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Net-Zero Energy vs. Zero Energy Ready Homes
Not all net-zero energy (NZE) homes are created equal, well, they aren’t all built the same that is. Also, slight differences in wording can mean different things. Did you know that there are net-zero energy homes and Department of Energy (DOE) Zero Energy Ready Homes (ZERH). What’s the difference? DOE defines a net-zero energy, or “zero energy,” building as “an energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.” [1] And a Zero Energy Ready home is “a high performance home which is so energy efficient, that a renewable energy system can offset all or most of its annual energy consumption.”[2] So, a ZERH is a building that has maximized its energy efficiency and has the potential to be a NZE home if renewable energy sources are added. Those are the definitions, but how they are built, and what products are used, to meet those standards, is up to the builder.
Whether the target is NZE or ZERH, insulation and air sealing decisions are critical. This raises the question: which insulation is the right one to meet these stringent performance targets?
Insulation Used in High Performance Homes
DOE maintains a list of many homes around the country that meet its ZERH standard. This list also contains many specifics on how the homes meet the target, including both the products and practices used. In reviewing the case studies to see which insulation options are employed, you will find that homes use fiber glass (both batts and blown), spray foam, cellulose, insulated panels or a combination of multiple insulation types. The Adaptation ZERH in Geneva, IL is an example where a combination of insulation types was used; blown fiber glass insulation is in the walls and cellulose is found in the attic. With this insulation package, the 4,798 sqft home earned a HERS score of 30 (without solar panels) and averages an $84 monthly energy bill. Sounds pretty good, right?
There are also examples of Zero Energy Ready Homes insulated with just fiber glass. The Hale Plan home in Denver, CO is 3,560 sqft, has a HERS score of 41 (without solar panels) and has a projected annual utility cost of $1,146. This home is insulated with R-36 blown-in fiber glass in the walls, R-50 blown-in fiberglass on the attic floor, and R-19 fiber glass batt in the basement walls.
The Whole House Approach
Across the 77 homes currently in their database there are many variations of insulation and air sealing employed. So, is there a best type of insulation to use in a NZE home or a ZERH? The dataset indicates that multiple insulation types can be used to build these ZERH houses. The only constant is the need to take a whole-house systems approach. This approach is essential to building a NZE home or a ZERH home, because it considers a home to be made up of independent systems that affect one another’s performance and prioritizes the maximization of energy efficiency. So, if you hear someone saying how you have to use a specific insulation product to reach high performance targets, whether that target is NZE, ZERH or just a low HERS score, you ought to consider those claims and evaluate the evidence available to support them.
Want to see more case studies of high performance homes? Check out our Case Studies page.
[1] http://energy.gov/eere/buildings/articles/doe-releases-common-definition-zero-energy-buildings-campuses-and
[2] http://energy.gov/eere/buildings/zero-energy-ready-home