In March 2012, North Carolina introduced a new residential energy code. It includes the usual tougher standards, but also has new requirements for testing and an expanded role for third-party inspection and testing. Additionally, a new High Efficiency Residential Option path provides an option to improve your household systems’ efficiency by as much as 15 percent. The new standards form the basis for some exciting new utility rebates from Progress Energy as well. With the new code in effect since last year, we know what’s working well, what could be working better and how to get the best results with the least frustration.
Duct testing is one of the highlights in the new code. Any duct system outside conditioned space (in attics, crawlspaces and unconditioned basements) must be tested for leakage of less than 6 CFM25 per 100 square feet of floor area served, or 6 percent total leakage. (CFM25 stands for the cubic feet per minute of airflow required to create a 25-Pascal unit pressure change in the duct system.) This measure applies to new homes and new duct systems for existing homes. Leaky ducts are a major cause of energy waste and problems with thermal comfort and indoor air quality in homes. Based on our company’s experience, most new duct systems would have failed this test prior to the new code — unless the home was already pursuing ENERGY STAR or Green Built N.C. certification (both of which have required testing for a long time). It’s great that all new systems will now meet this level of quality.
Whole-house air sealing
Unfortunately, the new code hasn’t been as successful in the area of whole-house air tightness. The new code allows builders to fill out an air-sealing checklist or perform a blower-door test to demonstrate whole-house air tightness. Most builders choose the checklist option, and it’s not clear that more air sealing is taking place as a result. Unless you test with a blower door, there’s simply no way to know that a home is airtight.
As a third-party testing service, most of the code-compliance blower-door tests we’ve been asked to perform for the new code are for modular homes. Since it’s difficult to easily or honestly track all of the air-sealing tasks back through the plant, builders of modular homes are among the few who are opting to test their homes. Scores vary based on the modular plant of origin, but we do see failures that can be traced back to fairly obvious leakage paths. Without a requirement that every home be tested with a blower door, the checklist option is unlikely to result in tighter homes. This is one area where ENERGY STAR and Green Built N.C. homes have a significant advantage, since all homes in those programs are blower-door-tested.
Increased insulation and window requirements
The new code has also upgraded requirements for walls and windows. In Western North Carolina, walls now need to have insulation with an R value of 15 (up from R-13 in the previous code), which can be easily achieved in 2-by-4-foot walls with a high-density batt. Windows now need to have a lower U-value and a much lower solar heat gain coefficient (SHGC) than before. In both cases, the code is definitely driving large numbers of builders to install components that were better than before, and that’s great news.
However, builders do need to be aware that the window SHGC is difficult to trade off if you happen to buy the wrong window. And, if you have a passive-solar house (where you want a lot of solar-heat gain), you’ll need to have your design analyzed by a licensed design professional who can perform energy analysis and develop a “simulated performance alternative” method of meeting code. For a new home, this isn’t difficult to do. But for a renovation, you’ll likely be hamstrung by the existing building components. Unless it’s a gut-rehab, you’re better off sticking to the prescriptive requirements for windows.
Air barriers for air-permeable insulation
New requirements for air barriers are yielding mixed results, but we’re hopeful that the situation will improve over time. It’s been well-known for years that air-permeable insulation (fiberglass, cellulose etc.) needs to be in contact with a hard, air-impermeable surface (sheathing, drywall, rigid insulation etc.) on all six sides to work properly. This requires builders to add a rigid surface on the attic side of kneewalls, behind tub enclosures, under stair platforms, in duct chases and a number of other locations where the insulation might not be fully encased. We call it the “no naked fiberglass” rule, and it’s been part of the ENERGY STAR New Homes program since 2006.
Code inspectors are now looking for these details, but enforcement varies by jurisdiction, as does interpretation of what constitutes an acceptable “air barrier.” With time, education and experience, this will hopefully become more uniform.
The new code also clarifies vertical-edge insulation for slabs on grade. Heat loss through floor slabs tends to take the shortest path, which is out through the vertical edge. The easiest place to put insulation is horizontally under the slab, which unfortunately misses this entire vertical edge. There are two big challenges with installing vertical-slab edge insulation properly. First, the code shows only one insulation detail, with the insulation placed on the outside of the foundation wall. Second, because termites can tunnel through some types of rigid insulation, there is a 2-inch required “view strip” that must be left uninsulated at the top of this vertically applied insulation. This leaves about half the slab edge uninsulated, preventing it from achieving its full energy-savings potential. In reality, there are several ways to detail this that are both termite-durable and allow the slab edge to be insulated, but some code officials are interpreting the single detail in the code to mean that only one method is acceptable. We’re afraid that this detail will continue to frustrate efficiency-seekers for the next few years.
New financial incentives for energy efficiency
Finally, the above-code HERO pathway provides an optional compliance path to achieve even more savings. The N.C. Utility Commission has just approved a new $1,000-$4,000 Progress Energy builder rebate that is based on this HERO compliance path, so we expect to see more people pursuing it in the near future.
In addition to HERO requirements like R-19 walls and above-code windows for Asheville’s climate zone, this program also requires that the home be tested by a third-party HERS rater, passing both the duct-blaster and blower-door tests. Progress Energy feels so confident that this will result in a highly efficient house that they will even offer a bill guarantee to the homeowner in addition to the rebate. We expect the HERO code to become a lot more popular locally because of the significant utility incentives.
There is no doubt that the new residential-energy code raises the bar for builders. While there will be a significant learning curve, it’s all feasible to incorporate into standard building practices. It will also significantly improve the performance of new homes in the area. In addition, new utility rebates will reward builders for going even further with these energy efficiency measures, reducing the out-of-pocket investment for high-performing homes. New homes will be substantially more efficient than older homes, providing savings and value that homeowners will enjoy for their entire time living in the home. It’s the very definition of a win-win!
Amy Musser, Matthew Vande and Emily Boyd bring diverse points of view to Vandemusser Design PLLC, an Asheville-based home-energy-efficiency company. Musser is a mechanical engineer; Vande is an architect; and Boyd is a general contractor and former home inspector. Together, they provide design assistance, certification and audits to support high-performance homes.