ASHRAE 90.1 – 99 (Energy Standard for Buildings Except Low-Rise Residential Buildings)

Regulatory requirements:

All fenestration performance criteria (U-factor, Solar Heat Gain Coefficient, Emissivity, Visible Transmittance and Air Leakage) must be determined in accordance with NFRC (National Fenestration Rating Council) standards as follows:

• NFRC 100   Procedure for Determining Fenestration Product U-factors
• NFRC 200   Procedure for Determining Fenestration Product Solar Heat Gain Coefficient at Normal Incidence
• NFRC 300   Procedure for Determining Solar Optical Properties of Simple Fenestration Products
• NFRC 400   Procedure for Determining Fenestration Product Air Leakage   

In addition, ASHRAE 90.1-99 requires that all performance criteria “shall be determined by a laboratory accredited by a nationally recognized accredited organization, such as the National Fenestration Rating Council and shall be labeled and certified by the manufacturer.”

Prescriptive Requirements

The Prescriptive Building Envelope Option recognizes the advances made in fenestration performance over the last ten years by allowing vertical fenestration area up to 50% of the gross wall area. If a building has greater than 50% glazing area (or greater than 5% fenestration area in the roof) another compliance path must be used (i.e., the Trade-Off Option, Energy Cost Budget Method).

The prescriptive option has maximum requirements for U-factor and Solar Heat Gain Coefficient. These requirements are based on local climate (heating and cooling degree days) and building orientation. Fenestration U-factors requirements range from 0.29 to 1.27, while SHGC requirements range from 0.14 to no requirement. In addition, the standard provides a credit for overhangs that provide shading.

Air leakage shall not exceed 1.0 cfm/ft 2 (cubic feet per minute per square foot of door area) for glazed winging entrance doors and 0.4 cfm/ft 2 for all other products (note that field glazed products are exempt from these air leakage requirements).

The Building Envelope Trade-Off Option

ASHRAE 90.1-99 provides a trade-off option to assist in compliance. This option allows the performance of one envelope component to be improved to make-up for another component that does not meet minimum requirement (note that this option cannot be used to trade-off between the building envelope and the lighting or mechanical system). When using the trade-off option, the visible transmittance of the fenestration systems must also be specified. This is used for determining the potential for daylighting.

Software is available of for calculating these trade-offs. The “ENVSTD” program meets all the Building Envelope Trade-Off Option requirements. ASHRAE publishes a user's manual and the ENVSTD software.

System or Design Requirements

The system or design requirements are in ASHRAE 90.1 and outlined in the Energy Cost Budget Method. It is used to evaluate the compliance of proposed building designs and is intended for use with simulation programs that model building efficiency (ENVSTD). The models must report energy usage by lights, internal equipment loads, and service water heating equipment, space cooling and heat rejection equipment, fans and other HVAC equipment. Using this approach, architects, builders, and designers can increase the use of glass in buildings by specifying high performance glass products (i.e., spectrally selective tinted and coated glass) to control solar heat gain and take advantage of lighting energy savings from daylighting.

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