Attic Eave Minimum Insulation
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, and criteria to meet national programs such as ENERGY STAR Certified Homes, DOE’s Zero Energy Ready Home program, and EPA’s Indoor airPLUS.
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.
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 on 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 (DOE 2002; Straube 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), if 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
- Order and install oversized or raised heel trusses, or install a site-built rafter roof with a raised top plate. Specify 2- to 2½-foot overhangs, which provide room for insulation at the wall junction and additional window shading.
- Install baffles at each rafter bay to prevent wind washing of thermal insulation and to prevent insulation from blocking ventilation in vented roof assemblies.
- 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 (Image courtesy of PNNL).
- Install rafter baffles to prevent ventilation from covering soffit vents and use insulation dams at the soffit, porch, garage, and attic access to prevent the insulation from spilling.
- Install attic rulers to show proper blown depth (facing the attic entrance, one ruler for every 300 ft2).
- Fill the attic with blown, spray foam, or batt insulation to at least the required minimum insulation level. The insulation should cover the tops of the ceiling joists. Make certain batts completely fill the joist cavities. Shake batts to ensure proper loft. If joist spacing is uneven, patch gaps in the insulation with scrap pieces. Do not compress the insulation with wiring, plumbing, or ductwork (cut slits in the insulation if necessary).
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. For insulated ceilings with attic space above (i.e., non-cathedralized), Grade I insulation should extend to the inside face of the exterior wall below at these levels: CZ 1-5: >= R-21; CZ 6-8: >= R-30. This may require a raised-heel truss, alternate framing that provides adequate space, and/or high-density insulation.
Thermal Enclosure Checklist, Reduced Thermal Bridging. For insulated ceilings with attic space above (i.e., non-cathedralized ceilings), Grade I insulation extends to the inside face of the exterior wall below at these levels: CZ 1 to 5: >= R-21; CZ 6 to 8: >= R-30.
International Energy Conservation Code (IECC) Climate Regions
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
(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.
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3. Exhibit 2: DOE Zero Energy Ready Home Target Home. Insulation levels shall meet the 2012 IECC and achieve Grade 1 installation, 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.
Section 402.2.3, Access hatches and doors. Access doors separating conditioned from unconditioned space are weather-stripped and insulated (without insulation compression or damage) to at least the level of insulation on the surrounding surfaces. Where loose fill insulation exists, a baffle or retainer is installed to maintain insulation application.*
Section N1102.2.3, Access hatches and doors. Access doors separating conditioned from unconditioned space are weather-stripped and insulated (without insulation compression or damage) to at least the level of insulation on the surrounding surfaces. Where loose fill insulation exists, a baffle or retainer is installed to maintain insulation application.*
Section R402.2.3, Eave baffle. Where air permeable insulation exists in vented attics, a baffle (of solid material) is installed adjacent to soffit and eave vents. Baffles maintain an opening equal or greater than the size of the vent. The baffle extends over the top of the attic insulation.*
Section N1102.2.3, Eave baffle. Where air permeable insulation exists in vented attics, a baffle (of solid material) is installed adjacent to soffit and eave vents. Baffles maintain an opening equal or greater than the size of the vent. The baffle extends over the top of the attic insulation.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.
High-R Attic Insulation = High-Efficiency or Ultra-Efficient Attic Insulation