ASHRAE/IESNA 90.1 and the IECC are divided into prescriptive and performance requirements. Prescriptive requirements are provided in both codes for the building envelope; heating, ventilating and air conditioning (HVAC) system; hot water and steam production; and electric power and lighting. These prescriptive requirements are given in sections 5 through 9 in ASHRAE/IESNA 90.1 and sections 802 through 805 in the IECC.

The prescriptive requirements in these codes provide the minimum performance requirements for equipment and materials used in these systems as well as system- operating requirements. Each building system is addressed independently in the prescriptive requirements and the focus is on energy use and not on energy production. Therefore, PV is not addressed in the prescriptive performance requirements of either ASHRAE/IESNA 90.1 or the IECC.

The performance requirements of both ASHRAE/IESNA 90.1 and the IECC involve the comparison of the proposed building design to a base building design that meets the prescriptive requirements of the respective code used. In ASHRAE/IESNA 90.1, the performance requirements are outlined in Section 11 and referred to as the energy cost budget (ECB) method.

Section 806 in the IECC addresses the total building performance requirements. ASHRAE/IESNA 90.1 and the IECC require that the annual energy cost of the proposed building design be shown to be less than or equal to that predicted for an equivalent base building meeting the prescriptive code requirements. The predicted annual energy cost for both the proposed and base building designs are based on a computer simulation that uses local climate data and energy rates.

The prescriptive method requires only that the materials, equipment and systems incorporated into the building meet code minimums. The prescriptive method does not recognize the building as a collection of systems that are interdependent and together determine overall building-energy use.

For example, the amount of artificial lighting in the building has an impact on the building heat load and the size of the HVAC system. Reducing artificial lighting by increasing the amount of incoming daylight will also increase solar heating that will in turn impact the size and operation of the HVAC system. The performance method in both codes recognizes these interrelationships and allows trade-offs between building systems. One trade-off recognized by both ASHRAE/IESNA 90.1 and the IECC in their performance requirements is that the incorporation of PV into the proposed building design will reduce the building’s annual energy needs from outside sources and that PV-generated energy can be used to achieve code compliance.

Annual energy costs for the proposed building design are determined under both codes by multiplying the predicted energy use by purchased energy rates for each building energy source such as electricity and natural gas. The exception to paragraph 11.2.3 in ASHRAE/IESNA 90.1 permits the annual energy use of the proposed design to be reduced by the amount of energy that will be produced on site by PV.

Similarly, the IECC in Paragraph 806.2.4 states that energy produced on site by PV can be deducted directly from the annual energy cost of the proposed building design. The IECC takes it a step further than ASHRAE/IESNA 90.1 in that it also allows the predicted annual energy cost to be reduced by any energy received from off-site PV sources which would include agreements with utilities or independent power producers (IPP) that use PV to produce green power.

The advantage that PV provides in meeting energy codes is flexibility. Energy codes establish the minimum requirements for building efficiency and the criterion for either the ASHRAE/IESNA 90.1’s ECB method or the IECC’s total building performance method is the annual energy use of the proposed building.

Incorporating PV into the proposed building’s design allows the energy produced by the PV system to directly reduce the predicted annual energy use. As a result, the use of PV gives more leeway in the selection of building materials, equipment and systems for aesthetic, operational or economic reasons while the building will still meet energy code performance requirements. EC

GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or [email protected].