Learning the vocabulary is the first step to truly understanding any subject, and mechanical insulation is no exception. Specialized terms abound in the industry, some of which are frequently misunderstood or misused, and some that people may find confusing. Technical experts who are members of the National Insulation Association have compiled the following list of definitions to help you better understand mechanical insulation.

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 less per coat.

Facing

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

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.

Lagging—Insulation

A block material for insulating tanks and boilers, usually curved or tapered; can be made from any of several insulation materials.

Lagging—Jacketing

Jacketing installed over insulation.

Jacket

A covering installed over insulation.

Conditioned Space

Building area supplied with conditioned air that is heated or cooled to a certain temperature and may be mechanically controlled to provide a certain humidity level.

Unconditioned Space

An unconditioned space is one without conditioned air.

Mean Temperature

Sum of the cold surface temperature and the hot surface temperature divided by two. (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.

Apparent Thermal Conductivity

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

Conductivity, Thermal

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.

C-Value (Thermal Conductance)

A measure of the rate of heat flow for the actual thickness of a material (either more or less than 1 inch), 1 square foot in area, at a temperature difference of 1°F. If the K-value of a material is known, the C-value can be determined by dividing the K-value by the thickness. The lower the C-value, the higher the insulating value.

K-Value (Conductivity)

The measure of heat in Btus that pass through one square foot of a homogeneous substance, 1 inch thick, in an hour, for each degree F temperature difference. The lower the K-value, the higher the insulating value.

Apparent Thermal Resistivity

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

R-Value (Resistance)

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.

Resistance, Thermal, 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.

Resistivity, Thermal, r

The quantity determined by the temperature difference, at steady state, between two 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 ft² F/Btu in.)

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.

Moisture Barrier

A plastic coated paper or polymeric coating applied to the inner surface of a metal jacket for the purpose of reducing corrosion of the jacketing.

Note: moisture barriers are not water vapor retarders.

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 two specific surfaces, under specified temperature and humidity. Permeability is measured in the IP system in perm inches.

Water Vapor Barrier

See Water Vapor Retarder.

Water Vapor Retarder

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

Water Vapor Retarder (Barrier)

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

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.

Weather Vapor-Retarder (barrier)

A vapor retarder that also protects from atmospheric conditions.

For more definitions, consult the Insulation Science Glossary. You’ll also find other handy reference tools throughout the Techs & Specs section of Insulation.org.