRAE/IES Standard 90.1-2010, Normative Appendix A, Rated R-value of Insulation and Assembly U-factor, C-factor, and F-factor Determinations

A3.4 Wood-framed Walls, A3.4.1 General. The base assembly is a wall where the insulation is installed between 2 inch nominal wood framing. Cavity insulation is full depth, but values are taken from Table A9.4C for R-19 insulation, which is compressed when installed in a 5.5 inches cavity. Headers are double 2 inches nominal wood framing. The U-factors include R-0.17 for exterior air film, R-0.08 for stucco, R-0.56 for 0.625 inches gypsum board on the exterior, R-0.56 for 0.625 inches gypsum board on the interior, and R-0.68 for interior air film, vertical surfaces. Additional assemblies include continuous insulation, uncompressed and uninterrupted by framing. U-factors are provided for the following configurations:

Standard framing: wood framing at 16 inches on center with cavities filled with 14.5 inches wide insulation for both 3.5 inches deep and 5.5 inches deep wall cavities. Double headers leave no cavity. Weighting factors are 75% insulated cavity, 21% studs, plates, and sills, and 4% headers.

Advanced framing: wood framing at 24 inches on center with cavities filled with 22.5 inches wide insulation for both 3.5 inches deep and 5.5 inches deep wall cavities. Double headers leave uninsulated cavities. Weighting factors are 78% insulated cavity, 18% studs, plates, and sills, and 4% headers.

Advanced framing with insulated headers: wood framing at 24 inch on center with cavities filled with 22.5 inch wide insulation for both 3.5 inch deep and 5.5 inch deep wall cavities. Double header cavities are insulated. Weighting factors are 78% insulated cavity, 18% studs, plates and sills, and 4% headers.

  • Air Sealing/Air Leakage Control. The building thermal envelope should be constructed to limit air leakage.

    — 2015 IRC/IECC, Air Barrier and Insulation Installation Table R402.4.1.1/N1102.4.1.1

    • Continuous air barrier4 – Confirm that construction documents specify a continuous air barrier for the building components that include the wall intersections. Exterior thermal envelope insulation for framed walls should be in substantial contact and continuous alignment with the air barrier.

 

1"Thermal Bridging" is defined as when a more conductive (or poorly insulating) material allows an easier pathway for heat transfer across a thermal barrier.

2"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.

3ASHRAE – American Society of Heating, Refrigerating, Air-Conditioning Engineers, Inc., http://www.ashrae.org

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.

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 on-site 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 insulation of interior/exterior wall intersections where one or more specific type of inspection per the IECC or IRC may be necessary to confirm compliance. To confirm code compliance, framing and rough-in would be the typical type of inspection performed.

  • Ensure that the appearance of insulation of the interior/exterior wall in the field matches what is on the approved construction documents.
  • Ensure that the advanced framing techniques identified in approved construction documents matches what was installed in the field.
  • If R-value or U-factor approach for wall compliance was used in the documentation, ensure that the insulation installed meets the minimum R-value or maximum U-factor required for the type of wall and climate zone per the approved construction documents.
  • If the UA tradeoff approach for wall compliance was used in the documentation, ensure that the areas and U-factors associated with the interior/exterior wall intersection match the areas and insulation levels used on the approved construction documents. Also check that the areas and U-factors associated with whatever envelope component was improved to make up for the thermal bridge at the interior/exterior wall intersection.
  • Joints, seams, holes, and penetrations are caulked, gasketed, weather-stripped, or otherwise sealed.
  • Continuous air barrier is properly installed. Confirm the insulation for framed walls is installed in substantial contact and continuous alignment with the air barrier.
  • Corners and headers are insulated and the junction of the foundation and sill plate is sealed. The junction of the top plate and top of exterior walls are sealed.
Technical Validation(s)

This section provides additional related information and references to materials that are applicable to the provision.