Radiant heat transfer – the transfer of heat by radiation. Heat radiation is a form of electromagnetic radiation. Radiant heating due to infrared radiation is commonly employed in passive systems.

Radiant temperature – the average temperature of surfaces surrounding a person or surface, with which the person or surface can exchange thermal radiation.

Reference design – a detailed specification of the passive solar features of a hypothetical passive solar building used as the subject of performance analysis.

Reflectance – the ratio of radiation reflected by a surface to the radiation incident on it. The range is 0–1.0.

Reflection – process by which radiation is returned by a surface or a medium, without change of frequency of its monochromatic component.

Relative humidity – the percentage of water vapour in the atmosphere relative to the maximum amount of water vapour that can be held by the air at a given temperature.

Resistivity – the thermal resistance of unit area of a material of unit thickness to heat flow caused by a temperature difference across the material.

Reverse thermocirculation – thermocirculation in the reverse direction, that is, from the heated space to the solar collector. This can occur at night when the heated space is warmer than the collector. In the reference design, the process is assumed to be prevented by dampers.

Roof pond system – an indirect gain heating and cooling system in which the mass, which is water in plastic bags, is located on the roof of the space to be heated or cooled and covered with a movable insulation. A roof pond system absorbs solar radiation for heating in the winter and radiates heat to the sky for cooling in the summer.

Selective coating – finishes applied to materials to improve their performance in relation to radiation of different wavelengths. Those applied to solar absorbers have a high absorptance of solar radiation accompanied by a low emittance of long wave radiation, while those for glazing have a high transmittance to solar radiation and high reflectance of long wavelengths.

Selective surface – a surface used to absorb and retain solar heat in a solar heating system such as a Trombe wall or in a solar collector. Selective surfaces have high absorptance and low emittance.

Sensible heat – heat that results in a change in air temperature, in contrast with latent heat.

SI units – Standard International units; the metric system.

Sky component – the portion of the daylight factor (at a point indoors) contributed by luminance from the sky, excluding direct sunlight.

Sky cover – a measure of the fraction of the sky covered in clouds. Range is 0–10 tenths.

Skylight – a roof window, horizontal or sloped.

Sol-air temperature – an equivalent temperature which will produce the same heating effect as the incident radiation in conjunction with the actual external air temperature.

Solar absorptance – the fraction of incident solar radiation that is absorbed by a surface. The radiation not absorbed by an opaque surface is reflected. The range is 0–1.0.

Solar gain – heat transferred to a space by solar radiation through glazing.

Solar heat gain coefficient (SHGC) – the fraction of incident solar radiation (for the full spectrum) which passes through an entire window assembly, including the frame, at a specified angle. Range is 0–0.85. A higher SHGC is preferred in solar heating applications to capture maximum sun, whereas in cooling applications, a low SHGC reduces unwanted solar heat gain.

Solar load – the demand for energy required at any moment to compensate for the difference between desired indoor conditions and heat gains from solar radiation.

Solar radiation – radiation emitted by the sun, including infrared radiation, ultraviolet radiation, and visible light. The radiation received without change of direction is called beam or direct radiation. The radiation received after its direction has been changed by scattering and reflection is called diffuse radiation. The sum of the two is referred to as global or total radiation.

Specific heat – a measure of the ability of a material to store heat. Specifically, the quantity of heat required to raise the temperature of unit mass of a substance by one degree (kJ/kg °C).

Stack ventilation – the cooling process of natural ventilation induced by the chimney effect, where a pressure differential occurs across the section of a room. Air in the room absorbs heat gained in the space, loses density, thus rising to the top of the space. When it exits through high outlet openings, a lower pressure is created low in the space, drawing in cooler outside air from low inlets.

Sunlight – beam daylight from the sun only, excluding diffuse light from the sky dome.

Surface resistance – the surface resistance is the resistance to heat flow at the surface of a material. It has two components: the surface resistance for convection and for conduction.

Task light – lighting on a specific area used for a specific task. Task lighting is usually from an electric source and is of a higher illuminance level than the surrounding ambient light level. It is a good strategy to combine task light with ambient daylight.

Temperature swing – the range of indoor temperatures in the building between the day and night.

Thermal break (thermal barrier) – an element of low thermal conductivity placed within a composite envelope construction in such a way as to reduce the flow of heat across the assembly.

Thermal bridge – an element of high thermal conductivity within a construction of otherwise low thermal conductivity. Small areas of materials that conduct heat at high rates can substantially reduce the insulating effectiveness of an assembly. Examples are metal frame windows without thermal breaks and metal stud walls, where the metal conducts heat at a much higher rate than the insulation in between.

Thermal conductivity (k) – a measure of the ease with which heat flows through a unit thickness of a material by conduction; specifically, the heat flow rate in Watt per metre of material thickness, and degree of temperature difference. Units are W/m-°C.

Thermal radiation – energy transfer in the form of electromagnetic waves from a body by virtue of its temperature, including infrared radiation, ultraviolet radiation, and visible light.

Thermal resistance – a measure of the insulation value or resistance to heat flow of building elements or materials; specifically, the reciprocal of the thermal conductance.

Thermal storage mass – high-density building elements, such as masonry or water in containers, designed to absorb solar heat during the day for later release when heat is needed.

Thermal storage wall – a Trombe wall or water wall.

Thermocirculation – the circulation of a fluid by convection. For example, the convection from a warm zone (sunspace or Trombe wall air space) to a cool zone through openings in a common wall.

Thermosyphon – the convective circulation of a fluid which occurs in a closed system where warm fluid rises and is replaced by a cooler fluid in the same system.

Tilt – the angle of a plane relative to a horizontal plane.

Time-lag – the period of time between the absorption of solar radiation by a material and its release into a space. Time-lag is an important consideration in sizing a thermal storage wall or Trombe wall.

Transmittance – the ratio of the radiant energy transmitted through a substance to the total radiant energy incident on its surface.

Ultraviolet radiation – electromagnetic radiation having wavelengths shorter than those of visible light. This invisible form of radiation is found in solar radiation and plays a part in the deterioration of plastic glazing materials, paint and furnishing fabrics.

U-value (coefficient of heat transfer) – the number of Watts that flow through one square metre of building component (e.g. roof, wall, floor, glass), in one second, when there is a 1 °C difference in temperature between the inside and outside air, under steady state conditions. The U-value is the reciprocal of the resistance.

Ventilation load – the energy required to bring outdoor air to the desired indoor conditions. In this context, ventilation load refers to fresh air ventilation, which may be provided either naturally or by a mechanical system. The rate of required ventilation varies with the use of the space and the number of occupants. Ventilation load depends on the rate of fresh air ventilation and on the temperature difference between inside and outside. It may be reduced by pre-tempering or the use of heat exchangers.

Ventilation losses – the heat losses associated with the continuous replacement of warm, stale air by fresh cold air.

Ventilation (natural) – air flow through and within a space stimulated by either the distribution of pressure gradients around a building, or thermal forces caused by temperature gradients between indoor and outdoor air.

Visible spectrum – that part of the solar spectrum which is visible to the human eye; radiation with wavelength roughly between 380 and 700 nm.

Visible transmittance (VT) – the fraction of incident visible light that passes through glazing.

Watt (W) – a measure of power commonly used to express heat loss or heat gain, or to specify electrical equipment. It is the power required to produce energy at the rate of one joule per second.

Weather stripping – narrow or jamb-width sections of thin metal or other material to prevent infiltration of air and moisture around windows and doors.

Wet-bulb temperature – the air temperature measured using a thermometer with a wetted bulb moved rapidly through the air to promote evaporation. The evaporating moisture and changing phase lowers the temperature measured relative to that measured with a dry bulb. Wet bulb temperature accounts for the effects of moisture in the air. It can be used along with the dry-bulb temperature on a psychrometric chart to determine relative humidity.

Zenith – the top of the sky dome. A point directly overhead, 90° in altitude angle above the horizon.

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