Commissioned by the city of Cincinnati, the first-ever comparison of the two national green building rating systems has found similarities as well as a few major differences.
The detailed review of the programs--USGBC’s LEED for Homes and the ANSI-National Green Building Standard (NGBS)--was completed by AIA Cincinnati for the city’s Office of Environmental Quality to help determine if the NGBS should be adopted as part of the city’s tax abatement program for green homes.
Since January 2008, new LEED Certified, Silver, and Gold houses built in the city limits receive 15 years of city and county tax relief for dwellings valued up to $515,000; LEED Platinum projects have no maximum-value limit. A 10-year tax abatement period also is offered for residential remodeling projects as small as $2,500 that meet LEED certification standards.
The incentive has spurred green residential building in Cincinnati: Since its launch, more than 28 LEED-certified houses have been built and more are in the midst of the design or construction process, the report states.
After the local NAHB chapter asked the city to consider including NGBS in the tax abatement program last year, AIA Cincinnati was tasked with reviewing both rating systems to determine whether they are equal in performance and rigor of proof.
The committee’s research took about four months and included interviews with local raters and verifiers for both programs as well as discussions with USGBC and NAHB officials. Chairperson Andy Corn of RWA Architects says he believes the report is the first in-depth comparison of LEED and the one-year-old NGBS, although past studies looked at LEED and the NAHB’s Model Green Home Building Guidelines, which are being phased out this year.
The four-member committee found the two programs are similar in their breakdown of divisions such as water, site, and energy. A side-by-side comparison of credits showed further similarities.
“We felt when we were done that the credits addressed through the NGBS were ample enough and similar enough to say that the two programs have very similar intents,” says Corn.
Differences arose when they looked at the two systems’ mandatory minimum performance level and mandatory site testing requirements. While LEED mandates a minimum site-tested performance that is equivalent to Energy Star certification--either by a prescriptive or a performance path--the NGBS does not require a site-tested minimum performance.
For example, the LEED Prescriptive Energy Path requires meeting minimum levels for an envelope leakage test, an HVAC ductwork pressure test, window values, and lighting. LEED also mandates erosion controls, a minimum indoor air filter rating, fireplace doors, closed combustion or power vented exhaust, and prohibits locating HVAC equipment in the garage. NGBS makes minimum performance levels for these measures optional.
“It can be said that LEED is too restrictive in this requirement and NGBS is more flexible
since NGBS has fewer mandatory measures,” the report says. “While the LEED mandatory measures do limit the flexibility of choice, in the opinion of the committee they ensure a minimum and effective level of performance.”
In addition, the committee put a high priority on post-construction testing. While there are many credits in both rating systems that include optional site testing and verification, only LEED mandates that high-priority practices must be site tested after construction, the report says. These include a HERS score, envelope leakage test, and HVAC ductwork pressure test that meet Energy Star levels of performance.