Term Description
British thermal unit (Btu)

The heat energy required to raise the temperature of one pound of water 1 degree Fahrenheit.
Coefficient of thermal expansion

The coefficient of change in dimension of a material per unit of dimension per degree change in temperature.
Conductance, thermal (C)

See "a href="https://cottermanroofing.com/knowledgebase?field_category_value=All&fiel... conductance (C)">thermal conductance (C)."
Conductivity, thermal (k)

See "thermal conductivity (k)."
Long-term thermal resistance (LTTR)

A method for determining and reporting estimated long-term R-values of unfaced or permeably faced rigid gas-filled closed-cell plastic foam insulations by reducing the specimen thickness to accelerate aging under controlled conditions. This method is reported to account for the diffusion of the blowing agent from the foam insulation‚Äôs cells over time, resulting in a recognized reduction of thermal resistance of some plastic foam insulations as they age. The R-values produced by the LTTR method correspond to the thermal resistances measured for insulation materials stored in controlled laboratory conditions until they have aged five years. LTTR values are reported for polyisocyanurate rigid board insulation in accordance with ASTM C1289, ‚"Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.‚"
Thermal barrier

In torched membrane applications over combustible substrates, an above-deck barrier incorporated into a roof system before torching as a fire-safety practice. Acceptable thermal barriers include: minimum 3/4-inch-thick perlite board insulation, minimum 3/4-inch-thick fiberglass or mineral wool board insulation or minimum 1/4-inch-thick glass-faced gypsum board.
Thermal bridge

The penetration of a material of high thermal conductivity (e.g., a metal insulation or roof membrane fastener) through a material of low thermal conductivity (e.g., thermal insulation); the result is a lowered thermal resistance for the assembly.
Thermal conductance (C)

The time rate of steady-state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces. In English (inch•pound) units of measurement, the number of British thermal units (Btu) that pass through a specified thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. In English (inch•pound) units, it is expressed as Btu/h•ft2•F.
Note 1: A thermal conductance (C) value applies to a specific thickness of a specific material.

Note 2: It is mathematically incorrect to multiply or divide the thermal conductance (C) value for a specific thickness of a material to determine the thermal conductance value of a different thickness of the same material.

Note 3: It is mathematically incorrect to add thermal conductance (C) values to determine overall thermal performance. If it is necessary to determine the overall thermal performance of a construction, it is appropriate to convert the individual thermal conductance (C) values to thermal resistance (R) values (i.e., R = 1/C) and then add the thermal resistance values (i.e., Rt = R1 + R2 + ...).
Thermal conductivity (k)

The time rate of steady-state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area. In English (inch•pound) units of measurement, it is the number of British thermal units (Btu) that pass through a 1-inch thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. In English (inch•pound) units, it is expressed as Btu•inch/h•ft2•F.
Note 1: A thermal conductivity (k) value applies to 1-inch thickness of a specific material.

Note 2: It is mathematically incorrect to add, multiply or divide the thermal conductivity (k) value of a material to determine the thermal performance value of a different thickness of the same material. If it is necessary to determine the thermal performance of a specific thickness of a material, it is appropriate to convert the thermal conductivity (k) of the material to a thermal resistance (R) value (i.e., R = 1/k) and then perform the mathematical calculation.
Thermal cycling

A fluctuation in material, system component or system stress mode, such as a change from tension to compression and back to tension, and any related displacements caused by recurring temperature fluctuations.

Thermal expansion

The increase in the dimension or volume of a body because of temperature variations.
Thermal insulation

A material applied to reduce the flow of heat.
Thermal movement

Changes in dimension of a material as a result of temperature changes.
Thermal resistance (R)

The quantity determined by the temperature difference at steady state between two defined surfaces of a material or construction that induces a unit heat flow rate through a unit area. In English (inch•pound) units, it is expressed as h•ft2•F/Btu.
Note 1: A thermal resistance (R) value applies to a specific thickness of a material or construction.

Note 2: The thermal resistance (R) of a material is the reciprocal of the thermal conductance (C) of the same material (i.e., R = 1/C).

Note 3: Thermal resistance (R) values can be added, subtracted, multiplied and divided by mathematically appropriate methods.
Thermal resistance ratio (TRR)

(1) The ratio of a material‚Äôs ‚"wet‚" thermal resistance to its air-dry thermal resistance; (2) the ratio of a material‚Äôs thermal resistance determined from conditioned sample measurements to its reference thermal resistance typically determined at equilibrium moisture content at specific temperature and relative humidity.
Thermal shock

The stress-producing phenomenon resulting from sudden temperature changes in a roof membrane when, for example, a cold rain shower follows brilliant sunshine.
Thermal stress

Stress introduced by uniform or nonuniform temperature change in a structure or material that is contained against expansion or contraction.
Thermal transmittance (U or U-factor)

The heat transmission in unit time through unit area of a material or construction and the boundary air films induced by unit temperature difference between the environments on each side. In English (inch•pound) units, it is expressed as Btu/h•ft2•F.
Note 1: A thermal transmittance (U) value applies to the overall thermal performance of a system (e.g., roof assembly).

Note 2: Thermal transmittance (U) is sometimes called the overall coefficient of heat transfer.

Note 3: Thermal transmittance (U) is the reciprocal of the overall thermal resistance (Rt) of a system (i.e., U = 1/Rt).