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.
Figure 1 - Standard roof trusses are narrow at the eaves, preventing full insulation coverage over the top plate of the exterior walls.
Figure 2 - A standard site-built roof of rafters may pinch the insulation at the eaves.
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
1. 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.
2. Install baffles at each rafter bay to prevent wind washing of thermal insulation and to prevent insulation from blocking ventilation in vented roof assemblies.
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.
Figure 4 - A site-built rafter roof with a raised top plate allows for more insulation underneath.
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.
4. 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.
5. Install attic rulers to show proper blown depth (facing the attic entrance, one ruler for every 300 ft2).
6. 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).