Attic Eave Minimum Insulation

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Scope

Design the roof with raised heel trusses to allow full insulation over the top plates of the exterior walls.
Design the roof with raised heel trusses to allow full insulation over the top plates of the exterior walls.

Design the roof to allow full insulation over the top plates of the exterior walls.

  • Install raised heel trusses or use another roof framing method that allows space to install insulation over the top plates of the exterior walls, or install high-density insulation, to achieve an R-value that meets or exceeds code minimums for attic insulation.
  • ENERGY STAR requires ≥ R-21 in climates zones 1 through 5 and ≥ R-30 in climates zones 6-8. The insulation must extend to the inside face of the exterior wall and be installed using Grade 1 insulation techniques (ENERGY STAR 2015).

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

In vented attics, insulation is laid on the ceiling deck of the top floor of the home. Maintaining the insulation level throughout the entire plane of the ceiling and over the top of the perimeter walls is key to preventing heat flow through the ceiling and into or out of the home. When the roof pitch is low at the eaves, insulation may be compressed or lacking, causing cold spots in winter along exterior walls and possibly contributing to ice dam formation in snowy climates. This can be true of roofs built with pre-made trusses and roof rafters constructed on site.

Standard roof trusses are narrow at the eaves, preventing full insulation coverage over the top plate of the exterior walls

Figure 1 - Standard roof trusses are narrow at the eaves, preventing full insulation coverage over the top plate of the exterior walls.  Reference

A standard site-built roof of rafters may pinch the insulation at the eaves

Figure 2 - Standard site-built roof rafters may pinch the insulation at the eaves.  Reference

Builders and architects have several options for designing pitched, vented roofs that allow the insulation to achieve its full thickness over the plate line of the exterior walls: elevating the heel (sometimes referred to as an energy truss, raised-heel truss, or Arkansas truss), use of cantilevered or oversized trusses, lowering the ceiling joists, or framing with a raised rafter plate (BECP 2011). For a truss roof, raised heel energy trusses or oversized (cantilevered) trusses that form elevated overhangs, in combination with rafter baffles and soffit dams, will provide clearance for both full-height insulation and ventilation.

In stick-built roofs where rafters and ceiling joists are cut and installed at the construction site, laying an additional top plate across the top of the ceiling joists at the eave will raise the roof height, prevent compression of the attic insulation, and permit ventilation. When installing a raised top plate, place a band joist at the open joist cavities of the roof framing. The band joist also serves as a soffit dam, helping to prevent wind washing of the attic insulation (where air entering the soffit vents flows through the attic insulation, which can reduce attic insulation R-values on extremely cold days or add moisture to the insulation) (Southface and ORNL 2000Straube and Grin 2010).

With a cathedral ceiling, a vaulted parallel chord truss roof can be constructed. Cathedral ceilings must provide space between the roof deck and ceiling for adequate insulation and ventilation. The 2009 IECC requires at least R-30 in areas where the roof-ceiling design doesn’t allow for more. Insulation levels of R-30 or higher can be achieved through the use of truss joists, scissor truss framing, or sufficiently large rafters. For example, cathedral ceilings built with 2x12 rafters have space for standard 10-inch, R-30 batts and ventilation.

The designer should specify energy trusses or other constructions that will allow full height construction and baffles on building plans. These designs will be implemented by the framer. The insulation contractor should install the insulation correctly to full depth and install rulers. This task should be included in the contract for the appropriate trade, depending on the workflow at a specific job site.

See the “compliance” tab for 2009 IECC-specified wall insulation levels.  Some building scientists note that fully vented, pitched attic assemblies can be the lowest cost, highest R-value, and most durable roofs in all climates zones (except perhaps IECC Zone 1 and Zone 2 with high coastal humidity), as long as no major sources of potential air leakage (e.g., HVAC ducts or recessed light fixtures) are present in the ceiling plane. Given the low cost, high insulation levels (R-60 to R-100) are affordable and economically justified in Zones 5 through 8 and the only change required to meet these high levels, other than an airtight ceiling, is to construct raised heel trusses or rafter designs to accommodate the increased amount of insulation (Straube and Grin 2010).

How to Construct a Roof with Full Insulation at the Eaves

  1. Order and install oversized or raised heel trusses (Figure 3), or install site-built rafters with raised top plates (Figure 4). Specify 2- to 2½-foot overhangs, which are higher, providing more room for insulation at the top plate and additional window shading.
  2. Install baffles and soffit dams at each rafter bay to provide a clear path for ventilation air above the insulation and to prevent insulation from falling into the soffit vents.

    Raised heel, energy trusses extend further past the wall and are deeper at the wall allowing room for full insulation coverage over the top plate of the exterior walls

    Figure 3 - Raised heel energy trusses extend further past the wall and are deeper at the wall, allowing room for full insulation coverage over the top plate of the exterior walls.  Reference

    A site-built rafter roof with a raised top plate allows for more insulation underneath

    Figure 4 - A site-built rafter roof with a raised top plate allows for more insulation underneath.  Reference

  3. For cathedral roofs, specify and install parallel chord trusses (Figure 5).

    In cathedral ceilings, parallel chord trusses allow thicker insulation levels over the exterior wall top plates

    Figure 5 - In cathedral ceilings, parallel chord trusses allow thicker insulation levels over the exterior wall top plates. 

  4. Install attic rulers to show that blown insulation is installed to the proper depth. (The rules should be installed with numbering facing the attic entrance; one ruler for every 300 ft2).
  5. Fill the attic with blown, spray foam, or batt insulation to at least the required minimum insulation level. Blown or spray foam insulation should cover the tops of the ceiling joists. Batts should completely fill the joist cavities; an additional layer of batts can be layed over top of and running perpendicular to the joists. Shake batts to ensure proper loft. If joist spacing is uneven, fill any gaps with scrap pieces of insulation. Cut slits in batt nsulation to fit around wiring, plumbing, or ductwork without compressions.

Ensuring Success

The quality of the insulation installation should be visually inspected by the site supervisor. It may be possible to detect heat loss at the tops of exterior walls with an infrared camera if a sufficient temperature difference exists between the outside and the conditioned space of the house. Attic rulers should be installed upright from the ceiling deck and facing the attic entrance, one ruler for every 300 ft2 including at the eaves, to make it easier for the inspector to confirm that proper insulation depth has been achieved.

Climate

Install insulation in amounts that meet or exceed code-required levels for your climate zone. See for example Table R402.1.1 in the International Energy Conservation Code (2009 IECC, 2012 IECC or Table R402.1.2 in the 2015 IECC).

ENERGY STAR Certified Homes 

ENERGY STAR Certified Homes (Ver. 3/3.1 Ver 08) Rater Field Checklist, 3. Reduced Thermal Bridging. For insulated ceilings with attic space above (i.e., non-cathedralized), Grade I insulation extends to the inside face of the exterior wall below and is ≥ R-21 in CZ 1-5; ≥ R-30 in CZ 6-8.

DOE Zero Energy Ready Home

DOE Zero Energy Ready Home (Rev 05) Exhibit 1:  Envelope: Ceiling, wall, floor, and slab insulation shall meet or exceed 2012 or 2015 IECC levels.

 

climate zone map

International Energy Conservation Code (IECC) Climate Regions

Training

Right and Wrong Images

Presentations

None Available

Videos

  1. Attic Eave Minimum Insulation (1)
    Publication Date: July, 2015
    Courtesy Of: NAIMA

    Video describing how to install batt insulation. 

  2. Attic Eave Minimum Insulation (2)
    Publication Date: July, 2015
    Courtesy Of: NYSERDA

    Video describing how to properly insulate attic eaves.

  3. Attic Eave Minimum Insulation (3)
    Publication Date: July, 2015
    Courtesy Of: Zero Energy Homes

    Video describing raised heel trusses for minimum attic eave insulation.

CAD Images

Compliance

ENERGY STAR Certified Homes

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

Thermal Enclosure System:

3. Reduced Thermal Bridging

3.1 For insulated ceilings with attic space above (i.e., non-cathedralized), Grade I insulation extends to the inside face of the exterior wall below and is ≥ R-21 in CZ 1-5; ≥ R-30 in CZ 6-812 

Footnotes:

(12) The minimum designated R-values must be achieved regardless of the trade-offs determined using an equivalent U-factor or UA alternative calculation, with the following exception:
For homes permitted through 12/31/2012: CZ 1-5: For spaces that provide less than 5.5 in. of clearance, R-15 Grade I insulation is permitted. CZ 6-8: For spaces that provide less than 7.0 in. of clearance, R-21 Grade I insulation is permitted. 
For homes permitted on or after 01/01/2013: Homes shall achieve Item 3.1 without exception.
Note that if the minimum designated values are used, then higher insulation values may be needed elsewhere to meet Item 1.2. Also, note that these requirements can be met by using any available strategy, such as a raised-heel truss, alternate framing that provides adequate space, and / or high-density insulation.

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:  Envelope: Ceiling, wall, floor, and slab insulation shall meet or exceed 2012 or 2015 IECC levels, per RESNET standards. Compliance can be determined by meeting Zero Energy Ready Home requirements based on prescriptive insulation requirements, or U-factor alternatives. Steel-frame ceilings, walls, and floors shall meet the insulation requirements of the 2012 IECC – Table 402.2.6. The calculation for a steel-frame envelope assembly shall use the ASHRAE zone method or a method providing equivalent results, and not a series-parallel path calculation method.

2009 IECC

Table 402.1.1 lists ceiling R values by climate zone: CZ 1-3: R-30, CZ 4-5: R-38, CZ 6-8: R-49.
Section 402.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation.

2009 IRC

Table 1102.1.1 lists ceiling R values by climate zone: CZ 1-3: R-30, CZ 4-5: R-38, CZ 6-8: R-49.
Section 1102.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation.  

2012 IECC

Table 402.1.1 lists ceiling R values by climate zone: CZ 1: R-30, CZ 2-3: R-38, CZ 4-8: R-49.
Section 402.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation

2012 IRC

Table 1102.1.1 lists ceiling R values by climate zone: CZ 1: R-30, CZ 2-3: R-38, CZ 4-8: R-49.
Section 1102.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation

2015 IECC

Table 402.1.1 lists ceiling R values by climate zone: CZ 1: R-30, CZ 2-3: R-38, CZ 4-8: R-49.
Section 402.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation.

2015 IRC

Table 1102.1.2 lists ceiling R values by climate zone: CZ 1: R-30, CZ 2-3: R-38, CZ 4-8: R-49.
Section 1102.2.1, Ceilings with attic spaces. R-30 will satisfy the requirement for R-38 insulation on the ceiling if the R-30 is uncompressed and extends over the top plates at the eaves. In the same way, R-38 suffices in climate zones requiring R-49 attic insulation.

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

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Contributors to this Guide

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

Case Studies

None Available

References and Resources*

  1. Author(s): Southface Energy Institute
    Organization(s): Southface Energy Institute
    Publication Date: January, 2009

    Document intended to help graphically demonstrate the air leakage provisions of section 402.4 of the 2009 IECC.

  2. Author(s): NAHB, Southface Energy Institute, ORNL, NREL
    Organization(s): NAHB, Southface Energy Institute, ORNL, NREL
    Publication Date: January, 2002

    Information sheet about advanced wall framing.

  3. Author(s): Seifert
    Organization(s): University of Alaska
    Publication Date: January, 2003

    Document describing approaches to energy efficiency and moisture considerations for roofs in northern climates.

  4. Author(s): Southface Energy Institute, ORNL
    Organization(s): Southface Energy Institute, ORNL
    Publication Date: February, 2000

    Information sheet with information about insulating and ventilating attics.

  5. Author(s): DOE
    Organization(s): DOE
    Publication Date: May, 2015

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  6. Author(s): EPA
    Organization(s): EPA
    Publication Date: September, 2015

    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).

  7. Author(s): Georgia Department of Community Affairs
    Organization(s): Georgia Department of Community Affairs
    Publication Date: January, 2011

    Georgia state's minimum standard energy code, including state supplements and amendments.

  8. Author(s): Straube, Grin
    Organization(s): BSC
    Publication Date: March, 2009

    Report that considers a number of promising wall systems that can meet the requirement for better thermal control.

  9. Author(s): DOE
    Organization(s): DOE
    Publication Date: September, 2011

    Information sheet with the definition and uses for a raised truss.

  10. 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.

Building Science-to-Sales Translator

High-R Attic Insulation =
High-Efficiency or Ultra-Efficient Attic Insulation

Technical Description: 

Because most heat loss in a home is through the attic, increasing attic insulation is one of the most cost-effective steps home builders can take to improve home energy efficiency. High-efficiency insulation meets or exceeds the insulation levels required by the 2012 International Energy Conservation Code (IECC), which is ~15% more efficient than 2009 IECC. Ultra-efficient insulation levels exceed the minimum 2009 IECC levels by 50% or more. High-efficiency and ultra-efficient attic insulation systems include vented attics that are insulated over the ceiling deck with increased amounts of blown fiberglass or blown cellulose, unvented attics that are insulated on the underside of the roof deck with blown spray foam, and vented or unvented attics that are insulated above the roof deck with rigid foam.

Alternate Terms

Enhanced Comfort Attic Insulation
Enhanced Quiet Ceiling Insulation
Advanced Attic Insulation
High-Efficiency or Ultra-Efficient Attic Insulation
Sales Message
High-efficiency attic insulation helps provide added thermal protection. What this means to you is less wasted energy along with enhanced comfort and quiet. Knowing there is one opportunity to optimize performance during construction, wouldn’t you agree it’s a great opportunity to meet or exceed future codes?
Last Updated: 03/14/2016

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