Good Bones: Structural Framing Tips for Better Energy Performance Under the Latest Codes
It is virtually impossible to achieve the standards of the 2009 International Energy Conservation Code (IECC) by accident, let alone those of the 2012 IECC, which is even more stringent.
The 2012 IECC reaches into almost every part of the home, including how it is framed. The building code allows for a wide variety of structural framing approaches, but some framing approaches and practices play a role in a home’s total energy performance.
Several U.S. jurisdictions have already adopted 2012 IECC, and more will soon follow. Maryland was the first to adopt the 2012 IECC statewide, and local governments in Missouri, Washington, and other states are using it. Minnesota and Oregon have had above-code programs for a while.
Regardless of your jurisdiction’s IECC adoption status, taking a closer look at your framing approach now can help you prepare for the day when the latest codes take effect—and help boost energy performance to reduce homeowners’ energy bills today.
2012 IECC and Framing
The 2012 IECC (which is now incorporated by reference into the 2012 International Residential Code) includes changes that address traditional energy-related topics and systems, such as hot-water piping, duct leakage, and windows. To help achieve an estimated 30% improvement in energy savings compared to the 2006 IECC, the latest energy code also touches on structural framing in several ways.
- It specifies more stringent air leakage limits, as verified by a mandatory blower door test (Section R402.4.1.2):
< 5 ACH50 in climate zones 1 and 2; and
< 3 ACH50 in climate zones 3–8.
- It specifies more rigorous wall insulation requirements in climate zones 3–8 (Table R402.1.1):
- R-20 cavity insulation or R-13 cavity plus R-5 insulated siding/continuous insulation (Zones 3, 4, 5); and
- R-20 cavity insulation plus R-5 insulated siding/continuous insulation or R-13 cavity plus R-10 insulated siding/continuous insulation (Zones 6, 7, 8).
- Like the 2009 code, the 2012 IECC calls for corners, headers, and rim joists to be insulated (Table R402.4.1.1).
General Framing Tips
Structural framing materials and practices to improve home energy efficiency fall into three broad categories. They consist of
- steps to help improve airtightness;
- actions to reduce thermal bridging; and
- assemblies that make possible better insulation.
Successful framing in each of these categories requires effective planning and design, rather than reliance on framers’ rules of thumb. It is crucial to figure out framing details during the planning phase of the project, since the frame interfaces with so many other building systems.
Historically, there has been a lot of overbuilding within residential structural frames, especially in walls. Cripple studs, extraneous jack studs, and so forth create thermal bridges and make insulating the home more difficult.
An important framing action you can take is to question every stick that is framed into the wall and scrutinize how it functions with the rest of the structure. Is any given board truly part of the structural load path, or was it added just to make framing more convenient? Each board should be analyzed to ensure that it serves a purpose, even if that purpose is not structural.
To achieve this, find building materials dealers who have the tools to support energy-efficient framing. This includes design software that enables builders to see every framing member before construction begins, and to track loads throughout the structure to determine which sticks can be removed from the design.
Appropriately equipped dealers are also a valuable resource for the fabrication of framing components. Precut and labeled framing members, for example, are more precise, which helps to ensure a straight and true structural frame with fewer potential leak points, and supports better-fitting insulation. Dealers who can precut holes in joists also help make it easier to locate HVAC in conditioned spaces.
HVAC in Conditioned Spaces
The 2012 IECC doesn’t specifically require HVAC to be located within conditioned spaces, but chapter 16 of the 2012 IRC states that “stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.” In most climates there is an incentive to locate HVAC in conditioned spaces, since additional air-sealing tests and extra insulation may be required if ducts are located outside the thermal envelope—in uninsulated attics, for example.
Aside from code requirements, routing mechanical ducts through conditioned spaces makes tremendous sense from a cost-benefit perspective. The National Renewable Energy Laboratory (NREL) notes that it is relatively inexpensive to place ducts in conditioned spaces during construction, and that doing so can reduce electricity demand for cooling by about 15%.
NREL analysis also shows that locating ducts within conditioned spaces is a much more cost-effective investment than other common energy efficiency improvement measures, such as R-15 or R-19 walls, R-40 or R-50 attics, low-SHGC windows, and 15 SEER or 17 SEER air conditioners. That’s not to say that those other measures shouldn’t be taken, but that ducts in the conditioned space provide the greater financial return.
In practice, placing HVAC inside the thermal envelope often means running ducts between floors. The challenge, of course, is that other things, such as plumbing, electrical, sprinkler systems, and central vacuums, typically occupy the same area. Space is too limited not to plan out all the elements of HVAC installment in advance.
Building materials dealers who use advanced structural frame design programs such as Weyerhaeuser Javelin design software, can identify and resolve framing conflicts with mechanical systems before construction begins. Such tools also provide framing crews with a precise picture of what needs to be done.
Taking technology a step further, some dealers also use component fabrication software to precut holes in joists, which greatly simplifies work for framers and mechanical installers on the jobsite. In addition to helping reduce jobsite labor, precut holes reduce the risk of crews compromising the structure by cutting holes outside of a joist’s design parameters (that is,holes that are too large or too close to I joist flanges or joist ends).
In certain climate zones, where 2 x 4 exterior walls have been used, framing changes are needed to meet the 2012 IECC’s more stringent insulation requirements. For example, climate zone 3 requirements for R-20 cavity insulation need an alternative framing approach, such as converting to 2 x 6 walls to allow for thicker insulation. If 2 x 4 walls are still desired, the code allows for R-13 cavity insulation plus R-5 continuous insulation. Other engineered solutions are possible that can also meet the new target R-values.
A host of more energy-efficient framing options are available to builders. So-called advanced or optimum-value engineering (OVE) framing is not difficult, especially when planned in advance with design software.
The energy savings from advanced framing can be substantial. Weyerhaeuser built a 2 x 6 demonstration wall at the 2013 International Builders’ Show that achieved an 86% increase in insulation volume and 58% increase in the assembly’s R-value compared to traditional framing practices. At the same time, the wall used 8% less wood volume, making more efficient use of natural resources.
Following are some advanced framing methods to consider. Check with your local authority for any requirements or restrictions that might apply in implementing any of these measures.
Two-by-six studs at 24-inch OC spacing. Increasing stud spacing from the traditional 16 inches on center to 24 inches and increasing the stud depth allows for 2 more inches of insulation between the exterior and interior walls. In addition, the wider stud spacing decreases the number of sticks within a given wall, reducing thermal bridges.
Two-stud corners. The typical corner framing practice of three studs set back-to-back increases thermal bridging and makes it difficult to install insulation deep in the corner of the wall. An easy-to-build alternative is the California corner, with two studs placed at a right angle to each other. One of the studs provides an attachment point for the drywall along one wall, while drywall clips allow simple attachment along the other wall. This configuration allows insulation to penetrate farther into the corner, reduces thermal bridging, and saves on materials by eliminating a stud at each corner.
Ladder blocking. Where interior walls join an exterior wall in a T intersection, the traditional practice of adding extra studs in the exterior wall creates narrow, difficult-to-insulate spaces and increases thermal bridging. A more energy-efficient alternative is to place short, horizontal pieces of lumber between the regular run of studs—providing an attachment point that resembles a ladder in the exterior wall. This configuration makes it possible to place the full width of insulation between the exterior wall studs, and when 2 x 6 framing is used, still allows for several inches of insulation between the blocking and the exterior wall.
Resources for Energy-Efficient Framing
NAHB Research Center’s ToolBase.org is a great resource for technical information on building products, materials, new technologies, business management, and housing systems.
While there, I recommend using the search bar to download a PDF of the documents, “Ducts in Conditioned Space TechSpecs” and “Advanced Framing Techniques ToolBase TechSpecs,” both of which include basic information about the costs, benefits and drawbacks, and real-world feedback from field evaluations.
The Weyerhaeuser Learning online resource helps customers get the most out of the company’s products and services. I recommend taking their continuing education workshop, Residential Framing for Green Certification.
Alternate headers. In some cases, it is possible to use smaller window or door headers or remove them from certain exterior wall openings. Building materials dealers with advanced structural frame design software can help make this determination, or you can consult a structural engineer. This is a key place to consider for energy-efficient framing, since headers create wide thermal bridges. Depending on the overall wall configuration and loading, an alternative to sawn-lumber headers is to use along the top of the wall a double thickness of rim board that has structural bending capacity. Adding ½-inch-thick insulating foam to the outside face of the rim provides an effective and simple barrier against heat gain and loss. Laminated strand lumber (LSL) rim board, such as TimberStrand LSL, works well for such applications, since it is strong, dimensionally stable, and warp resistant. And without a full header, there is more space for insulation above the window or door.
Additional advanced framing options to consider, where possible, for reducing thermal bridging include 1) using one, rather than two, top plates, and 2) minimizing trimmers or jack studs. Again, a structural engineer or materials supplier with advanced structural frame design software should be consulted if you are considering these options.
Insulated Framing Components
Table R402.4.1.1 of the 2012 IECC (Table 402.4.2 in 2009 IECC) requires insulated headers, rim board, and corners. Builders can either assemble these on site or use prefabricated insulated members. The latter are faster and easier to install than field-assembled components. Following is a summary of the available options (and performance characteristics) of one such line of prefabricated members manufactured by Weyerhaeuser—TJ Insulated products:
- Insulated headers (R-17). Composed of 1½-inch-thick insulating foam sandwiched between 3½-inch-thick LSL and a 7/16-inch-thick oriented strand board face. Unlike traditional sandwich headers, in which loads are carried through individual layers of laminated veneer lumber separated by foam, this configuration allows loads to be carried directly through the LSL into the wall framing. Builders can thus size and specify the product similar to traditional LSL headers, while avoiding the additional calculations required with other insulated headers.
- Insulated rim board (R-10). Preassembled units combine 1¼-inch-thick LSL rim board with 1 inch of insulation, and fit and fasten much like standard LSL rims.
- Insulated corners (R-30). Options include either LSL or specialty sawn-lumber wall studs sized for 2 x 6 exterior walls, combined with 4 inches of rigid foam insulation.
Roberts, David, and Jon Winkler, “Ducts in the Attic? What Were They Thinking?” Conference paper NREL/CP-550-48163, presented at the annual meeting of the National Renewable Energy Laboratory, August 2010.
Although it takes careful planning to achieve a more energy-efficient structural frame, the actual construction is not complex, and many technical supports are available. Regardless of code requirements, attention to the frame can help to give builders the marketing advantage: a home that costs less to operate and is more comfortable. To learn more, contact your framing products manufacturer or dealer (see “Resources for Energy-Efficient Framing”). Some manufacturers have technical experts available to consult on framing practices.
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