The National Insulation Association (NIA) publishes a glossary of insulation science terminology, which is updated quarterly by NIA’s Technical Information Committee. Below is a list of new and commonly used terms. Visit insulation.org/about-insulation/system-design/techs-specs to download the full glossary or see NIA’s other technical resources.

Thermal Properties of Insulation
Usually expressed as C-value, K-value, R-value, and U-value.

Conductance, Thermal, C-value
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.

Conductivity, Thermal (K-value)
The measure of heat that passes through a unit area of a homogeneous substance, through a unit thickness, in a unit of time, for each unit temperature difference.
The lower the K-value, the higher the insulating value. Note: I-P units are Btu – in / hr – ft2 – °F and typical SI units are Watts / m – °C.
Definition Two: The time rate of steady heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.

Resistance, Thermal, (R-value)
A measure of the ability to retard heat flow rather than the ability to transmit heat. R-value is the numerical reciprocal of “U” or “C,” thus R = 1/U or 1/C.
Thermal resistance R-value is used in combination with numerals to designate thermal resistance values: R-11 equals 11 resistance units. The higher the “R,” the higher the insulating value. The I-P units are °F – ft2 – hr / Btu; the SI units are °C – m2 / W.

Resistivity, Thermal, r
The quantity determined by the temperature difference, at steady state, between 2 defined parallel surfaces of a homogeneous material of unit thickness, that induces a unit heat flow rate through a unit area. (r in SI units: m K/W.) (r in inch-pound units: h ft F/Btu or, h ft ² F/Btu in.)

Transmittance, Thermal (U-value)
The combined thermal value of all the materials in an insulation system, air spaces, and surface air films. The heat transmission in unit time through the unit area of a material or construction and boundary air films, induced by unit temperature difference between the environments on each side. The I-P units are Btu / (hr – sq ft – deg F temperature difference) and the SI units are W / (sq m – deg C temperature difference). Note: This heat transmission rate has been called the overall coefficient of heat transfer.

Thermal Capacity
The quantity of heat required to change the temperature of the body one degree. For a homogeneous body, it is the product of mass and specific heat. For a non-homogeneous body, it is the sum of the products of mass and specific heat of the individual constituents. (May also be seen as heat capacity.)

Thermal Insulation
Definition 1: Insulation applicable within the general temperature range of –300°F to 1800°F.
Definition 2: A material or assembly of materials used to provide resistance to heat flow.

Thermal Insulation System
Applied or installed thermal insulation complete with any accessories, vapor retarder, and facing required.

Transference, Thermal
The steady-state heat flow from (or to) a body through applied thermal insulation and to (or from) the external surroundings by conduction, convection, and radiation. It is expressed as the time rate of heat flow per unit area of the body surface per unit temperature difference between the body surface and the external surroundings.

Transmission, Heat
The quantity of heat flowing through unit area due to all modes of heat transfer induced by the prevailing conditions.

Cladding (as Related to Insulation Jacketing)—Synonymous with Jacketing
Discussion—The three terms “jacketing,” “lagging,” and “cladding” are considered synonymous in most metal jacket related applications and geographies. However, in some cases in the power industry in North America the term “lagging” has a different meaning than “jacketing” or “cladding” and refers specifically to a heavier gauge of jacketing.

Jacket (as Related to Insulation Jacketing)
A protective covering installed over thermal insulation.

Facing
A thin covering adhered to the surface of insulation prior to field installation.

Finish (as Related to Insulation Metal Jacketing)
The texture of the metal surface.

Finishes
Jackets, mastics, or strong films used for aesthetics or to protect insulation from at least one or more of the following: weather, mechanical, and/or
personnel abuse.

Lag
(v.) To apply lagging. (n.) A single piece of covering material.

Lagging (as Related to Insulation Jacketing) Synonymous with Jacketing
Discussion—The 3 terms “jacketing,” “lagging,” and “cladding” are considered synonymous in most metal jacket related applications and geographies. However, in some cases in the power industry in North America the term “lagging” has a different meaning than “jacketing” or “cladding” and refers specifically to a heavier gauge of jacketing.

Lagging Adhesive
Water-based resin emulsion products that are used to adhere lagging cloth to the insulation and to itself at the lap joints. They also seal and size the fabric and shrink it tightly to the surface. They can be brushed or sprayed.

Lagging—Insulation
Definition 1: A block material for insulating tanks and boilers, usually curved or tapered, and can be made from any of several insulation materials.
Definition 2: Insulation used on pipe, tanks, ducts, vessels, or other mechanical equipment. Discussion—Lagging insulation is usually applied in the form of cut, pieced together, or mitered parts.

Lagging—Jacketing
Jacketing installed over insulation.  Also see “Jacket.”

Laminate
A product made by bonding together 2 or more layers of material or materials.

Laminate Jacket
A thin, flexible sheet material intended for use as a jacket over thermal insulation on pipe, duct, or equipment, and consisting of multiple layers of polymer film and aluminum foil bonded together.
Discussion—A laminate jacket is available with or without a factory applied pressure sensitive adhesive.
Discussion—Laminate jacket is commercially available in different widths, it typically is provided in approximate widths of pipe insulation sections.

Laminate Tape
A thin, flexible sheet material intended for use as a tape to seal and secure a laminate jacket over thermal insulation on pipe, duct, or equipment.
Discussion—Laminate tape always has a factory applied, pressure sensitive adhesive which first requires removal of a release liner.
Discussion—Laminate tape is commercially available in several different widths.
Discussion—A laminate tape can also include a polymer coating as a top surface.

Lap Adhesive
The adhesive used to seal the butt joints and laps of insulation jackets.

ASJ: All Service Jacket; (Traditional Paper Type)
A white, flexible reinforced lamination with Paper as exposed layer which is used as a vapor retarder and finish for pipe, tank, and equipment insulation.

ASJ: All Service Jacket; (Polymeric Film Type)
A white, flexible reinforced lamination with Polymeric film as exposed layer which is used as a vapor retarder and finish for pipe, tank, and equipment insulation.

Flexible Cellular Material
N—A cellular material that will not rupture within a specified time when bent around a mandrel at a specified uniform temperature and rate.
Discussion—Test Methods D3574 “Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams” provides a standard procedure for assessing whether an insulation material is a flexible cellular material.

Slag Wool
A mineral wool made usually from molten blast-furnace slag by the action of jets of steam under high pressure.

Emissivity
Emissivity is for pure materials that are perfectly smooth. The ratio of the radiant flux given off (emitted) by a surface to that given off (emitted) by a blackbody at the same temperature and under the same conditions.

Emittance
Emittance is for rough and contaminated surfaces (practical surfaces). The ratio of the radiant flux given off (emitted) by a surface to that given off (emitted) by a blackbody at the same temperature and under the same conditions.

Emittance, Directional
The ratio of the radiance from a surface in a particular direction to the radiance from a blackbody at the same temperature under the same conditions.

Emittance, Hemispherical
The average directional emittance over a hemispherical envelope covering a surface.

Emittance, Spectral
An emittance based on the radiant energy emitted per unit wavelength interval (monochromatic radiant energy).

Emittance, Total
An emittance that is an integrated average over all wavelengths of radiant energy emitted.

Absorptance
The ratio of the radiant flux absorbed by a body to that incident upon it.

Absorption
Transformation of radiant energy to a different form of energy by interaction with matter.

Adsorption
Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule.

Sorption
Sorption refers to the process in which one substance takes up or holds another (by either absorption or adsorption).

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