Having a strong knowledge of the terminology in the insulation industry is the foundation upon which a more complex understanding is built. This article reviews and defines some of the basic terms that are vital to a more nuanced understanding of insulation systems.

TYPES OF INSULATION

Cellular Insulation

Insulation composed of small, individual cells separated from each other. The cellular material may be glass or plastic such as polystyrene, polyurethane, polyisocyanurate, or elastomeric.

Fibrous Insulation

Insulation composed of small-diameter fibers that finely divide the air space. Fibers used are silica, rock wool,
slag wool, or alumina silica.

Granular Insulation

Insulation composed of small nodules that contain voids or hollow spaces. The material may be calcium silicate, diatomaceous earth, expanded vermiculite, perlite, cellulose, or microporous insulations.

TEMPERATURE TERMS

Ambient Temperature

The average temperature of the medium, usually air, surrounding the object under consideration.

Application Temperature Limits

Minimum and maximum temperatures between which it is usually safe to service finishes, adhesives, and sealants without endangering the integrity of the material.

Mean Temperature

The sum of the cold surface temperature and the hot surface temperature, divided by 2. (Thermal conductivity charts are calculated to use mean temperatures.)

Service Temperature Limits

The temperature to which the jacket or coating may be subjected when applied over insulation. It does not refer to the operating temperature of the equipment, vessel, or pipe.

COMMON INSULATION VALUES

Apparent Thermal Conductivity

A thermal conductivity assigned to a material that exhibits thermal transmission by several modes of heat transfer, resulting in property variation with specimen thickness or surface emittance. (See Conductivity, Thermal.)

Apparent Thermal Resistivity (R-value)

A thermal resistivity assigned to a material that exhibits thermal transmission by several modes of heat transfer, resulting in property variation with specimen thickness or surface emittance. (See Resistivity, Thermal.)

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.

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.

F-Rating

A rating usually expressed in hours, indicating a specific length of time that a fire-resistive barrier can withstand fire before being consumed or permitting the passage of flame through an opening in the assembly, as determined by ASTM E 814 (UL 1479).

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.)

Thermal Capacity

The quantity of heat required to change the temperature of the body 1 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 Properties of Insulation

Usually expressed as C-value, K-value, R-value, and U-value.

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 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 – degree F temperature difference), and the SI units are W / (sq m – degree C temperature difference).
Note: This heat transmission rate has been called the overall coefficient of heat transfer.

TERMS RELATED TO FINISHING ON INSULATION SYSTEMS

Bands

Strapping used to fasten insulation and/or jacketing in place.

Coating

A liquid or semi-liquid that dries or cures to form a protective finish, suitable for application to thermal insulation or other surfaces in a dry thickness of 30 mils or fewer per coat.

Facing

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

Film (Wet)

The applied layer of mastic or coating before curing or drying.

Finish (AS RELATED TO INSULATION METAL JACKETING)

The texture of a metal surface.

Finishes

Jackets, mastics, or strong films used for aesthetics or to protect insulation from 1 or more of the following: weather, mechanical, and/or personnel abuse.

Jacket (AS RELATED TO INSULATION JACKETING)

A protective covering installed over thermal insulation.

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 Adhesives

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. It 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. (SEE JACKET.)

Mastic

A protective coating applied by spray or trowel to weatherproof or otherwise prevent deterioration of the insulation to which it is applied.

Securement

Any device, wire, strap, or adhesive used to fasten insulation into its service position and hold it there.

Seal

To make water-tight or airtight.

Sealant

Sealants in insulation function primarily as water and vapor seals. They may also be used as adhesives and for expansion joints for metal, masonry, cellular glass, etc. They must exhibit low shrinkage, excellent adhesion, and permanent flexibility.

Sealer

A liquid coating used to prevent excessive absorption of finish coats into porous surfaces.

WATER AND MOISTURE TERMS

Moisture Barrier (AS RELATED TO INSULATION JACKETING)

A polymeric film or coating applied to the inner surface of metal jacketing for the primary purpose of reducing electrolytic, pitting, or crevice corrosion of the jacketing. Discussion—moisture barriers are not water vapor barriers or water vapor retarders.

Moisture Retarder (AS RELATED TO INSULATION JACKETING)

A layer of plastic film or other material applied to the inner side of metal jacketing to inhibit jacket corrosion by interfering with the formation of a galvanic cell between the dissimilar metals of the pipe and jacket or preventing crevice corrosion.

Discussion—A moisture retarder is not an insulation system water vapor retarder and does not perform the same function.

Water Absorption

The increase in weight of a material expressed as a percentage of its dry weight or volume after immersion in water for a specified time.

Water Vapor Permeability

The time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between 2 specific surfaces under specified temperature and humidity. Water vapor permeability is measured in the ingress protection (IP) system in perm inches.

Water Vapor Permeance

The time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between 2 specific surfaces under specified temperature and humidity conditions. Water vapor permeance is measured in IP systems in units of perm.

Water Vapor Pressure

The pressure of water vapor at a given temperature; also the component of atmospheric pressure contributed by the presence of water vapor.

Water Vapor Resistance

The steady-state vapor pressure difference that induces unit time rate of vapor flow through unit area of a flat material (or construction that acts like a homogeneous body) for specific conditions of temperature and relative humidity at each surface.

Water Vapor Resistivity

The steady-state vapor pressure difference that induces unit time rate of vapor flow through unit area and
unit thickness of a flat material (or construction that acts like a homogeneous body) for specific conditions of temperature and relative humidity at each surface.

Water Vapor Retarder (Barrier)

A material or system that significantly impedes the transmission of water vapor under specified conditions.

Water Vapor Retarder Jacket

Any material or composite meeting the requirements of a water vapor retarder and used for the jacketing of insulation material. It may be factory furnished or field applied and may or may not be adhered to the insulation material.

Water Vapor Transmission Rate (WVTR)

The steady-state water vapor flow in unit time through unit area of a body, normal to specific parallel surfaces, under specific conditions of temperature and humidity at each surface. The I-P units are lbs / hr – ft2; the SI units are grams / hr – m2.

Weather/Vapor Retarder (Barrier)

A vapor retarder that also protects from atmospheric conditions.

This brief introduction to insulation terminology is a starting point for those looking to develop their knowledge of insulation and mechanical systems. For a full listing of insulation terms, visit NIA’s Insulation Science Glossary. The Insulation Glossary is located in the Specs & Codes portion of NIA’s website, which contains other valuable resources, including the Insulation Materials Specification Guide that provides the physical and material properties of various insulation materials as specified by the ASTM. These tools can help beginners and more knowledgeable professionals develop their understanding of insulation and its properties.