The following is a compilation of building energy-code related terms and acronyms used on the Building Energy Codes website and throughout the building construction industry.

Select a letter to navigate through the glossary:


A (23) B (23) C (41) D (27) E (28) F (15) G (12) H (21) I (20) K (5) L (11) M (16) N (15) O (11) P (21) R (22) S (37) T (14) U (12) V (11) W (10)
Occupancy Type

The type of activity occurring within a building.

Occupant Sensing Device

A device that detects the presence or absence of people within an area and causes any combination of lighting, equipment, or appliances to be adjusted accordingly

Occupant Sensor

A device that detects the presence or absence of people within an area and causes lighting, equipment, or appliances to be regulated accordingly.


All areas in the building envelope, except fenestration and building service openings such as vents and grilles.

Opaque Areas

Opaque areas include all areas of the building envelope except openings for windows, skylights, doors, and building service systems. For example, although solid wood and metal doors are opaque, they should not be included as part of the opaque wall area (also referred to as the net wall area).

Operational Performance Requirements

A written document that details the functional requirements of a project and the expectations of how it will be used and operated. This includes project and design goals, measurable performance criteria, budgets, schedules, success criteria, and supporting information.

Optimum Start Controls

Controls that are designed to automatically adjust the start time of an HVAC system each day with the intention of bringing the space to desired occupied temperature levels immediately before scheduled occupancy.


The direction an envelope element faces, i.e., the direction of a vector perpendicular to and pointing away from the surface outside of the element.

Outdoor Air

Air taken from the outdoors and, therefore, not previously circulated through the system.


Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

Overhead Glazing Area

The area whose horizontal dimension, in each direction, is equal to the overhead glazing dimension plus either the floor to ceiling height or the dimension to the nearest 66" or higher opaque partition, or one-half the distance to the adjacent overhead or vertical glazing.