DOE's Challenge Home

December 29, 2013
January/February 2014
A version of this article appears in the January/February 2014 issue of Home Energy Magazine.
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Although our federal government is notorious for bad decision making (or no decision making), sometimes it does good work. One example is Energy Star.

Energy Star, a federal certification program with many facets, is one of the most recognized brands in the world. In the building world, the Energy Star Certified Homes program has been widely accepted by builders and consumers (see “Energy Star Homes—Predictions, Performance, and Real-World Results,” HE Jan/Feb ’11, p. 28). More than one million Energy Star 2.0 homes have been built since the inception of the voluntary program. (The current version, officially launched in January 2012, is 3.0.) EPA manages Energy Star.

For several years other government programs, such as Building America, have pushed building science beyond the basic Energy Star requirements. The latest leading-edge program from DOE is called Challenge Home. DOE’s intention with Challenge Home is to encourage builders to build net zero energy-ready homes that have “unparalleled levels of comfort, indoor air quality [IAQ], durability, and quality.” The program was officially launched in early 2012.

Steve Mann
is a HERS rater, LEED AP+ Homes, Certified Energy Analyst, serial remodeler, and longtime software engineer. (April Wise Photos)

Challenge Homes are homes of the not-too-distant future. They are generally 40–50% more energy efficient than a current code home. They must be solar thermal- and solar PV-ready. They must be verified by a third party, typically a HERS rater. Challenge Home builds heavily on Energy Star for Homes, and significantly ups the ante. On the surface, the requirements look pretty simple. The devil is in the details.

Prescriptive Versus Performance

There are two paths to attaining Challenge Home certification: prescriptive and performance. To qualify for the prescriptive path, the home has to

  • incorporate a set of mandatory measures shown in Table 1;
  • meet or exceed International Energy Conservation Code (IECC) climate zone-specific requirements (called the Challenge Home Target Home) shown in Table 2; and
  • meet the floor area and bedroom requirements shown in Table 3.

This photo from the EPA airPlus program describes the installation of a peel-and-stick membrane on a roof valley. (Building American Solution Center)

Table 1. DOE Challenge Home Mandatory Requirements for All Labeled Homes

Table 2. DOE Challenge Home Target Home

Table 3. Benchmark Home Size

EPA's Climate Zones for Fenestration and DOE's Proposed Climate Zones

Figure 1. Challenge Home uses IECC climate zones. Energy Star fenestration has its own climate zones that don’t exactly align with the IECC designations.

An insulator blows fiberglass insulation into netted wall cavities in a new home. (Building American Solution Center)

If you can’t comply using the prescriptive approach, you have to comply using the performance method. This might be required if, for instance, you don’t have at least 80% Energy Star light fixtures, if the U-value of your windows in a mixed climate is higher than allowed for your climate zone, or if you have too few bedrooms for the house size.

The performance method gives you much more flexibility. It requires using a HERS model and it must be one in which the estimated project index is less than the index of the same home built to the Challenge Home Target Home specifications, adjusted for home size based on Table 3. This size adjustment uses conditioned floor area (CFA) to adjust the target, making it harder for oversized houses to comply:

HERS Target index = Challenge Home Target Home HERS index x (CFA from Table 3 / CFA home to be built) 0.25

The beauty of the performance approach is that you can trade off energy-inefficient features against energy-efficient features as long as you meet the mandatory requirements. For instance, you might be able to have a higher window solar heat gain coefficient (SHGC) but a lower U-factor. That may make sense in a Passive House design that relies on solar heat gain for heating.

The downside to the performance approach is that you have to build a model using RESNET-approved modeling software that can do the Challenge Home analysis; more about the software later.

Mandatory Requirements

There are seven Challenge Home mandatory requirements. First, you have to have the home certified as an Energy Star 3.0 home. That sounds simple but it actually requires a fair amount of work compared to Energy Star 2.0. There isn’t room to describe all the details in this article (see “Energy Star for Homes 3.0,” HE Jan/Feb ’11, p. 6 for more). Most of the requirements amount to smart, high-quality building practices with third-party verification.

Here’s a summary of the other Challenge Home mandatory requirements:

  • Fenestration has to meet or exceed the latest Energy Star window requirements. This requirement can get complicated, as I’ll explain shortly.
  • All insulation has to meet or exceed 2012 IECC levels, which are not unreasonable. For instance, depending on climate zone, walls range from R-13 to R-25, ceilings from R-30 to R-49.
  • Forced-air ducts have to be located within conditioned space. There are exceptions, such as R-8 + 1.3 inches of spray foam on ducts buried under at least 2 inches of blown-in cellulose in a vented attic.
  • Hot water must be deliverable to all fixtures within 0.6 gallons of discharge, a WaterSense specification. That’s approximately the volume of water in 50 feet of ½-inch copper pipe from the source to the destination. Start-up delays on tankless water heaters can easily exceed that requirement. On-demand recirculation systems can qualify; other types of recirculation system do not.
  • All refrigerators, dishwashers, clothes washers, bath fans, and ceiling fans must be Energy Star-rated.
  • Eighty percent of the lighting fixtures or lamps must be Energy Star-rated.
  • You have to comply with all the provisions on the EPA Indoor airPLUS Checklist.
  • The home must be both solar PV- and solar thermal-ready. There are separate checklists for each of these requirements.

The Indoor airPLUS program has been around for several years. It’s designed to make sure that buildings have high IAQ. The checklist includes provisions for moisture control, radon-resistant construction, pest management, garage contaminants, forced-air filtering, and low-VOC and low-formaldehyde building materials. You can substitute the Energy Star Water Management Builder Checklist for the airPLUS provisions related to moisture control.

Stay in close communication with both the Challenge Home and the Energy Star program administrators if you have questions.

The solar-ready requirements are straightforward. For both solar thermal and solar PV, you have to do a site assessment, document structural considerations, install conduit and a plumbing chase, make sure there is adequate space for the hardware, do preliminary system designs, and provide the homeowner with all the documentation. Essentially you do everything to provide solar hot water and electricity except permitting, and buying and installing the equipment. The only wrinkle is that the solar-ready checklists, which come from an EPA program, require a certain type of shading analysis. Challenge Home requires a different type of shading analysis.

Altogether, the Challenge Home and Energy Star documentation alone entails 12 pages of checklists and 20 pages of explanatory footnotes. There are numerous exceptions in almost all categories (hence the footnotes). These page counts don’t include information from outside sources, such as ASHRAE documentation, Energy Star fenestration documentation, airPLUS specifications, or the IECC code. The devil lurks in the footnotes.

Devilish Details

If you follow the Energy Star and Challenge Home prescriptive paths, life is pretty easy. If you’re building unusual homes that can comply only if you use the performance path, things can get more interesting. Assuming that you grasp all the subtleties, which can be a challenge in itself, there are still some tricky areas.

The Energy Star requirements are, for the most part, rigorous but not unreasonable. You’re expected to have a high-quality thermal envelope with minimal thermal bridging, good air sealing, intelligent HVAC design and commissioning, and good water management practices, all verified by a certified third-party rater.

The most complicated aspect of Energy Star 3.0 certification, at least in California, is that HVAC contractors have to be Energy Star 3.0-certified. They are expected to do ACCA Manual J, D, and S designs. They are also expected to test refrigerant charge, electricity consumption of the air handler and condenser, and static pressure; and to do air balancing. In my experience, most HVAC installers either do not have these skills or don’t use them.

That may explain why as of September 2013, there are only 12 Energy Star certified HVAC installers in all of California, and none in the Bay Area, where I live. The closest is in Sacramento, about 60 miles away. That’s definitely a barrier to both Energy Star and Challenge Home adoption.

This appears to be less of a problem in other parts of the country. As one HERS colleague said to me a few months ago, “I see that there are currently only 13 certified contractors for all of California. By contrast, we have 26 in Virginia, with probably only 10% the number of homes being built. Part of the problem is that there is only a very small market (as I understand it) for the Energy Star certification in California, due to the strength of Title 24.”

The Challenge Home mandatory requirements present some challenges as well (perhaps that’s why they chose the name Challenge Home?). Here’s one example—the most vexing one I’ve encountered. Fenestration has to meet or exceed current Energy Star fenestration requirements. That sounds simple at first reading. Here are some of the devilish details.

Challenge Home uses IECC climate zones. Energy Star fenestration has its own climate zones that don’t exactly align with the IECC designations (see Figure 1). You have to make sure you are looking at the correct specifications.

There are various exceptions detailed in copious footnotes. For instance, triple-pane windows with thermal spacers are exempt. You can use area-weighted averages to meet the Energy Star U-factor and SHGC requirements. You can exclude 15 square feet of glazing and one door. You can also exclude fenestration that is part of a passive-solar design if you meet a very specific definition of passive solar.

If you refer to the Energy Star Thermal Enclosure System Rater Checklist, a prerequisite for Challenge Home, you see that fenestration must meet 2009 IECC requirements, not Energy Star requirements. There is also a group of exceptions to this requirement. If the windows don’t have NFRC ratings, you have to use Tables 4 and 14 from the ASHRAE 2005 Handbook of Fundamentals to determine your compliance requirements.

What About Software?

Both Energy Star and Challenge Home performance path certifications require modeling with RESNET-approved software to calculate a HERS index. The two candidates are REM/Rate and EnergyGauge. They both had their Energy Star 3.0 2012 modules in place in 2012 and REM/Rate released its Challenge Home module in 2013.

In order to get a Challenge Home certification, you have to provide a certified report that claims the home complies with Challenge Home requirements. The problem I’m encountering with unusual custom-home projects is that many of the Challenge Home and Energy Star footnoted exceptions are not supported by the modeling software (admittedly, it’s very difficult to accommodate so many exceptions). In order to achieve certification, I have to make sure the architects and builders I work with limit their design choices to those items supported by the software.

On top of that, in California we don’t use a HERS index like the rest of the country. We use software approved by the California Energy Commission (CEC). Currently, if we want a Challenge Home certificate, we have to build two models—CEC and HERS. If the project is a Passive House (Challenge Home is now a prerequisite for Passive House U.S. certification), we have to build three models–HERS, CEC, and Passive House Planning Package (PHPP). After a while, it really starts to feel like a government program.

In all fairness, the Challenge Home program implementers are working on a California-specific program, much like the Energy Star regional program for California, that uses CEC modeling instead of HERS modeling. This may be in place by the time you read this article. They may also be developing regional variations for unusual climate zones, such as Hawaii, or locations with stringent energy codes.

Whatever Happened to KISS?

The Keep It Simple Stupid (KISS) principle has been around for a long time. The Challenge Home program, and for that matter, Energy Star 3.0, could use a dose of it. I’m a huge advocate of high-performance building, but there’s got to be an easier way. I currently advise all my clients who want to get their homes Challenge Home-certified to start with a design with the simplest compliance path and then pay close attention to all the details. It’s easy to let one slip.

Additionally, stay in close communication with both the Challenge Home and the Energy Star program administrators if you have questions or are uncertain about any of the requirements. They are very willing to field questions, and they try to resolve complexities.

learn more

Learn more about the DOE Challenge Home program and other DOE home energy efficiency efforts.

The Challenge Home program designers are working on simplifying the program and making it work more readily with other programs, especially Passive House U.S. certification. They are also coordinating with the Energy Star folks to bring the programs into closer alignment. I’m just worried that by the time all the kinks are worked out, it will be time for Energy Star 4.0 and Challenge Home 2.0. In the meantime, the complexities may prevent builders from accepting the challenge. That would be unfortunate.

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