Radiant Barrier
A thin, reflective foil sheet that exhibits low radiant energy transmission and under certain conditions can block radiant heat transfer; installed in attics to reduce heat flow through a roof assembly into the living space.
Radiant Ceiling Panels
Ceiling panels that contain electric resistance heating elements embedded within them to provide radiant heat to a room.

Radiant Energy

Energy that transmits away from its source in all directions.

Radiant Floor

A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room heats from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.

Radiant Heating System

A heating system where heat is supplied (radiated) into a room by means of heated surfaces, such as electric resistance elements, hot water (hydronic) radiators, etc.


The transfer of heat through matter or space by means of electromagnetic waves.

Radiative Cooling

The process of cooling by which a heat absorbing media absorbs heat from one source and radiates the heat away.


A room heat delivery (or exchanger) component of a hydronic (hot water or steam) heating system; hot water or steam is delivered to it by natural convection or by a pump from a boiler.

Radiator Vent

A device that releases pressure within a radiator when the pressure inside exceeds the operating limits of the vent.

Radioactive Waste

Materials left over from making nuclear energy. Radioactive waste can living organisms if it is not stored safely.


A naturally occurring radioactive gas found in the U.S. in nearly all types of soil, rock, and water. It can migrate into most buildings. Studies have linked high concentrations of radon to lung cancer.


A construction element used for ceiling support.

Rammed Earth

A construction material made by compressing earth in a form; used traditionally in many areas of the world and widely throughout North Africa and the Middle East.

Rankine Cycle

The thermodynamic cycle that is an ideal standard for comparing performance of heat-engines, steam power plants, steam turbines, and heat pump systems that use a condensable vapor as the working fluid; efficiency is measured as work done divided by sensible heat supplied.

Rated Life

The length of time that a product or appliance is expected to meet a certain level of performance under nominal operating conditions; in a luminaire, the period after which the lumen depreciation and lamp failure is at 70% of its initial value.

Rated Power

The power output of a device under specific or nominal operating conditions.

Rate Schedule

A mechanism used by electric utilities to determine prices for electricity; typically defines rates according to amounts of power demanded/consumed during specific time periods.

Rayleigh Frequency Distribution

A mathematical representation of the frequency or ratio that specific wind speeds occur within a specified time interval.

Reactive Power

The electrical power that oscillates between the magnetic field of an inductor and the electrical field of a capacitor. Reactive power is never converted to non-electrical power. Calculated as the square root of the difference between the square of the kilovolt-amperes and the square of the kilowatts. Expressed as reactive volt-amperes.

Real Price

The unit price of a good or service estimated from some base year in order to provide a consistent means of comparison.

Recirculation Systems

A type of solar heating system that circulate warm water from storage through the collectors and exposed piping whenever freezing conditions occur; obviously a not very efficient system when operating in this mode.


The component of a central receiver solar thermal system where reflected solar energy is absorbed and converted to thermal energy.

Recirculated Air

Air that is returned from a heated or cooled space, reconditioned and/or cleaned, and returned to the space.


An electrical device for converting alternating current to direct current. The chamber in a cooling device where water is separated from the working fluid (for example ammonia).


A heat exchanger in which heat is recovered from the products of combustion.

Recurrent Costs

Costs that are repetitive and occur when an organization produces similar goods or services on a continuing basis.


The process of converting materials that are no longer useful as designed or intended into a new product.


The amount (percent) of light that is reflected by a surface relative to the amount that strikes it.

Reflective Coatings

Materials with various qualities that are applied to glass windows before installation. These coatings reduce radiant heat transfer through the window and also reflects outside heat and a portion of the incoming solar energy, thus reducing heat gain. The most common type has a sputtered coating on the inside of a window unit. The other type is a durable “hard-coat” glass with a coating, baked into the glass surface.

Reflective Window Films

A material applied to window panes that controls heat gain and loss, reduces glare, minimizes fabric fading, and provides privacy. These films are retrofitted on existing windows.

Reflective Glass

A window glass that has been coated with a reflective film and is useful in controlling solar heat gain during the summer.

Reflective Insulation (see also radiant barrier)

An aluminum foil fabricated insulator with backings applied to provide a series of closed air spaces with highly reflective surfaces.

Reflector Lamps

A type of incandescent lamp with an interior coating of aluminum that reflects light to the front of the bulb. They are designed to spread light over specific areas.


The change in direction of a ray of light when it passes through one media to another with differing optical densities.


The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and absorb heat.


The process of the absorption of heat from one location and its transfer to another for rejection or recuperation.

Refrigeration Capacity

A measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour or in tons, where one (1) ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 Btu per hour.

Refrigeration Cycle

The complete cycle of stages (evaporation and condensation) of refrigeration or of the refrigerant.

Refuse-Derived Fuel (RDF)

A solid fuel produced by shredding municipal solid waste (MSW). Noncombustible materials such as glass and metals are generally removed prior to making RDF. The residual material is sold as-is or compressed into pellets, bricks, or logs. RDF processing facilities are typically located near a source of MSW, while the RDF combustion facility can be located elsewhere. Existing RDF facilities process between 100 and 3,000 tons per day.

Regenerative Cooling

A type of cooling system that uses a charging and discharging cycle with a thermal or latent heat storage subsystem.

Regenerative Heating

The process of using heat that is rejected in one part of a cycle for another function or in another part of the cycle.


The replacement of a non-functional or ineffective lamp with a new, more efficient lamp.

Relative Humidity

A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.


This is the concept of how long a device or process can operate properly without needing maintenance or replacement.

Renewable Energy

Energy derived from resources that are regenerative or for all practical purposes can not be depleted. Types of renewable energy resources include moving water (hydro, tidal and wave power), thermal gradients in ocean water, biomass, geothermal energy, solar energy, and wind energy. Municipal solid waste (MSW) is also considered to be a renewable energy resource.


The inherent characteristic of a material to inhibit the transfer of energy. In electrical conductors, electrical resistance results in the generation of heat. Electrical resistance is measured in Ohms. The heat transfer resistance properties of insulation products are quantified as the R-value.

Resistance Heating

A type of heating system that provides heat from the resistance of an electrical current flowing through a conductor.

Resistive Voltage Drop

The voltage developed across a cell by the current flow through the resistance of the cell.


An electrical device that resists electric current flow.

Resource Recovery

The process of converting municipal solid waste to energy and/or recovering materials for recycling.


The process of changing the structure of the electric power industry from one of guaranteed monopoly over service territories, as established by the Public Utility Holding Company Act of 1935, to one of open competition between power suppliers for customers in any area.


The process of modifying a building’s structure.

Return Air

Air that is returned to a heating or cooling appliance from a heated or cooled space.

Return Duct

The central heating or cooling system contains a fan that gets its air supply through these ducts, which ideally should be installed in every room of the house. The air from a room will move towards the lower pressure of the return duct.

Retail Wheeling

A term for the process of transmitting electricity over transmission lines not owned by the supplier of the electricity to a retail customer of the supplier. With retail wheeling, an electricity consumer can secure their own supply of electricity from a broker or directly from the generating source. The power is then wheeled at a fixed rate, or at a regulated “non-discriminatory” rate set by a utility commission.

Reverse Thermosiphoning

When heat seeks to flow from a warm area (e.g., heated space) to a cooler area, such as a solar air collector at night without a reverse flow damper.

Reversing Valve

A component of a heat pump that reverses the refrigerant’s direction of flow, allowing the heat pump to switch from cooling to heating or heating to cooling.


See R-Value.

Ribbon (Photovoltaic) Cells

A type of solar photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

Rigid Insulation Board

An insulation product made of a fibrous material or plastic foams, pressed or extruded into board-like forms. It provides thermal and acoustical insulation strength with low weight, and coverage with few heat loss paths.

Rock Bin

A container that holds rock used as the thermal mass to store solar energy in a solar heating system.

Rock Wool

A type of insulation made from virgin basalt, an igneous rock, and spun into loose fill or a batt. It is fire resistant and helps with soundproofing.


A building element that provides protection against the sun, wind, and precipitation.

Roof Pond

A solar energy collection device consisting of containers of water located on a roof that absorb solar energy during the day so that the heat can be used at night or that cools a building by evaporation at night.

Roof Ventilator

A stationary or rotating vent used to ventilate attics or cathedral ceilings; usually made of galvanized steel, or polypropylene.


An electric generator consists of an armature and a field structure. The armature carries the wire loop, coil, or other windings in which the voltage is induced, whereas the field structure produces the magnetic field. In small generators, the armature is usually the rotating component (rotor) surrounded by the stationary field structure (stator). In large generators in commercial electric power plants the situation is reversed. In a wind energy conversion device, the blades and rotating components.

Run-of-River Hydropower

A type of hydroelectric facility that uses the river flow with very little alteration and little or no impoundment of the water.

Rural Electrification Administration (REA)

An agency of the U.S. Dept. of Agriculture that makes loans to states and territories in the U.S. for rural electrification and the furnishing of electric energy to persons in rural areas who do not receive central station service. It also furnishes and improves electric and telephone service in rural areas, assists electric borrowers to implement energy conservation programs and on-grid and off-grid renewable energy systems, and studies the condition and progress of rural electrification.


A measure of the capacity of a material to resist heat transfer. The R-Value is the reciprocal of the conductivity of a material (U-Value). The larger the R-Value of a material, the greater its insulating properties.