Thermal (or cold) bridge
A path of low thermal resistance between two surfaces.

Thermal comfort
The human body involuntarily regulates its production of internal heat to the thermal conditions of the environment, eventually creating a situation where the metabolic generation of heat is offset by the heat losses so the individual experiences only very small variations in the feeling of thermal comfort and thereby feels at ease.

Thermal conductance (W/(m² K))
The reciprocal of thermal resistance; a measure of heat flow rate through a layer of material per unit of area and unit temperature difference (see also U-value).

Thermal conductivity (W/(m K))
The rate of heat flow across the unit area of a homogeneous material of unit thickness with unit temperature difference between opposite sides of the material. The thermal conductivity of materials varies with density and moisture content. Still air and inert gases such as argon and krypton have the lowest values of thermal conductivity; thermal insulation materials achieve a low thermal conductivity by trapping air or other gases within their porous fabric. The conductivity of masonry materials is largely a function of their density (compare, for example, lightweight concrete to dense cast concrete); metals have the highest values of thermal conductivity.

Thermal inertia
The property of a material or structure to delay the effect of a change of thermal gradient.

Thermal insulation
Materials of low thermal conductivity; qualitative expression of the thermal resistance of an element or construction.

Thermal transmittance
See U-value.

Thermal resistance (m²K/W)
The ratio of thickness of a layer of homogeneous material over its thermal conductivity. The thermal resistance is a measure of the thermal insulation value of a construction layer; the higher the thermal resistance the better the insulation value.

Thermal resistivity (m K/W)
The reciprocal of thermal conductivity; the thermal resistance of unit thickness of material.

Flow induced by a warm fluid or gas being less dense than a cool medium and hence rising. In a thermosiphon solar air system, solar-warmed air in the collector rises and so can be self-transported to either a room or storage.

The angle between a surface and the horizontal plane.

Total heat transfer coefficient
See U-value

Total solar radiation
See Global solar radiation.