Posted in Architects
Roof Designs and the Home Energy Rating System (HERS)
EW: What is the Home Energy Rating System (HERS)?
AJ: The HERS Index has become an industry standard in measuring a home’s relative efficiency by looking at the home as a system. Any product or assembly that affects the flow of heat or energy usage is included in the analysis. Three of the most basic concepts used to analyze a home are conditioned space, thermal boundary and air barrier.
EW: Will you explain the differences between conditioned space, thermal boundary and air barrier?
AJ: Thermal boundary and air barrier are found on the same wall, roof, or floor that separates conditioned space from unconditioned space. Most people think of conditioned space as the area of a home that you live in every day. However, specific products and methods within a home design can increase conditioned space in other areas of the home.
EW: How does the HERS system rate radiant barrier roof sheathing?
AJ: The HERS system only has a check box for radiant barrier; it’s a simple yes or no. On a typical plan it can be a one point reduction, sometimes it’s a zero. On one of the plans in this study it was a two. Since the radiant barrier effect cannot be expressed as an R-value, and the rating system has yet to develop a specific radiant barrier scoring value, it’s hard to quantify. However, the cost of a radiant barrier roof sheathing is minimal in new construction because there’s no added labor. This makes it one of the most cost-effective ways to pursue an extra point on the HERS scale. We hope the HERS system will give more importance to radiant barrier sheathing in the future.
EW: How can foam encapsulation affect HERS scores and unconditioned space?
AJ: The HERS system only recognizes an insulation product as a certain R-Value. For instance, a foam that has R-3.6 per inch, and is sprayed 10 inches thick against the attic side of the roof deck would yield a R-36 insulation value. The real impact on HERS scores is not the R-Value of the insulation, as a value of R-36 is easily accomplished with traditional insulation, but rather the change of the HVAC air system and ductwork to conditioned space.
Locating the spray foam on the attic side of the roof deck moves the thermal and air barriers up, adding the attic’s volume to the total conditioned space. Therefore, the positive benefits of moving the HVAC system and ductwork into the conditioned space is partially offset by having a larger volume to condition.
Many contractors believe that the spray foam does a superior job of filling holes that would allow air infiltration. While I would agree that achieving the desired tightness of a home may be easier with spray foam, it is certainly not the only way. Builders also use code-required weather-resistant barriers or house wraps and use foam to seal penetrations with excellent results on blower door tests.
EW: Tell us about recent testing LP Building Products has done to quantify that information.
AJ: Not only did LP want to quantify that information, but we wanted to understand how radiant barriers performed against alternative designs when using the HERS index.
Cheldan Homes based in Fort Worth, Texas, loaned us three plans to analyze, a single story, a story and a half (single story plan with attic bonus room), and a two story plan. We worked directly with Fox Energy, a Texas-based energy efficiency services company that uses the HERS Index to evaluate energy-efficient building practices. We asked Fox Energy to calculate each of the three home plans with the following design aspects:
- Standard roof sheathing with blown cellulose insulation
- Radiant barrier roof sheathing with blown cellulose insulation
- Spray foam encapsulation of the entire attic space
- An alternative design that relocates the HVAC equipment and ductwork into a conditioned space while continuing to utilize radiant barrier roof sheathing
EW: What did you find?
AJ: The HERS analysis showed that homes using a radiant barrier compared to homes using foam encapsulation of the attic space had similar results. However, for each of the plans (1, 1-1/2, and 2-story), the design options that relocated the HVAC equipment and ductwork into a conditioned space without encapsulating the entire attic increased performance even further. Such a design allows for the added benefits of radiant barrier roof sheathing. Check with your designer to find the best way to utilize alternative designs when looking to achieve lower HERS scores.
Established by the Residential Energy Services Network (RESNET), the HERS Index has become the industry standard by which a home’s relative energy efficiency is measured. It’s also the nationally recognized system for inspecting and calculating a home’s energy performance. With this index, the lower the HERS score, the more efficient the home. Visit www.resnet.us to learn more.
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