LEED 2009 and Energy Modeling Changes February 24, 2009

One of the big changes in LEED 2009 is the scoring system, specifically in how it relates to energy modeling. Energy and Atmosphere Prerequisite 2 (minimum energy performance) and Credit 1 (optimize energy performance) have changed significantly.  First, the threshold for the prerequisite has changed from 14% to 10%.  Then the points awarded in the credit have changed from a 1-10 point scale, to a 1-19 point odd numbered scale, awarding basically double the points for same percentage improvement over the baseline building.  But the biggest change is in the fine print, as the baseline itself has changed.  Where the current LEED baseline is Appendix G of ASHRAE 90.1-2007, the new LEED 2009 is based on the 2007 update of ASHRAE 90.1.  So the question becomes what are the major differences in Appendix G for 2007?  In studying the documents side by side, first let’s note what has not changed: 

• Lighting power densities are IDENTICAL in both the space by space, and the building area method.  This is very surprising, given the abundance of T5 lighting in the market.  Lighting remains arguably the best way to pick up efficiency against the baseline, when HVAC system types are similar to baseline systems.
   
• While there are some minor changes, efficiencies of HVAC equipment are very similar between the documents.  This is however not a surprise, as no major upgrades in unit efficiencies have happened in the industry lately.  Most of the HVAC manufacturers are focusing their attention on the R-22 phase out, and are trying to maintain current efficiency levels using more environmentally friendly refrigerants.

But some glaring differences are also noted: 

• Building construction values have gotten stricter.  For example in Charlotte (climate zone 3A), minimum compliance for roof insulation has increased from R-15 to R-20 and wall insulation has increased from R-13 to R-16.8.  Glass compliance has remained basically the same, but different U-values have been introduced based on the type of glass (nonmetal framing, curtain wall / storefront, entrance door, and all others), but the former assembly values of U-0.57 and SHGC-0.25 remain consistent.  So using efficient glass remains a good way to earn percentage points against the baseline.  The importance of this change will be magnified in building types where skin loads are a high percentage of peak HVAC load (i.e. office buildings), but less important in spaces where people and ventilation loads dominate the sizing of the HVAC equipment (i.e. assembly areas and schools).
   
• The calculation for supply fan power in the baseline calculations has changed slightly, to account for commonly used airflow restrictions used in LEED systems.  It now more accurately models baseline systems when proposed systems include components like return/exhaust control devices (i.e. CO2 sensors for demand control ventilation), or filtration media with high MERV ratings.  Using filters that complied with the requirements of EQc5 (indoor chemical and pollutant source control) used to hurt energy modeling performance, but this change will make that credit more attractive, though many common HVAC systems cannot overcome MERV 13 filtration.  As an example, a 10,000 cfm supply fan in a VAV system would have been modeled as 10.56 kW in the old system, now will be modeled as a 13 kW fan.  Fan energy will remain as another strong means to pick up efficiency against the baseline systems, especially when VAV systems are installed in single zone baseline applications.
   
• Lastly, and potentially most significantly, ASHRAE has made a change to the baseline HVAC system types themselves.  In the 2004 version, the cutoff in baseline criteria between commercial systems 3-4 (single zone rooftop equipment) and 5-6 (VAV rooftop equipment) was 3 floors or less, and < 75,000 square feet.  The new standard drastically reduces that square footage limit to 25,000 sf.  So where a 60,000 square foot office building that used VAV HVAC could pick up 10-15% in fan energy savings alone using the old system, it will now be modeled against a similar baseline HVAC system.  This is likely one of the major reasons the prerequisite percentage has been reduced from 14% to 10%.

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