Glossary of Terms

Acceptance Requirements

Test procedures that include equipment and systems to be tested, functions to be tested, conditions under which the test shall be performed, the scope of the tests, results to be obtained, and measurable criteria for acceptable performance. (California)



An addition is the extension or increase in the conditioned floor area and volume of a building. This includes converting an existing unconditioned space to a conditioned space, such as remodeling a basement, garage or attic. The standards require energy compliance analysis and documentation for all additions that increase the conditioned space and volume of the building.

AFUE (Annual Fuel Utilization Efficiency)

The ratio of annual output energy to annual input energy, which includes any non-heating pilot input loss and, for gas- or oil-fired furnaces or boilers, does not include electric energy.


Airflow Across the Evaporator

The efficiency of air conditioners and heat pumps is affected by the airflow across the evaporator (or condenser in the case of a heat pump). Measuring the temperature split or temperature drop across the evaporator and comparing this measurement to reference tables can verify proper airflow. See also Thermostatic Expansion Valves (TXV).



An alteration is any change to a building's water heating system, space conditioning system, lighting system, or envelope that is not an addition.


Alternative Calculation Method (ACM)

An alternative calculation method is one of the Commission's Public Domain Computer Programs or any other calculation method approved by the Commission. (California)


Alternative Component Packages

An alternative component package is one of the sets of prescriptive requirements contained in the standards. Each package is a set of measures that achieve a level of performance, which meets the standards.  These are often referred to as the prescriptive packages or packages. Buildings that comply with the prescriptive standards shall be designed, constructed and equipped to meet all of the requirements of one of the alternative packages of components for the appropriate climate zone. (California)


Automatic Time Switch

An automatic time switch is a device capable of automatically turning loads off and on based on time schedules.


Btu/hr (Btu/h)

British thermal unit per hour, also abbreviated Btu/h. One Btu equals the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. Used for measuring heating and cooling equipment output.


Building Envelope

The building envelope is made up of the elements of a building that enclose conditioned spaces and through which thermal energy may be transferred to or from the exterior.


Climate Zone

The Energy Commission established 16 climate zones that represent a geographic area for which an energy budget is established. These energy budgets are the basis for the Title 24 energy standards. (California)


Coefficient of Performance, Cooling (COP)

The coefficient of performance for cooling is the ratio of the rate of net heat removal to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units.


Coefficient of Performance, Heating (COP)

The coefficient of performance for heating is the ratio of the rate of net heat output to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units.


Combined Hydronic Space/Water Heating

A combined hydronic space conditioning and water heating system is one in which both domestic hot water and space heating is supplied from the same water heating equipment. Combined hydronic space heating includes both radiant floor systems and convective or fan coil systems.


Conditioned Floor Area (CFA)

Conditioned floor area is the total floor area (in square feet) of enclosed conditioned space on all floors of a building, as measured at the floor level of the exterior surfaces of exterior walls enclosing the conditioned space.


Conditioned Footprint Area

The conditioned footprint is the total area of the building footprint, in square feet, not including unconditioned space. The conditioned footprint area may be equal to the first floor area, or it may be greater. The footprint area is the total area of floor over unconditioned space (not over conditioned space), ambient air and slab-on-grade. One way to think of the conditioned footprint area is as the area of the largest conditioned floor in the building plus the conditioned floor area of any projections from other stories that extend beyond the outline of that largest floor.


Conditioned Space

In residential compliance, conditioned space is space in a building that is either directly conditioned or indirectly conditioned.


Controlled Ventilation Crawl Space (CVC)

The Energy Commission has approved an exceptional method for analyzing the energy impact of buildings with raised floors which use foundation wall insulation and have automatically controlled crawl space vents. The method is available as an option using an approved computer method with unique modeling criteria. (California)


Cooling Load

The rate at which heat must be extracted from a space to maintain a desired room condition.


Cool Roof

A cool roof is a roof surface with a high reflectivity and high emittance. A cool roof rejects solar heat before it enters the building by reflecting it back to the atmosphere.


Crawl Space

A crawl space is a space immediately under the first floor of a building adjacent to grade. The thermal characteristics of a crawl space (or any similar vented unheated space below a raised floor) tend to reduce heat loss and heat gain into the building compared with an open, unprotected space below the floor. Compliance credit for the crawl space is assumed to be equivalent to an additional R-6 insulation value. R-6 insulation is not modeled when a raised floor is over an open area or over a garage. (California)


Daylight Area

The floor area under skylights or next to windows. The daylight area includes primary sidelit daylight area, secondary sidelit daylight area, and skylit daylight area.


Daylighting Control, Automatic Multi-level

A multi-level lighting control that automatically reduces lighting in multiple steps or continuous dimming in response to available daylight. This control uses one or more photosensors to detect changes in daylight illumination and then change the electric lighting level in response to the daylight changes.


Decorative Gas Appliance

A decorative gas appliance is a gas appliance that is designed or installed for visual effect only, cannot burn solid wood, and simulates a fire in a fireplace.  A decorative gas appliance installed in a new residential building or addition cannot contain a continuously burning pilot light, and cannot use indoor air for cooling a firebox jacket if the indoor air is vented to the outside of the building. (California)


Demand Response

Controlling electricity loads in buildings in response to an electronic signal sent by the local utility requesting their customers to reduce electricity consumption.


Demising Partition

A demising partition is a wall, fenestration, floor, or ceiling that separates conditioned space from enclosed unconditioned space.


Design Heat Gain

The design heat gain is the total calculated heat gain through the building envelope under design conditions.


Design Heat Loss

The design heat loss is the total calculated heat loss through the building envelope under design conditions.


Dimming, Continuous

A lighting control method that is capable of varying the light output of lamps over a continuous range from full light output to minimum light output.


Dimming, Stepped

A lighting control method that varies the light output of lamps in one or more pre-determined discrete steps between full light output and off.


Directly Conditioned Space

Directly conditioned space is an enclosed space that is provided with wood heating, is provided with mechanical heating that has a capacity exceeding 10 Btu/(hr×ft²), or mechanical cooling that has a capacity exceeding 5 Btu/(hr×ft²), unless the space conditioning system is designed and thermostatically controlled to maintain a process environment temperature less than 55°F or to maintain a process environment temperature greater than 90°F for the whole space that the system serves, or unless the space conditioning system is designed and controlled to be incapable of operating at temperatures above 55°F or incapable of operating at temperatures below 90°F at design conditions. (California)


Dominant Occupancy

In mixed occupancy buildings, the dominant occupancy is the occupancy type with the greatest percentage of total conditioned floor area.


Dual-Glazed Greenhouse Windows

Dual-glazed greenhouse windows are a type of dual-glazed fenestration product which adds conditioned volume but no conditioned floor area to a building.


Duct Sealing

A procedure for installing a space conditioning distribution system that minimizes leakage of air from or to the distribution system. Minimum specifications for installation procedures, materials, diagnostic testing and field verification are contained in the Reference Residential Appendix RA3 and Reference Nonresidential Appendix NA1. (California)


Economizer, Air

A ducting arrangement, including dampers, linkages, and an automatic control system that allows a cooling supply fan system to supply outside air to reduce or eliminate the need for mechanical cooling.


EER (Energy Efficiency Ratio)

The energy efficiency ratio (EER) is the ratio of net cooling capacity (in Btu/hr) to total rate of electrical energy (in watts), of a cooling system under designated operating conditions, as determined using the applicable test method in the Appliance Efficiency Regulations or §112. (California)


Electric Resistance Heating

Electricity is inherently less efficient than gas as a heating energy source because it must account for losses associated with generation from depletable fossil fuels and transmission to the building site. A source energy multiplier of 3.0 (representing a net efficiency of 33 percent) is assigned to electricity by the standards. (California)


Enclosed Space

Enclosed space is space that is substantially surrounded by solid surfaces.


Energy Budget

Energy budget is the maximum amount of source energy that a proposed building, or portion of a building, can be designed to consume, calculated with the approved procedures specified in Title 24, Part 6." The low-rise residential standards are based upon the concept of an annual energy budget. This is the measure of source energy used per year in a building. The energy budget for low-rise residential buildings includes space heating, space cooling and domestic water heating. To comply with the standards, the energy use of the proposed building design must be less than the annual energy budget. (California)


Energy Efficiency Standards

The California state Title 24 energy standards as set forth in the California Code of Regulations, Title 24, Part 6. (California)


Energy Factor (EF)

Used to measure the efficiency of water heaters, the Energy Factor (EF) is the ratio of energy output to energy consumption of a water heater, expressed in equivalent units, under designated operating conditions over a 24-hour use cycle, as determined using the applicable test method in the Appliance Efficiency Regulations. (California)


Energy Management Control System (EMCS)

Often a computerized control system designed to regulate the energy consumption of a building by controlling the operation of energy consuming systems, such as the heating, ventilation and air conditioning (HVAC), lighting and water heating systems. The EMCS is also capable of monitoring environmental and system loads, and adjusting HVAC operations in order to optimize energy usage and respond to demand response signals.


Energy Obtained From Depletable Sources

Energy obtained from depletable sources is electricity purchased from a public utility, or energy obtained from burning coal, oil, natural gas, or liquefied petroleum gases.


Energy Obtained From Nondepletable Sources

Also referred to as renewable energy, including solar and wind power, energy from nondepletable sources is defined as energy that is not obtained from depletable sources.


Enforcing Agency

The enforcing agency is the city, county, or state agency responsible for issuing a building permit.


Evaporative Cooler

Evaporative coolers may be installed as an alternative to air conditioning, particularly in climate zones with dry air. These systems use water evaporation and air circulation to provide cooling. Evaporative coolers use less energy for cooling than minimum efficiency air conditioners.



Uncontrolled outward air leakage from inside a building, including leakage through cracks and interstices, around windows and doors, and through any other exterior partition or duct penetration.


Exterior Door

An exterior door is any openable opaque surface that separates conditioned and unconditioned space. A door with one half or less of the surface area as glazing is an exterior door. A door with more than 50 percent of its surface area made up of glazing is a fenestration product.


Fenestration Area (Glazing Area)

Fenestration area is defined as the area of all fenestration products (i.e., windows, skylights and glass doors) in exterior openings, including the sash or frame area. The nominal area (from nominal dimensions such as 4o4o) or rough opening is also acceptable. For details on calculating fenestration area for glass doors, see Exterior Door. Where the term "glazing area" is used in the standards it means the entire fenestration area, not just the area of glazing, unless stated otherwise.


Fenestration Product

A fenestration product is any transparent or translucent material plus any sash, frame, mullions, and dividers, in the envelope of a building, including, but not limited to: windows, sliding glass doors, French doors, skylights, curtain walls, garden windows, and other doors with a glazed area of more than one-half of the door area.


Fenestration Product, Field-Fabricated

A fenestration product including a glazed exterior door 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 with a label certificate or products required to have temporary or permanent labels.


Fenestration Product, Manufactured

A fenestration product constructed of materials which are factory cut or otherwise factory formed with the specific intention of being used to fabricate a fenestration product. A manufactured fenestration product is typically factory-assembled before delivery to a job site.


Fenestration Product, Site-Built

Fenestration designed to be 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.



A fireplace is a hearth and fire chamber or similar prepared place in which a solid fuel fire may be burned, as defined in UBC Section 3102; these include but are not limited to factory-built fireplaces, masonry fireplaces, and masonry heaters.


Gas Log

A gas log is a self-contained, free-standing, open-flame, gas-burning appliance consisting of a metal frame or base supporting simulated logs and designed for installation only in a vented fireplace.


Geothermal Heat Pump

A geothermal heat pump uses the earth as a source of energy for heating and a sink for energy when cooling. Some systems pump water from an aquifer in the ground and return the water to the ground after transferring heat from or to the water. A few systems use refrigerant directly in a loop of piping buried in the ground. Those heat pumps that use either a water loop or pump water from an aquifer have efficiency test methods that are accepted by the Energy Commission. These efficiency values are certified to the Energy Commission by the manufacturer and are expressed in terms of heating Coefficient of Performance (COP) and cooling Energy Efficiency Ratio (EER). (California)



Glazing is defined as the translucent portion of any fenestration product, typically glass.


Ground Floor Area

For compliance, the ground floor area is defined as the slab-on-grade area of a slab-ongrade building and the conditioned footprint area of a raised floor building.



The designation of a lamp holder and socket configuration, based on a coding system by the International Energy Consortium, where “G” indicates the broad type of two or more projecting contacts, such as pins or posts, “U”distinguishes between lamp and holder designs of similar type but that are not interchangeable due to electrical or mechanical requirements, and “24” indicates 24 millimeters center to center spacing of the electrical contact posts.


Habitable Story

A habitable story in a building is a story that contains space in which people may work or live in reasonable comfort. A habitable story is defined as having at least 50 percent of its volume above grade. The standards use this definition to determine whether a building is high-rise or low-rise.


Heat Capacity (HC)

The heat capacity of an assembly is the amount of heat necessary to raise the temperature of all the components of a unit area in the assembly one degree F. It is calculated as the sum of the average thickness times the density times the specific heat for each component, and is expressed in Btu per square foot per degree F.


Heat Pump

A heat pump is an air conditioner capable of heating by refrigeration. It may or may not include a capability for cooling. Outside air or water is used as a heat source or heat sink, depending upon whether the system is heating or cooling.


Heating, Ventilating and Air Conditioning (HVAC)

The mechanical heating, ventilating and air conditioning system of the building is also known as the HVAC system. The standards use various measures of equipment efficiency defined according to the type of equipment installed. Gas (fossil fuel) heating equipment is rated according to its Annual Fuel Utilization Efficiency (AFUE). The heating efficiency of electric heat pumps with less than 65,000 Btu/h cooling capacity is rated according to Heating Seasonal Performance Factor (HSPF). The heating efficiency of heat pumps with cooling capacity of 65,000 Btu/h or more is rated according to Coefficient of Performance (COP). Electric resistance heating is rated according to its HSPF. All electric cooling with less than 65,000 Btu/h output capacity is rated according to the Seasonal Energy Efficiency Ratio (SEER). Electric cooling with an output capacity of 65,000 Btu/h or more is rated according to its Energy Efficiency Ratio (EER). (Heat pump cooling is rated according to its SEER or EER, and heat pump heating by the HSPF or COP). Since few residential buildings use air conditioners or heat pumps with an output capacity greater than 65,000 Btu/h, the Low-Rise Residential Standards only use SEER and HSPF ratings, not EER or COP.


High-Rise Residential

A high-rise residential building is a building, other than a hotel/motel, of occupancy group R-1 with four or more habitable stories. All hotels and motels, regardless of the number of stories, and multi-family residential buildings with four or more habitable stories must comply with §120 through 149 of the standards (Nonresidential and High-Rise Residential Title 24 Energy Standards), rather than §150 through 152 (Low-Rise Residential Title 24 Energy Standards).


HSPF (Heating Seasonal Performance Factor)

HSPF is the total heating output of a heat pump (in British thermal units) during its normal usage period for heating divided by the total electrical energy input (in watt-hours) during the same period, as determined using the applicable test method in the Appliance Efficiency Regulations.



See Heating, Ventilating and Air Conditioning.


Hydronic Space Heating

A hydronic space heating system uses water-heating equipment, such as a storage tank water heater or a boiler, to provide space heating. Hydronic space heating includes both radiant floor systems and convective or fan coil systems.


Indirectly Conditioned Space

Indirectly conditioned space is enclosed space including, but not limited to, unconditioned volume in atria, that (1) is not directly conditioned space; and (2) either (a) has an areaweighted heat transfer coefficient to directly conditioned space exceeding that to the outdoors or to unconditioned space, or (b) is a space through which air from directly conditioned spaces is transferred at a rate exceeding 3 air changes per hour. Indirectly conditioned space must be included when calculating total conditioned floor area. Examples of areas that may be indirectly conditioned space include enclosed porches, enclosed sunrooms, laundry rooms and furnace closets.



Infiltration is uncontrolled inward air leakage from outside a building, or unconditioned space, including leakage through cracks and interstices, around windows and doors, and through any other exterior or demising partition or pipe or duct penetration.



Insulation must be placed within or contiguous with a wall, ceiling or floor, or over the surface of any appliance or its intake or outtake mechanism for the purpose of reducing heat transfer or reducing adverse temperature fluctuations of the building, room or appliance. Insulation may be installed in wall, ceiling/roof and raised floor assemblies and at the edge of a slab-on-grade. Movable insulation is designed to cover windows and other glazed openings part of the time to reduce heat loss and heat gain.


Insulation R-Value

The R-value of insulation or any material or building component is the measure of its thermal resistance expressed in ft2-hr-°F/Btu (see R-Value). This value may be obtained from Appendix B or from manufacturer's literature. The rated R-value of mineral fiber (batt) insulation is based upon its fully expanded thickness. When the insulation is compressed, the R-value is reduced. For example, an R-19 batt of insulation expands to a thickness of 6 inches. If it is compressed into 2x6 framing with an actual depth of 5.5 inches, the insulation R-Value is lowered to R-17.8.


Interior Partition

An interior wall or floor/ceiling that separates one area of conditioned space from another within the building envelope.


Knee Wall

A knee wall is a sidewall separating conditioned space from attic space under a pitched roof. Knee walls should be insulated as an exterior wall as specified by the chosen method of compliance.



The low-rise residential standards have mandatory measures for kitchen and bathroom lighting, and for incandescent lighting fixtures recessed into insulated ceilings. Both kitchens and rooms containing a bathtub or shower are required to have at least one luminaire with lamps that have an efficacy of at least 40 lumens per watt.


Light Emitting Diode (LED)

A pn junction semiconductor device that emits incoherent optical radiation when biased in the forward direction. The acronym “LED” typically refers to an LED component, LED device, or LED package.


Low-Rise Residential Building

A building, other than a hotel/motel that is of Occupancy Group R, Division1, and is multi-family with three stories or less, or a single family residence of Occupancy Group R, Division 3, or an Occupancy Group U building located on a residential site.



A lumen is a measure of the amount of light available from a given light source. A watt is a measure of the power requirement for that light source. The efficacy of a light source is measured by dividing the lumens by the wattage. The more usable light that a light source provides per watt, the greater its energy efficiency.



A complete lighting unit consisting of a lamp(s) and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply; commonly referred to as "lighting fixtures."


Mechanical Cooling

Lowering the temperature within a space using refrigerant compressors or absorbers, desiccant dehumidifiers, or other systems that require energy from depletable sources to directly condition the space. In non-residential, high-rise residential, and hotel/motel buildings, cooling of a space by direct or indirect evaporation of water alone is not considered mechanical cooling.


Mechanical Heating

Raising the temperature within a space using electric resistance heaters, fossil fuel burners, heat pumps, or other systems that require energy from depletable sources to directly condition the space.


Mixed Occupancy

A building designed and constructed for more than one type of occupancy, such as a three story building with ground floor retail and second and third floor residential apartments.


Motion Sensor, Lighting

A device that automatically turns lights off soon after an area is vacated. The term motion sensor applies to a device that controls outdoor lighting systems. When the device is used to control indoor lighting systems, it is termed an occupant sensor. The device also may be called an occupancy sensor, occupant-sensing device, or vacancy sensor.



A dwelling unit of occupancy type R, as defined by the UBC, sharing a common wall and/or ceiling/floor with at least one other dwelling unit.


Multi-Level Lighting Control

A lighting control that reduces lighting power in multiple steps while maintaining a reasonably uniform level of illuminance throughout the area controlled.


Newly Conditioned Space

Any space being converted from unconditioned to directly conditioned or indirectly conditioned space. Newly conditioned space must comply with the requirements for an addition.



North-facing is oriented to within 45 degrees of true north, including 45°0'0" east of north (NE), but excluding 45°0'0" west of north (NW). This definition applies only to the prescriptive packages and master plans analyzed according to the multiple orientation alternative. In the computer methods the actual building orientation must be used, except in the case of master plans as stated above.


Occupant Sensor, Lighting

A device that automatically turns lights off soon after an area is vacated. The term occupant sensor applies to a device that controls indoor lighting systems. When the device is used to control outdoor lighting systems, it is termed a motion sensor. The device also may be called an occupancy sensor, occupant-sensing device, or vacancy sensor.


Outside Air

Outdoor air is air taken from outdoors and not previously circulated in the building.



An electric device that detects changes in illumination levels then controls lighting load at predetermined illumination levels.


Process Space

A space that is thermostatically controlled to maintain a process environment temperature less than 55º F or to maintain a process environment temperature greater than 90º F for the whole space that the system serves, or that is a space with a space-conditioning system designed and controlled to be incapable of operating at temperatures above 55º F or incapable of operating at temperatures below 90º F at design conditions.


Radiant Barriers

A radiant barriers is a shiny metallic surface that is applied to the roof of the attic and its end walls. In attics, the radiant barrier is typically installed on the underside of the attic roof. Often the radiant barrier is pre-applied to the structural deck of the attic roof.


Raised Floor

A raised floor is a floor (partition) over a crawl space, or an unconditioned space, or ambient air.


Refrigerant Charge

The term refrigerant charge refers to the amount of refrigerant that is installed or charged into an air conditioner or heat pump. The refrigerant is the working fluid in an air conditioner or heat pump. It is compressed and becomes a liquid as it enters the condenser. The hot liquid is cooled in the condenser and flows to the evaporator where it released through the expansion valve. When the pressure is released, the refrigerant expands into a gas and cools. Air is passed over the evaporator to provide the space cooling. When an air conditioner or heat pump has too much refrigerant (overcharged) the compressor may be damaged. When an air conditioner has too little refrigerant (undercharged), the efficiency of the unit is reduced. A thermostatic expansion valve (TXV) can mitigate the impact of improper refrigerant charge.


Refrigerated Warehouse

A building or a space constructed for storage of products, where mechanical refrigeration is used to maintain the space temperature at 55 degrees F. or less.


R-Value (Thermal Resistance)

The R-value of a material is the thermal resistance of a material or building component to the passage of heat in (hr-ft2-ºF)/Btu. The R-value indicates how well a material prevents heat from flowing through it. R-19 insulation, for example, is only half as effective at slowing heat transfer as R-38 insulation. When more than one material is put in series with another in a construction assembly (such as exterior siding, insulation and interior gypsum board), the thermal resistance of the assembly is equal to the sum of the individual resistances.


Relative Solar Heat Gain

The ratio of solar heat gain through a fenestration product (corrected for external shading) to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation, which is then reradiated, conducted, or convected into the space.


SEER (Seasonal Energy Efficiency Ratio)

The total cooling of a central air conditioner or heat pump in Btu during 12 months divided by the total electric energy input in watt-hours during the same period


Single Family Attached

A multi-family building whose dwelling units share common walls but do not share any common floors/ceilings is considered single family attached.


Single Family Building

A single dwelling unit of occupancy type R, as defined in the UBC, which stands separate and unattached from other dwelling units, but may have an attached garage. A dwelling unit that is separated only by a property line and double wall construction (with a space between the walls) from another dwelling unit and that shares no common floor/ceiling is also treated as single family.


Solar Heat Gain Coefficient (SHGC)

The solar heat gain coefficient is a measure of the effectiveness of a fenestration product or window covering to stop solar gains through the window. Solar heat gain coefficient (SHGC) is the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation, which is then reradiated, conducted, or convected into the space.



A skylight is glazing having a slope less than 60 degrees from the horizontal with conditioned space below.


Solar Reflectance Index (SRI)

A measure of the roof's ability to reject solar heat which includes both reflectance and emittance.


Space-Conditioning System

A system that may consist of but not limited to chiller/compressor, air handler unit, cooling and heating coils, air and water cooled condenser, economizers, and the air distribution system, which provide either collectively or individually heating, ventilating, or cooling within or associated with conditioned spaces in a building.


Time Dependent Valuation (TDV)

TDV Energy is the time varying energy caused to be used by the building to provide space conditioning and water heating and for specified buildings lighting. TDV energy accounts for the energy used at the building site and consumed in producing and in delivering energy to a site, including, but not limited to, power generation, transmission and distribution losses.


Title 24 Energy Standards

The California Title 24 Energy Efficiency Standards as set forth in the California Code of Regulations, Title 24, Part 6.


Standby Loss

Standby loss is the ratio of heat lost per hour to the heat content of the stored water above room temperature. It is one of the measures of efficiency of water heaters required for water heating energy calculations for some types of water heaters. Standby loss is expressed as a percentage.


Thermal Mass

Thermal mass is solid or liquid material used to store heat for later heating use or for reducing cooling requirements. Commonly used thermal mass materials include concrete, masonry, brick, tile, rock and water. These materials are readily available and have excellent thermal properties. The more mass a building has, the slower its interior temperatures will change. Thermal mass can conserve energy in a residential building in two basic ways:

  • In the winter, solar radiation entering a building through glass is absorbed directly or indirectly into areas of thermal mass exposed to indoor air. The mass material tends to store its heat during the day and release it into the room air during the evening and night when more heating is required.
  • In the summer months, the mass material can be cooled down at night by natural ventilation. During the daytime, the mass absorbs excess heat and helps keep down indoor air temperatures. If glazing is well shaded to keep out direct sunlight, the mass can substantially reduce or eliminate completely the need for mechanical cooling.

Thermostatic expansion valves (TXV)

A thermostatic expansion valve is a metering device for refrigerant flow into the evaporator of an air conditioner or heat pump. Refrigerant flow is metered in response to the temperature of the refrigerant leaving the evaporator. A TXV improves efficiency and mitigates the effect a system with improper refrigerant charge. The valve is placed upstream from the evaporator inlet and is connected to a temperature sensing bulb and pressure tap that are located at the evaporator outlet. As the gaseous refrigerant leaves the evaporator the TXV senses its temperature and pressure (superheat) and adjusts the flow rate to maintain the super heat at a constant value. Eligible systems must provide a removable door or other method of access for verifying the valve is installed and must include verification by a HERS rater. Package D requires either a TXV or testing of refrigerant charge and airflow in climate zones 2 and 8 through 15.



The U-factor is the overall coefficient of thermal transmittance of a construction assembly, in Btu/(hr x ft2 x ºF), including air film resistances at both surfaces.


Unconditioned Space

Unconditioned space is enclosed space within a building that is not conditioned space. A space is unconditioned if:

  • It is not provided with space conditioning;
  • It can be isolated from conditioned space by closeable doors; and
  • It is not indirectly conditioned

Common unconditioned spaces include garages, attics, crawl spaces, mechanical closets and sunspaces.


Vacancy Sensor, Lighting

An occupant sensor for which the lights must be manually turned on but the sensor automatically turns the lights off soon after an area is vacated. The device also may be called a manual-on occupant sensor.


Vapor Barrier

A vapor barrier is a material with a permeance of one perm or less which provides resistance to the transmission of water vapor.


Variable Air Volume (VAV) System

A space-conditioning system that maintains comfort levels by varying the volume of supply air to the zones served.


Visible Light Transmittance (VLT)

The ratio (expressed as a decimal) of visible light that is transmitted through a glazing to the light that strikes the material as calculated in NFRC 200.


Zonal Control

Zonal control refers to the practice of dividing a residence into separately controlled HVAC zones. This may be done by installing multiple HVAC systems that condition a specific part of the building, or by installing one HVAC system with a specially designed distribution system that permits zonal control.


Zone, Space-Conditioning

A space or group of spaces within a building with sufficiently similar comfort conditioning requirements so that comfort conditions can be maintained throughout the zone by a single controlling device.


(Excerpted from the CEC Title 24 Energy Standards)