Definition Categories

Above-grade wall

An above-grade wall is a wall that is more than (>) 50 percent above grade (not covered by earth on the outside) and encloses conditioned space. This includes the area between floors, attic and basement knee walls, and skylight shafts.


Something is accessible if access requires removal of an access panel or similar obstruction removal of which does not damage permanent construction. Something is readily accessible if access does not require removal of any obstruction.

Accessible (Readily)

Electrical components are readily accessible if they can be reached quickly without moving or climbing over obstructions (such as pictures and work benches) and without using a portable ladder. Locks may be used to secure readily accessible components; however, the key or combination should be readily accessible to all occupants at all times. Panelboards and service disconnect equipment must be readily accessible in case circuits need to be shut off during an emergency.

Accessible (Wires)

Wires are accessible if they can be exposed without removing or damaging permanent parts of the building and if a person can reach them for inspection, repair, or maintenance. Examples: wires are accessible if they are behind suspended ceiling panels, or if access requires opening a door, removing an access panel, or climbing a ladder. Wires are not accessible if you must cut drywall to expose the wires or if they are located in an area that cannot be reached for repair, inspection, or maintenance.

Accessory structure

An accessory structure is another building on the same lot, the use of which is incidental to the main building. Examples of accessory structures are detached garages, storage sheds, and guest houses. An accessory structure must be not more than (≤) 3,000 square feet in floor area and not more than (≤) 2 stories tall.

Air admittance valve

An air admittance valve is a one-way valve attached to a plumbing vent pipe. It is used when extending a vent to the roof or to another vent is impractical or not desirable. The valve allows air into the vent system when there is negative pressure in the vent pipe and closes to limit the flow of sewer gas into the home.

Air barrier

An air barrier is any material or combination of materials that prevent the flow of air from unconditioned areas into the thermal envelope.

Air intake opening (gravity)

A gravity air intake opening is any opening that allows air to flow into the home through natural means. Operable windows and doors are the most common examples. Less common examples include combustion air openings, makeup and ventilation air openings, and soft ventilation openings.

Air intake opening (mechanical)

A mechanical air intake opening is one that draws air into the home using a fan, blower, or other powered means. One example is an outside air intake duct connected to the return boot of a forced-air HVAC system. These ducts are being installed in some new homes to mix outdoor air with the return air to increase air changes per hour in the home and to improve indoor air quality. Another example is a heat recovery or an energy recovery ventilation system.

Air-entrained concrete

Air-entrained concrete is concrete that contains air bubbles. It is used in severe weathering environments to reduce wear caused by freeze and thaw cycles and wear caused by chemical deicers. It also improves the concrete’s workability during pouring and finishing.


Ampacity describes the current-carrying capacity of a wire, measured in amperes. The ampacity of a wire depends on how it is used. A wire’s ampacity is higher when used as a service or feeder wire than when used as a branch circuit wire. Refer to Chapter 36 for wire service and feeder ampacities and to Chapter 37 for wire branch circuit ampacities.


An ampere is a measure of the rate of flow of electricity. One volt acting on one ohm of resistance equals one ampere of current flow. An ampere is similar to the gallon per minute rate of flow in a water pipe. Ampere is abbreviated amp and is expressed in Ohm’s Law as the letter I.


A manufactured device that uses energy and that is regulated by the IRC. Examples of appliances include furnaces and air conditioning condensers. Plumbing appliances include water heaters, dishwashers, and clothes washing machines. See also: equipment.


An electrical appliance is a device that uses electricity. Examples of appliances include ranges, ovens, furnaces, air handlers, air conditioner condensers, water heaters, clothes dryers, and plug-and-cord connected window and through-the-wall air conditioners. Ceiling fans are appliances, not light fixtures.

Aspect ratio

The ratio of the longest to the shortest perpendicular dimensions. The aspect ratio of old televisions is 4:3. The aspect ratio of wide screen televisions is 16:9.

Attic With Limited Storage

An attic with limited storage, built with joists and rafters, has at least (≥) 42 inches between the top of the ceiling joists and the bottom of the rafters. An attic without storage, built with trusses, has at least (≥) 3 adjacent trusses with the same web configuration that could contain a cube more than (>) 42 inches wide and 24 inches tall located in the same plane (area) of the truss.


Attic, habitable

A habitable attic is a habitable space located directly under the roof. Habitable attics have no exterior walls. Interior walls, if any, are knee walls between the rafters above and the floor below. An example of a habitable attic is an attic area that is accessible by permanent stairs. A habitable attic is not considered a story so it does not add to the number of stories above grade and it does not place additional load requirements on the supporting framing. A habitable attic is designed as a sleeping area for determining floor joist spans. A habitable attic must have the minimum floor space (70 square feet) and minimum ceiling height as required in Sections R304 and R305.

Attics Without Storage

An attic without storage, built with joists and rafters, has less than (<) 42 inches between the top of the ceiling joists and the bottom of the rafters. An attic without storage, built with trusses, has not more than (≤) 2 adjacent trusses with the same web configuration that could contain a cube not more than (≤) 42 inches wide and 24 inches tall located in the same plane (area) of the truss.

Balloon framing

Balloon framing is a building system where walls extend from the foundation to the ceiling joists. Intermediate floor joists are attached to the walls and are often supported by a ledger. Balloon framing is rare in modern residential construction.



A baluster (also called a picket) is a vertical piece found in guard rails and handrails that supports the rail and provides protection against occupants falling through or being caught between the balusters.