Sidebar: LEED: A Look at the Rating System That's Changing the Way America Builds
LEED Energy Credits and ASHRAE 90.1
Both Prerequisite 2 and the two to ten points available in Credit 1 are defined in relation to ASHRAE Standard 90.1–1999. While there are distinct advantages to referencing such a widely recognized concensus-based standard, doing so also introduces some tricky issues, which the Committee has yet to resolve. Some of these issues stem from the fact that, unlike LEED 1.0, LEED 2.0 references the 1999 version of 90.1, which was adopted just weeks before LEED was approved. The 1999 version is widely regarded as a better standard, but it includes some changes that make its use in LEED more difficult. Standard 90.1 includes two compliance paths: a user can meet the requirements on a component-by-component basis—meeting the requirement for thermal performance of the building envelope, HVAC systems, lighting, and other components individually—or by modeling the energy use of the entire building using the “Energy Cost Budget” method. The latter method basically defines a base-case building that matches the orientation and shape of the actual design, but just meets the requirements for each component. The actual design is then considered compliant if it matches this base case in annual energy use, regardless of how each component performs on its own. LEED Prerequisite 2, as written, requires the building to comply with 90.1 on a component-by-component basis (or meet the local energy code if that is more stringent). If interpreted literally, this means that designers will not be allowed to trade-off performance of one component against others to meet the prerequisite. LEED 1.0 referenced the 1989 version of the Standard for this requirement, and many jurisdictions base their energy code on that Standard, so acceptance by local officials was taken as sufficient for meeting the prerequisite, according to Paladino. With LEED 2.0 it may take a while for local energy codes to catch up, and designers may be required to document compliance with 90.1 for each component. Credit 1, on the other hand, requires the use of the Energy Cost Budget method. “You’re going to have to do a simulation to get a credit beyond the prerequisite,” says Paladino, adding: “There is really no way around it, particularly in a complex building, with energy moving from zone to zone.” This requirement has serious cost implications for buildings in which energy modeling was not already budgeted, as energy modeling of a large building typically costs on the order of $15,000. Paladino notes that it should be a goal of the program to encourage design teams to begin this sort of modeling early, when there is still time to use it to inform the design, rather than running the simulation after-the-fact as a measurement tool. Standard 90.1 is very specific about how the base-case building is to be set up in an energy model, and even more details are yet to be published in a forthcoming ASHRAE Supplement. For a number of reasons, it may not work for LEED to follow ASHRAE’s rules in this matter, however, which leaves a lot of unknowns for the LEED 2.0 Reference Guide to determine. One problem with following ASHRAE’s rules is simply that they make it almost impossible to achieve more than 4 of the 10 possible points in the LEED system. “Following rules of the Energy Cost Budget method it’s going to be very difficult to achieve savings greater than 30%, even in the most innovative designs,” says Charles Eley, FAIA PE, of Eley Associates, a leading expert on energy-efficient commercial buildings and author of the ASHRAE 90.1 Users Manual. LEED requires 40% savings to get six points, and 60% savings to get the full ten points in a new building. Eley notes that the 1999 component guidelines amount to a base case that is 15% to 20% more efficient than the 1989 base case. There are also stipulations about how the base case is determined that reduce available savings. For example, in the 1999 version, the Standard 90.1 base case must match the design building in massing and orientation, which means that designers don’t get any credit for shaping or orienting a building to save energy. Finally, Standard 90.1 does not recognize natural ventilation as a design strategy that might expand the comfort zone of occupants, so it requires that every building be modeled for energy use as though it were mechanically air conditioned. In addition to all these issues, there is a lot of energy used in buildings for applications that Standard 90.1 doesn’t address, such as plug loads for office equipment and appliances, elevators, and outdoor lighting. These can amount to as much as half the energy use in a commercial office building, according to Eley. The outdoor lighting use is addressed to some extent by LEED in the Light Pollution credit, and the plug loads are a more fitting topic for the LEED Operations and Maintenance Rating System. Elevator energy use appears to have fallen through the cracks, however.
