IECC International Conservation Code 2021 PDF

**Chapter 1 Scope and Admnistration** **Part 1 - Scope And application** **C101 Scope and General Requirements** **C102 Alternate Materials -- Method of Construction, Design or insulating Systems** **Part 2 Administration and enforcement** **C103 Construction Documents** **C104 Inpections** *...

**Chapter 1 Scope and Admnistration** **Part 1 - Scope And application** **C101 Scope and General Requirements** **C102 Alternate Materials -- Method of Construction, Design or insulating Systems** **Part 2 Administration and enforcement** **C103 Construction Documents** **C104 Inpections** **C105 Validity** **C106 Referenced Standards** **C107 Fees** **C108 Stop Work Orders** **C109 Board of Appeals** **Chapter 2 Defintions** **C201 General** **C202 General Defintions** **Chapter 3 General Requirements** **C301 Climate Zones** **C302 Design Conditions** **C303 Materials, Systems and Equipment** **Chapter 4 Commercial Energy Efficiency** **C401 General** **C402 Building Envelope Requirements** **C403 Building Mechanical Systems** **C404 Service Water Heating** **C405 Electrical Power and Lighting Systems** **C406 Additional Efficiency Package Options** **C407 Total Building Performance** **C408 System Comissioning** **Chapter 5 Existing Building** **C501 General** **C502 Additions** **C503 Alterations** **C504 Repairs** **C505 Change of occupancy or Use** **Chapter 6 Referenced Standards** **ACCESS (TO).** **That which enables a device, appliance or equipment to be reached by *ready access* or by a means that first requires the removal or movement of a panel or similar obstruction.** **ADDITION.** **An extension or increase in the *conditioned space* floor area, number of stories or height of a building or structure.** **AIR BARRIER.** **One or more materials joined together in a continuous manner to restrict or prevent the passage of air through the *building thermal envelope* and its assemblies.** **AIR CURTAIN.** **A device, installed at the *building entrance*, that generates and discharges a laminar air stream intended to prevent the infiltration of external, unconditioned air into the conditioned spaces, or the loss of interior, conditioned air to the outside.** **ALTERATION.** **Any construction, retrofit or renovation to an existing structure other than repair or addition. Also, a change in a building, electrical, gas, mechanical or plumbing system that involves an extension, addition or change to the arrangement, type or purpose of the original installation.** **APPROVED.** **Acceptable to the code official.** **APPROVED AGENCY.** **An established and recognized agency that is regularly engaged in conducting tests or furnishing inspection services, or furnishing product certification, where such agency has been approved by the *code official*.** **AUTOMATIC.** **Self-acting, operating by its own mechanism when actuated by some impersonal influence, as, for example, a change in current strength, pressure, temperature or mechanical configuration (see "[*[Manual]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefMANUAL/2218)").** **BELOW-GRADE WALL.** **See "[*[Wall, below-grade]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefWALL_BELOW_GRADE/2218)."** **BIOGAS.** **A mixture of hydrocarbons that is a gas at 60°F (15.5°C) and 1 atmosphere of pressure that is produced through the anaerobic digestion of organic matter.** **BIOMASS.** **Nonfossilized and biodegradable organic material originating from plants, animals and/or microorganisms, including products, by-products, residues and waste from agriculture, forestry and related industries as well as the nonfossilized and biodegradable organic fractions of industrial and municipal wastes, including gases and liquids recovered from the decomposition of nonfossilized and biodegradable organic material.** **BOILER, MODULATING.** **A boiler that is capable of more than a single firing rate in response to a varying temperature or heating load.** **BOILER SYSTEM.** **One or more boilers, their piping and controls that work together to supply steam or hot water to heat output devices remote from the boiler.** **BUBBLE POINT.** **The refrigerant liquid saturation temperature at a specified pressure.** **BUILDING.** **Any structure used or intended for supporting or sheltering any use or occupancy, including any mechanical systems, service water-heating systems and electric power and lighting systems located on the building site and supporting the building.** **BUILDING COMMISSIONING.** **A process that verifies and documents that the selected building systems have been designed, installed and function according to the owner's project requirements and construction documents, and to minimum code requirements.** **BUILDING ENTRANCE.** **Any door, set of doors, doorway or other form of portal that is used to gain access to the building from the outside by the public.** **BUILDING SITE.** **A contiguous area of land that is under the ownership or control of one entity.** **BUILDING THERMAL ENVELOPE.** **The basement walls, exterior walls, floors, ceilings, roofs and any other building element assemblies that enclose *conditioned space* or provide a boundary between *conditioned space* and exempt or unconditioned space.** **CAPTIVE KEY OVERRIDE.** **A lighting control that will not release the key that activates the override when the lighting is on.** **CAVITY INSULATION.** **Insulating material located between framing members.** ***C*-FACTOR (THERMAL CONDUCTANCE).** **The coefficient of heat transmission (surface to surface) through a building component or assembly, equal to the time rate of heat flow per unit area and the unit temperature difference between the warm side and cold side surfaces (Btu/h × ft2 × °F) \[W/(m2 × K)\].** **CHANGE OF OCCUPANCY.** **A change in the use of a building or a portion of a building that results in any of the following:** - **CIRCULATING HOT WATER SYSTEM.** **A specifically designed water distribution system where one or more pumps are operated in the service hot water piping to circulate heated water from the water-heating equipment to the fixture supply and back to the water-heating equipment.** **CLIMATE ZONE.** **A geographical region based on climatic criteria as specified in this code.** **CODE OFFICIAL.** **The officer or other designated authority charged with the administration and enforcement of this code, or a duly authorized representative.** **COEFFICIENT OF PERFORMANCE (COP) − COOLING.** **The ratio of the rate of heat removal to the rate of energy input, in consistent units, for a complete refrigerating system or some specific portion of that system under designated operating conditions.** **COEFFICIENT OF PERFORMANCE (COP) − HEATING.** **The ratio of the rate of heat delivered to the rate of energy input, in consistent units, for a complete heat pump system, including the compressor and, if applicable, auxiliary heat, under designated operating conditions.** **COMMERCIAL BUILDING.** **For this code, all buildings that are not included in the definition of "[*[Residential building]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefRESIDENTIAL_BUILDING/2218)."** **COMPUTER ROOM.** **A room whose primary function is to house equipment for the processing and storage of electronic data which has a design total information technology equipment (ITE) equipment power density less than or equal to 20 watts per square foot (20 watts per 0.092 m2) of conditioned area or a design total ITE equipment load less than or equal to 10 kW.** **CONDENSING UNIT.** **A factory-made assembly of refrigeration components designed to compress and liquefy a specific refrigerant. The unit consists of one or more refrigerant compressors, refrigerant condensers (air-cooled, evaporatively cooled or water-cooled), condenser fans and motors (where used) and factory-supplied accessories.** **CONDITIONED FLOOR AREA.** **The horizontal projection of the floors associated with the *conditioned space*.** **CONDITIONED SPACE.** **An area, room or space that is enclosed within the *building thermal envelope* and is directly or indirectly heated or cooled. Spaces are indirectly heated or cooled where they communicate through openings with conditioned spaces, where they are separated from conditioned spaces by uninsulated walls, floors or ceilings, or where they contain uninsulated ducts, piping or other sources of heating or cooling.** **CONTINUOUS INSULATION (ci).** **Insulating material that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.** **CRAWL SPACE WALL.** **The opaque portion of a wall that encloses a crawl space and is partially or totally below grade.** **CURTAIN WALL.** **Fenestration products used to create an external nonload-bearing wall that is designed to separate the exterior and interior environments.** **DATA CENTER.** **A room or series of rooms that share data center systems, whose primary function is to house equipment for the processing and storage of electronic data and that has a design total ITE equipment power density exceeding 20 watts per square foot (20 watts per 0.092 m2) of conditioned area and a total design ITE equipment load greater than 10 kW.** **DATA CENTER SYSTEMS.** **HVAC systems and equipment, or portions thereof, used to provide cooling or ventilation in a data center.** **DAYLIGHT RESPONSIVE CONTROL.** **A device or system that provides automatic control of electric light levels based on the amount of daylight in a space.** **DAYLIGHT ZONE.** **That portion of a building's interior floor area that is illuminated by natural light.** **DEMAND CONTROL VENTILATION (DCV).** **A ventilation system capability that provides for the automatic reduction of outdoor air intake below design rates when the actual occupancy of spaces served by the system is less than design occupancy.** **DEMAND RECIRCULATION WATER SYSTEM.** **A water distribution system where one or more pumps prime the service hot water piping with heated water upon a demand for hot water.** **DIRECT DIGITAL CONTROL (DDC).** **A type of control where controlled and monitored analog or binary data, such as temperature and contact closures, are converted to digital format for manipulation and calculations by a digital computer or microprocessor, then converted back to analog or binary form to control physical devices.** **DUCT.** **A tube or conduit utilized for conveying air. The air passages of self-contained systems are not to be construed as air ducts.** **DUCT SYSTEM.** **A continuous passageway for the transmission of air that, in addition to ducts, includes duct fittings, dampers, plenums, fans and accessory air-handling equipment and appliances.** **DWELLING UNIT.** **A single unit providing complete independent living facilities for one or more persons, including permanent provisions for living, sleeping, eating, cooking and sanitation.** **DYNAMIC GLAZING.** **Any fenestration product that has the fully reversible ability to change its performance properties, including *U*-factor, solar heat gain coefficient (SHGC) or visible transmittance (VT).** **ECONOMIZER, AIR.** **A duct and damper arrangement and automatic control system that allows a cooling system to supply outside air to reduce or eliminate the need for mechanical cooling during mild or cold weather.** **ECONOMIZER, WATER.** **A system where the supply air of a cooling system is cooled indirectly with water that is itself cooled by heat or mass transfer to the environment without the use of mechanical cooling.** **ENCLOSED SPACE.** **A volume surrounded by solid surfaces such as walls, floors, roofs and openable devices, such as doors and operable windows.** **ENERGY ANALYSIS.** **A method for estimating the annual energy use of the *proposed design* and *standard reference design* based on estimates of energy use.** **ENERGY COST.** **The total estimated annual cost for purchased energy for the building functions regulated by this code, including applicable demand charges.** **ENERGY RECOVERY VENTILATION SYSTEM.** **Systems that employ air-to-air heat exchangers to recover energy from exhaust air for the purpose of preheating, precooling, humidifying or dehumidifying outdoor ventilation air prior to supplying the air to a space, either directly or as part of an HVAC system.** **ENERGY SIMULATION TOOL.** **An *approved* software program or calculation-based methodology that projects the annual energy use of a building.** **ENTHALPY RECOVERY RATIO.** **Change in the enthalpy of the *outdoor air* supply divided by the difference between the *outdoor air* and entering exhaust air enthalpy, expressed as a percentage.** **ENTRANCE DOOR.** **A vertical fenestration product used for occupant ingress, egress and access in nonresidential buildings, including, but not limited to, exterior entrances utilizing latching hardware and automatic closers and containing over 50 percent glazing specifically designed to withstand heavy-duty usage.** **EQUIPMENT ROOM.** **A space that contains either electrical equipment, mechanical equipment, machinery, water pumps or hydraulic pumps that are a function of the building's services.** **EXTERIOR WALL.** **Walls including both above-grade walls and basement walls.** **FAN, EMBEDDED.** **A fan that is part of a manufactured assembly where the assembly includes functions other than air movement.** **FAN ARRAY.** **Multiple fans in parallel between two plenum sections in an air distribution system.** **FAN BRAKE HORSEPOWER (BHP).** **The horsepower delivered to the fan's shaft. Brake horsepower does not include the mechanical drive losses, such as that from belts and gears.** **FAN ENERGY INDEX (FEI).** **The ratio of the electric input power of a reference fan to the electric input power of the actual fan as calculated in accordance with [[AMCA 208]](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch06_PromAMCA_RefStd208_18/2218).** **FAN NAMEPLATE ELECTRICAL INPUT POWER.** **The nominal electrical input power rating stamped on a fan assembly nameplate.** **FAN SYSTEM BHP.** **The sum of the fan brake horsepower of all fans that are required to operate at fan system design conditions to supply air from the heating or cooling source to the *conditioned spaces* and return it to the source or exhaust it to the outdoors.** **FAN SYSTEM DESIGN CONDITIONS.** **Operating conditions that can be expected to occur during normal system operation that result in the highest supply fan airflow rate to conditioned spaces served by the system, other than during air economizer operation.** **FAN SYSTEM ELECTRICAL INPUT POWER.** **The sum of the fan electrical power of all fans that are required to operate at fan system design conditions to supply air from the heating or cooling source to the conditioned spaces and/or return it to the source or exhaust it to the outdoors.** **FAN SYSTEM MOTOR NAMEPLATE HP.** **The sum of the motor nameplate horsepower of all fans that are required to operate at design conditions to supply air from the heating or cooling source to the *conditioned spaces* and return it to the source or exhaust it to the outdoors.** **FAULT DETECTION AND DIAGNOSTICS (FDD) SYSTEM.** **A software platform that utilizes building analytic algorithms to convert data provided by sensors and devices to automatically identify faults in building systems and provide a prioritized list of actionable resolutions to those faults based on cost or energy avoidance, comfort and maintenance impact.** **FENESTRATION.** **Products classified as either skylights or vertical fenestration.** **FENESTRATION PRODUCT, FIELD-FABRICATED.** **A fenestration product whose frame is made at the construction site of standard dimensional lumber or other materials that were not previously cut or otherwise formed with the specific intention of being used to fabricate a fenestration product or exterior door. Field-fabricated does not include site-built fenestration.** **FENESTRATION PRODUCT, SITE-BUILT.** **A fenestration designed to be made up of field-glazed or field-assembled units using specific factory cut or otherwise factory-formed framing and glazing units. Examples of site-built fenestration include storefront systems, curtain walls and atrium roof systems.** ***F*-FACTOR.** **The perimeter heat loss factor for slab-on-grade floors (Btu/h × ft × °F) \[W/(m × K)\].** **FLOOR AREA, NET.** **The actual occupied area not including unoccupied accessory areas such as corridors, stairways, toilet rooms, mechanical rooms and closets.** **GENERAL LIGHTING.** **Interior lighting that provides a substantially uniform level of illumination throughout a space.** **GREENHOUSE.** **A structure or a thermally isolated area of a building that maintains a specialized sunlit environment exclusively used for, and essential to, the cultivation, protection or maintenance of plants. *Greenhouses* are those that are erected for a period of 180 days or more.** **GROUP R.** **Buildings or portions of buildings that contain any of the following occupancies as established in the [*[International Building Code]*](https://codes.iccsafe.org/lookup/IBC2021P1/2218):** - **HEAT TRAP.** **An arrangement of piping and fittings, such as elbows, or a commercially available heat trap that prevents thermosyphoning of hot water during standby periods.** **HEATED SLAB.** **Slab-on-grade construction in which the heating elements, hydronic tubing or hot air distribution system is in contact with, or placed within or under, the slab.** **HIGH SPEED DOOR.** **A nonswinging door used primarily to facilitate vehicular access or material transportation, with a minimum opening rate of 32 inches (813 mm) per second, a minimum closing rate of 24 inches (610 mm) per second and that includes an automatic-closing device.** **HISTORIC BUILDING.** **Any building or structure that is one or more of the following:** - **IEC DESIGN H MOTOR.** **An electric motor that meets all of the following:** - **IEC DESIGN N MOTOR.** **An electric motor that meets all of the following:** - **INFILTRATION.** **The uncontrolled inward air leakage into a building caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density or both.** **INFORMATION TECHNOLOGY EQUIPMENT (ITE).** **Items including computers, data storage devices, servers and network and communication equipment.** **INTEGRATED PART LOAD VALUE (IPLV).** **A single-number figure of merit based on part-load EER, COP or kW/ton expressing part-load efficiency for air-conditioning and heat pump equipment on the basis of weighted operation at various load capacities for equipment.** **INTERNAL CURTAIN SYSTEM.** **A system consisting of movable panels of fabric or plastic film used to cover and uncover the space enclosed in a *greenhouse* on a daily basis.** **ISOLATION DEVICES.** **Devices that isolate HVAC zones so that they can be operated independently of one another. *Isolation devices* include separate systems, isolation dampers and controls providing shutoff at terminal boxes.** **LABELED.** **Equipment, materials or products to which have been affixed a label, seal, symbol or other identifying mark of a nationally recognized testing laboratory, *approved agency* or other organization concerned with product evaluation that maintains periodic inspection of the production of the labeled items and whose labeling indicates either that the equipment, material or product meets identified standards or has been tested and found suitable for a specified purpose.** **LARGE-DIAMETER CEILING FAN.** **A ceiling fan that is greater than 7 feet (2134 mm) in diameter. These fans are sometimes referred to as High-Volume, Low-Speed (HVLS) fans.** **LINER SYSTEM (Ls).** **A system that includes the following:** - **For multilayer installations, the last rated *R*-value of insulation is for unfaced insulation draped over purlins and then compressed when the metal roof panels are attached.** **LISTED.** **Equipment, materials, products or services included in a list published by an organization acceptable to the *code official* and concerned with evaluation of products or services that maintains periodic inspection of production of *listed* equipment or materials or periodic evaluation of services and whose listing states either that the equipment, material, product or service meets identified standards or has been tested and found suitable for a specified purpose.** **LOW-SLOPED ROOF.** **A roof having a slope less than 2 units vertical in 12 units horizontal.** **LOW-VOLTAGE DRY-TYPE DISTRIBUTION TRANSFORMER.** **A transformer that is air-cooled, does not use oil as a coolant, has an input voltage less than or equal to 600 volts and is rated for operation at a frequency of 60 hertz.** **LUMINAIRE-LEVEL LIGHTING CONTROLS.** **A lighting system consisting of one or more luminaires with embedded lighting control logic, occupancy and ambient light sensors, wireless networking capabilities and local override switching capability, where required.** **MANUAL.** **Capable of being operated by personal intervention (see "[*[Automatic]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefAUTOMATIC/2218)").** **NAMEPLATE HORSEPOWER.** **The nominal motor output power rating stamped on the motor nameplate.** **NEMA DESIGN A MOTOR.** **A squirrel-cage motor that meets all of the following:** - **NEMA DESIGN B MOTOR.** **A squirrel-cage motor that meets all of the following:** - **NEMA DESIGN C MOTOR.** **A squirrel-cage motor that meets all of the following:** - **NETWORKED GUESTROOM CONTROL SYSTEM.** **A control system, with access from the front desk or other central location associated with a *Group R*-1 building, that is capable of identifying the rented and unrented status of each guestroom according to a timed schedule, and is capable of controlling HVAC in each hotel and motel guestroom separately.** **NONSTANDARD PART LOAD VALUE (NPLV).** **A single-number part-load efficiency figure of merit calculated and referenced to conditions other than IPLV conditions, for units that are not designed to operate at AHRI standard rating conditions.** **OCCUPANT SENSOR CONTROL.** **An automatic control device or system that detects the presence or absence of people within an area and causes lighting, equipment or appliances to be regulated accordingly.** **ON-SITE RENEWABLE ENERGY.** **Energy from renewable energy resources harvested at the building project site.** **OPAQUE DOOR.** **A door that is not less than 50-percent opaque in surface area.** **POWERED ROOF/WALL VENTILATORS.** **A fan consisting of a centrifugal or axial impeller with an integral driver in a weather-resistant housing and with a base designed to fit, usually by means of a curb, over a wall or roof opening.** **PROPOSED DESIGN.** **A description of the proposed building used to estimate annual energy use for determining compliance based on total building performance.** **RADIANT HEATING SYSTEM.** **A heating system that transfers heat to objects and surfaces within a conditioned space, primarily by infrared radiation.** **READY ACCESS (TO).** **That which enables a device, appliance or equipment to be directly reached without requiring the removal or movement of any panel or similar obstruction.** **REFRIGERANT DEW POINT.** **The refrigerant vapor saturation temperature at a specified pressure.** **REFRIGERATED WAREHOUSE COOLER.** **An enclosed storage space capable of being refrigerated to temperatures above 32°F (0°C) that can be walked into and has a total chilled storage area of not less than 3,000 square feet (279 m2).** **REFRIGERATED WAREHOUSE FREEZER.** **An enclosed storage space capable of being refrigerated to temperatures at or below 32°F (0°C) that can be walked into and has a total chilled storage area of not less than 3,000 square feet (279 m2).** **REFRIGERATION SYSTEM, LOW TEMPERATURE.** **Systems for maintaining food product in a frozen state in refrigeration applications.** **REFRIGERATION SYSTEM, MEDIUM TEMPERATURE.** **Systems for maintaining food product above freezing in refrigeration applications.** **REGISTERED DESIGN PROFESSIONAL.** **An individual who is registered or licensed to practice their respective design profession as defined by the statutory requirements of the professional registration laws of the state or jurisdiction in which the project is to be constructed.** **RENEWABLE ENERGY RESOURCES.** **Energy derived from solar radiation, wind, waves, tides, landfill gas, biogas, biomass or extracted from hot fluid or steam heated within the earth.** **REPAIR.** **The reconstruction or renewal of any part of an existing building for the purpose of its maintenance or to correct damage.** **REROOFING.** **The process of recovering or replacing an existing roof covering. See "[*[Roof recover]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefROOF_RECOVER/2218)" and "[*[Roof replacement]*](https://codes.iccsafe.org/lookup/IECC2021P2_CE_Ch02_SecC202_DefROOF_REPLACEMENT/2218)."** **RESIDENTIAL BUILDING.** **For this code, includes detached one- and two-family dwellings and multiple single-family dwellings (townhouses) and *Group R*-2, R-3 and R-4 buildings three stories or less in height above grade plane.** **ROOF ASSEMBLY.** **A system designed to provide weather protection and resistance to design loads. The system consists of a roof covering and roof deck or a single component serving as both the roof covering and the roof deck. A roof assembly includes the roof covering, underlayment, roof deck, insulation, vapor retarder and interior finish.** **ROOF RECOVER.** **The process of installing an additional roof covering over an existing roof covering without removing the existing roof covering.** **ROOF REPAIR.** **Reconstruction or renewal of any part of an existing roof for the purpose of its maintenance.** **ROOF REPLACEMENT.** **The process of removing the existing roof covering, repairing any damaged substrate and installing a new roof covering.** **ROOFTOP MONITOR.** **A raised section of a roof containing vertical fenestration along one or more sides.** ***R*-VALUE (THERMAL RESISTANCE).** **The inverse of the time rate of heat flow through a body from one of its bounding surfaces to the other surface for a unit temperature difference between the two surfaces, under steady state conditions, per unit area (h × ft2 × °F/Btu) \[(m2 × K)/W\].** **SATURATED CONDENSING TEMPERATURE.** **The saturation temperature corresponding to the measured refrigerant pressure at the condenser inlet for single component and azeotropic refrigerants, and the arithmetic average of the dew point and *bubble point* temperatures corresponding to the refrigerant pressure at the condenser entrance for zeotropic refrigerants.** **SERVICE WATER HEATING.** **Supply of hot water for purposes other than comfort heating.** **SLEEPING UNIT.** **A room or space in which people sleep that can include permanent provisions for living, eating, and either sanitation or kitchen facilities but not both. Such rooms and spaces that are part of a dwelling unit are not *sleeping units*.** **SMALL ELECTRIC MOTOR.** **A general purpose alternating-current single-speed induction motor.** **SOLAR HEAT GAIN COEFFICIENT (SHGC).** **The ratio of the solar heat gain entering the space through the fenestration assembly to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation that is then reradiated, conducted or convected into the space.** **STANDARD REFERENCE DESIGN.** **A version of the *proposed design* that meets the minimum requirements of this code and is used to determine the maximum annual energy use requirement for compliance based on total building performance.** **STOREFRONT.** **A system of doors and windows mulled as a composite fenestration structure that has been designed to resist heavy use. *Storefront* systems include, but are not limited to, exterior fenestration systems that span from the floor level or above to the ceiling of the same story on commercial buildings, with or without mulled windows and doors.** **TESTING UNIT ENCLOSURE AREA.** **The area sum of all the boundary surfaces that define the *dwelling unit*, *sleeping unit* or occupiable *conditioned space* including top/ceiling, bottom/floor and all side walls. This does not include interior partition walls within the *dwelling unit*, *sleeping unit*, or occupiable *conditioned space*. Wall height shall be measured from the finished floor of the *conditioned space* to the finished floor or roof/ceiling air barrier above.** **THERMAL DISTRIBUTION EFFICIENCY (TDE).** **The resistance to changes in air heat as air is conveyed through a distance of air duct. TDE is a heat loss calculation evaluating the difference in the heat of the air between the air duct inlet and outlet caused by differences in temperatures between the air in the duct and the duct material. TDE is expressed as a percent difference between the inlet and outlet heat in the duct.** **THERMOSTAT.** **An automatic control device used to maintain temperature at a fixed or adjustable setpoint.** **TIME SWITCH CONTROL.** **An automatic control device or system that controls lighting or other loads, including switching off, based on time schedules.** ***U*-FACTOR (THERMAL TRANSMITTANCE).** **The coefficient of heat transmission (air to air) through a building component or assembly, equal to the time rate of heat flow per unit area and unit temperature difference between the warm side and cold side air films (Btu/h × ft2 × °F) \[W/(m2 × K)\].** **VARIABLE REFRIGERANT FLOW SYSTEM.** **An engineered direct-expansion (DX) refrigerant system that incorporates a common condensing unit, at least one variable-capacity compressor, a distributed refrigerant piping network to multiple indoor fan heating and cooling units each capable of individual zone temperature control, through integral zone temperature control devices and a common communications network. Variable refrigerant flow utilizes three or more steps of control on common interconnecting piping.** **VEGETATIVE ROOF.** **An assembly of interacting components designed to waterproof a building's top surface that includes, by design, vegetation and related landscape elements.** **VENTILATION.** **The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.** **VENTILATION AIR.** **That portion of supply air that comes from outside (outdoors) plus any recirculated air that has been treated to maintain the desired quality of air within a designated space.** **VISIBLE TRANSMITTANCE (VT).** **The ratio of visible light entering the space through the fenestration product assembly to the incident visible light. Visible transmittance includes the effects of glazing material and frame and is expressed as a number between 0 and 1.** **VISIBLE TRANSMITTANCE, ANNUAL (VTannual).** **The ratio of visible light entering the space through the fenestration product assembly to the incident visible light during the course of a year, which includes the effects of glazing material, frame, and light well or tubular conduit, and is expressed as a number between 0 and 1.** **VOLTAGE DROP.** **A decrease in voltage caused by losses in the wiring systems that connect the power source to the load.** **WALK-IN COOLER.** **An enclosed storage space capable of being refrigerated to temperatures above 32°F (0°C) and less than 55°F (12.8°C) that can be walked into, has a ceiling height of not less than 7 feet (2134 mm) and has a total chilled storage area of less than 3,000 square feet (279 m2).** **WALK-IN FREEZER.** **An enclosed storage space capable of being refrigerated to temperatures at or below 32°F (0°C) that can be walked into, has a ceiling height of not less than 7 feet (2134 mm) and has a total chilled storage area of less than 3,000 square feet (279 m2).** **WALL, ABOVE-GRADE.** **A wall associated with the *building thermal envelope* that is more than 15 percent above grade and is on the exterior of the building or any wall that is associated with the *building thermal envelope* that is not on the exterior of the building. This includes, but is not limited to, between-floor spandrels, peripheral edges of floors, roof knee walls, dormer walls, gable end walls, walls enclosing a mansard roof and skylight shafts.** **WALL, BELOW-GRADE.** **A wall associated with the basement or first story of the building that is part of the *building thermal envelope*, is not less than 85 percent below grade and is on the exterior of the building.** **WATER HEATER.** **Any heating appliance or equipment that heats potable water and supplies such water to the potable hot water distribution system.** **ZONE.** **A space or group of spaces within a building with heating or cooling requirements that are sufficiently similar so that desired conditions can be maintained throughout using a single controlling device.**