It is a factor indicating the relative amount of radiation absorbed by a surface as compared to an absorbing black body under the same conditions. Its value is dependent upon the temperature of the source as also that of receiving surface*.
It is the ratio of the heat emitted by a surface as compared to that of an absolutely black surface under similar conditions. It varies with the temperature of the emitting surfaces*.
It is the ratio of the reflected heat to that of the total heat incident on a surface at a certain mean temperature range*.
Thermal conductance per unit area is the thermal transmission of a single layer structure per unit area divided by the temperature difference between the hot and cold faces. It is expressed in W/(m2K)*.
This is the quantity of heat in the ‘steady state’ conditions flowing the unit time through a unit area of a slab of uniform material of infinite extent and of unit thickness, when unit difference of temperature is established between its faces. Its unit is W/(mK)*.
It is reciprocal of thermal conductance. For a structure having plane parallel faces, thermal resistance is equal to thickness (L) divided by thermal conductivity (K).
The unit of thermal resistance is (m2K)/W*.
The usefulness of this quantity is that when heat passes in succession through two or more components of the building unit, the resistance may be added together to get the total resistance of the surface.
It is the sum of the surface resistance and the thermal resistance of the building unit itself*.
It is they may be added together to get the total resistance reciprocal of thermal conductivity. It is expressed in (mK)/ W*.
It is the thermal transmission through unit area of the given building unit divided by the temperature difference between the air or other fluid on either side of the building unit in ‘steady state’ conditions. It is reciprocal of total thermal resistance. Its unit is W/(m2K)*.
Thermal transmittance differs from ‘Thermal conductance’ in so far as temperatures are measured on the two surfaces of material or structure in the latter case and in the surrounding air or other fluid in the former. The conductance is a characteristic of the structure whereas the transmittance depends on conductance and surface coefficients of the structure under the conditions of use.
It is the sum of the surface resistance and the thermal resistance of the building unit itself*.
SP 41 (1987): Handbook on Functional Requirements of Buildings (Other than Industrial Buildings) [CED 12: Functional Requirements in Buildings]