Insulation Materials Guide

December 1, 2007

Definition of Insulation

Insulation is defined as those materials or combinations of materials that retard the flow of heat energy by performing one or more of the following functions:

  1. Conserve energy by reducing heat loss or gain.
  2. Control surface temperatures for personnel protection and comfort.
  3. Facilitate temperature control of a process.
  4. Prevent vapor flow and water condensation on cold surfaces.
  5. Increase operating efficiency of heating/ventilating/cooling, plumbing, steam, process, and power systems found in commercial and industrial installations.
  6. Prevent or reduce damage to equipment from exposure to fire or corrosive atmospheres.
  7. Assist mechanical systems in meeting U.S. Department of Agriculture (USDA) Food and Drug Administration (FDA) criteria in food and cosmetic plants.

The temperature range within which the term “thermal insulation” will apply is from -73.3°C (-100°F) to 815.6°C (1,500°F). All applications below -73.3°C (-100°F) are called cryogenic, and those above 815.6°C (1,500°F) are called refractory.

Thermal insulation is further divided into the following three general application temperature ranges:

  1. Low-Temperature Thermal Insulation
    1. 15.6°C to 0°C (60°F to 32°F)—cold or chilled water
    2. -0.6°C to -39.4°C (31°F to -39°F)—refrigeration or glycol
    3. -40.0°C to -73.3°C (-40°F to -100°F)—refrigeration or brine
    4. -73.9°C to -267.8°C (-101°F to -450°F)—cryogenic
  2. Intermediate Temperature Thermal Insulation
    1. 16.1°C to 99.4°C (61°F to 211°F)—hot water and steam condensate
    2. 100°C to 315.6°C (212°F to 600°F)—steam and high-temperature hot water
  3. High-Temperature Thermal Insulation
    1. 316.1°C to 815.6°C (601°F to 1,500°F)—turbines, breechings, stacks, exhausts, incinerators, and boilers

Generic Types and Forms of Insulation

Insulation will be discussed in this article according to its generic types and forms. The type indicates composition (such as glass or plastic) and internal structure (such as cellular or fibrous). The form implies overall shape or application (such as board, blanket, or pipe insulation).

Insulation Types

Fibrous insulation. This type of insulation is composed of small-diameter fibers, which finely divide the air space. The fibers may be either perpendicular or horizontal to the surface being insulated, and they may or may not be bonded together. Silica, rock wool, slag wool, and alumina silica fibers are used. The most widely used insulations of this type are the glass fiber and mineral wool types of insulation.

Cellular insulation. This type of insulation is composed of small, individual cells separated from each other. The cellular material may be glass or foamed plastic, such as polystyrene (closed cell), polyurethane, polyisocyanurate, plyolefin, and elastomeric.

Granular insulation. This is composed of small nodules that contain voids or hollow spaces. It is not considered a true cellular material since gas can be transferred between the individual spaces. It may be produced as a loose or pourable material, or combined with a binder and fibers to make a rigid insulation. Examples are calcium silicate, expanded vermiculite, perlite, cellulose, diatomaceous earth, and expanded polystyrene.

Insulation Forms

Insulation is produced in a variety of forms suitable for specific functions and applications. The combined form and type of insulation determine its proper method of installation. The forms most widely used include the following:

  • Rigid boards, blocks, sheets, and preformed shapes, such as pipe insulation, curved segment, and lagging: Cellular, granular, and fibrous insulations are produced in these insulation forms.
  • Flexible sheets and preformed shapes: Cellular and fibrous insulations are produced in these forms.
  • Flexible blankets: Fibrous insulations are produced in flexible blankets.
  • Cements (insulating and finishing): Produced from fibrous and granular insulations and cement, they may be of the hydraulic setting or air-drying type.
  • Foam: Poured or froth foam used to fill irregular areas and voids. Spray used for flat surfaces.

Properties of Insulation

Not all properties are significant for all materials or applications. Therefore, many are not included in manufacturers’ published literature. In some applications, however, omitted properties may assume extreme importance (like when insulations must be compatible with chemically corrosive atmospheres.)

If the property is significant for an application and the measure of that property cannot be found in manufacturers’ literature, effort should be made to obtain the information directly from the manufacturer, testing laboratory, or insulation contractors association.

The following properties are referenced only according to their significance in meeting design criteria of specific applications. (More detailed definitions of the properties themselves can be found in the online glossary of insulation terms at

Thermal properties of insulation. The following insulation properties are the primary consideration when choosing the type and form of insulation for specific projects:

  • Temperature limits: Upper and lower temperatures within which the material must retain all of its properties.
  • Thermal conductance “C”: The rate of heat flow for the actual thickness of a material.
  • Thermal conductivity “K”: The rate of heat flow based on a 25-mm (1-inch) thickness.
  • Emissivity “E”: This is significant when the surface temperature of the insulation must be regulated, as with moisture condensation or personnel protection.
  • Thermal resistance “R”: The overall resistance of a “system” to the flow of heat.
  • Thermal transmittance “U”: The overall conductance of heat flow through an insulation system.

Mechanical and chemical properties of insulation. Properties other than thermal must be considered when choosing materials for specific applications. These properties include the following:

  • Alkalinity (pH or acidity): Significant when corrosive atmospheres are present. Insulation must not contribute to system corrosion.
  • Appearance: Important in exposed areas and for coding purposes.
  • Breaking load: In some installations, the insulation material must “bridge” over a discontinuity in its support.
  • Capillarity: This must be considered when a material may be in contact with liquids.
  • Chemical reaction: Potential fire hazards exist in areas where volatile chemicals are present. Corrosion resistance must also be considered.
  • Chemical resistance: This is significant when the atmosphere is salt or chemical laden.
  • Coefficient of expansion and contraction: This enters into the design and spacing of expansion and contraction joints and/or the use of multiple-layer insulation applications.
  • Combustibility: This is one of the measures of a material’s contribution to a fire hazard.
  • Compressive strength: This is important if the insulation must support a load or withstand mechanical abuse without crushing. If, however, cushioning or filling in space is needed as in expansion and contraction joints, low-compressive-strength materials are specified.
  • Density: A material’s density affects other properties of that material, especially thermal properties.
  • Dimensional stability: This is significant when the material is exposed to atmospheric and mechanical abuse, such as twisting or vibration from thermally expanding pipe.
  • Fire retardancy: Flame spread and smoke developed ratings should be considered.
  • Hygroscopicity: The tendency of a material to absorb water vapor from the air.
  • Resistance to ultraviolet light: This is significant if the application is outdoors.
  • Resistance to fungal or bacterial growth: This is necessary in food or cosmetic process areas.
  • Shrinkage: This is significant on applications involving cements and mastics.
  • Sound absorption coefficient: This must be considered when sound attenuation is required, as it is in radio stations, some hospital areas, etc.
  • Sound transmission loss value: This is significant when constructing a sound barrier.
  • Toxicity: This must be considered in food processing plants and potential fire hazard areas.

Major Insulation Materials

The following is a general inventory of the characteristics and properties of major insulation materials used in commercial and industrial installations.

Calcium Silicate

Calcium silicate is a granular insulation made of lime and silica, reinforced with organic and inorganic fibers, and molded into rigid forms. Service temperature range covered is 37.8°C to 648.9°C (100°F to 1,200°F). Flexural strength is good. Calcium silicate is water absorbent. However, it can be dried out without deterioration. The material is noncombustible and used primarily on hot piping and surfaces. Jacketing is field applied.


  • Fibrous. This type is available as flexible blanket, rigid board, pipe insulation and other premolded shapes. Service temperature range is -40°C to 37.8°C (-40°F to 100°F). Fibrous glass is neutral; however, the binder may have a pH factor. The product is noncombustible and has good sound absorption qualities.
  • Cellular. This type is available in board and block form capable of being fabricated into pipe insulation and various shapes. Service temperature range is -267.8°C to 482.2°C (-450°F to 900°F). It has good structural strength, but poor impact resistance. The material is non-combustible, non-absorptive, and resistant to many chemicals.

Mineral Fiber (Rock and Slag Wool)

Rock and/or slag fibers are bonded together with a heat-resistant binder to produce mineral fiber or wool available in loose blanket, board, pipe insulation, and molded shapes. Upper temperature limits can reach 1,037.8°C (1,900°F). The material has a practically neutral pH, is noncombustible, and has good sound-control qualities.

Expanded Silica (Perlite)

Perlite is made from an inert siliceous volcanic rock combined with water. The rock is expanded by heating, causing the water to vaporize and the rock volume to expand. This creates a cellular structure of minute air cells surrounded by vitrified product. Added binders resist moisture penetration, and inorganic fibers reinforce the structure. The material has low shrinkage and high resistance to substrate corrosion. Perlite is noncombustible and operates in the intermediate and high temperature ranges. The product is available in rigid, preformed shapes and blocks.


Foamed resins combined with elastomers produce a flexible cellular material. Available in preformed shapes and sheets, elastomeric insulations possess good cutting characteristics and low water and vapor permeability. The upper temperature limit is 104.4°C (220°F). Elastomeric insulation is cost efficient for low-temperature applications with no jacketing necessary. Resiliency is high. Consideration should be made for fire retardancy.

Foamed Plastic

Insulation produced from foaming plastic resins creates predominately closed-cellular rigid materials. K-values decline after initial use as the gas trapped within the cellular structure is eventually replaced by air. Check manufacturers’ data for details. Foamed plastics are lightweight with excellent moisture resistance and cutting characteristics. The chemical content varies with each manufacturer. Available in preformed shapes and boards, foamed plastics are generally used in the low and lower intermediate service temperature range -182.8°C to 148.9°C (-297°F to 300°F). Consideration should be made for fire retardancy of the material.

Refractory Fiber

Refractory fiber insulations are mineral or ceramic fibers, including alumina and silica, bound with extremely high-temperature binders. The material is manufactured in blanket or rigid form. Thermal shock resistance is high. Temperature limits reach 1,648.9°C (3,000°F). The material is noncombustible. The use and design of refractory range materials is an engineering art in its own right and is not treated fully in this article, although some refractory products can be installed using application methods illustrated here.

Insulating Cement

Insulating and finishing cements are a mixture of various insulating fibers and binders with water and cement to form a soft, plastic mass for application on irregular surfaces. Insulation values are moderate. Cements may be applied to high-temperature surfaces. Finishing cements or one-coat cements are used in the lower intermediate range and as a finish to other insulation applications. Check each manufacturer for shrinkage and adhesion properties.

For more information, please see the online version of this article at