Types of Insulation

February 1, 2012

The Mechanical Insulation Design Guide (MIDG) contains extensive information on mechanical insulation materials and their properties, as well as other design information. Below is an overview of insulation materials, their properties, and fabrication as excerpted from the MIDG; for more information, visit www.wbdg.org/midg.

Insulation Materials

Insulation materials may be categorized into one of five major types: cellular, fibrous, flake, granular, and reflective.

Cellular insulations are composed of small individual cells either interconnecting or sealed from each other to form a cellular structure. Glass, plastics, and rubber may comprise the base material, and a variety of foaming agents are used.

Cellular insulations are often further classified as either open cell (i.e., cells are interconnecting) or closed cell (cells are sealed from each other). Generally, materials that have greater than 90 percent closed cell content are considered to be closed cell materials.

Fibrous insulations are composed of small diameter fibers that finely divide the air space. The fibers may be organic or inorganic and they are normally (but not always) held together by a binder. Typical inorganic fibers include glass, rock wool, slag wool, and alumina silica.

Fibrous insulations are further classified as either wool or textile-based insulations. Textile-based insulations are composed of woven and non-woven fibers and yarns. The fibers and yarns may be organic or inorganic. These materials are sometimes supplied with coatings or as composites for specific properties, e.g., weather and chemical resistance or reflectivity.

Flake insulations are composed of small particles or flakes that finely divide the air space. These flakes may or may not be bonded together. Vermiculite, or expanded mica, is flake insulation.

Granular insulations are composed of small nodules that contain voids or hollow spaces. These materials are sometimes considered open cell materials since gases can be transferred between the individual spaces. Calcium silicate and molded perlite insulations are considered granular insulation.

Reflective insulations and treatments are added to surfaces to lower the long-wave emittance, thereby reducing the radiant heat transfer to or from the surface. Some reflective insulation systems consist of multiple parallel thin sheets or foil spaced to minimize convective heat transfer. Low emittance jackets and facings are often used in combination with other insulation materials.

Another material sometimes referred to as “thermal insulating coatings” or paints is available for use on pipes, ducts, and tanks. These paints have not been extensively tested, and additional research is needed to verify their performance.

Insulation materials or systems may also be categorized by service temperature range. Understanding that some may have a different range of service temperature, the mechanical insulation industry has generally adopted the following category definitions:

  • Cryogenic Applications: -50°F and below
  • Thermal Applications
    • Refrigeration, chilled water, and below ambient applications): -49°F to + 75°F
    • Medium to high temperature applications: +76°F to +1,200°F
  • Refractory Applications: +1,200°F and above

Selecting an insulation material for a particular application requires an understanding of the physical properties associated with the various available materials.

Use Temperature is often the primary consideration in the selection of an insulating material for a specific application. Maximum temperature capability is normally assessed using ASTM C411 or C447. These test methods involve exposing samples to hot surfaces for an extended time and subsequently assessing the materials for any changes in properties. ASTM C411 is specified when exposing the insulation material to an ambient temperature surface and then utilizing a specified heat-up cycle. ASTM C447 requires the insulation material to be installed on a surface that has been pre-heated to the maximum operating temperature. Evidence of warping, cracking, delamination, flaming, melting, or dripping are indications that the maximum use temperature of the material has been exceeded. There is currently no industry-accepted test method for determining the minimum use temperature of an insulation material, but minimum temperatures are normally determined by evaluating the integrity and physical properties of the material after exposure to low temperatures.

Thermal Conductivity is defined in ASTM C168 as 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. The term apparent thermal conductivity is used for many insulation materials to indicate that additional non-conductive modes of heat transfer (i.e., radiation or free convection) may be present.

In the insulation industry, thermal conductivity is typically expressed as the symbol k, in units of Btu·in./(h ft² °F), or ?, in units of W/(m·°C).

The apparent thermal conductivity of insulation materials is a function of temperature. Many specifications call for insulation conductivity values evaluated at a mean temperature of 75°F. Most manufacturers provide conductivity data over a range of temperatures to allow evaluations closer to actual operating conditions. Conductivity of flat insulation products is measured per ASTM Test Method C177 or C518, while the conductivity of pipe insulation is generally determined using ASTM Test Method C335. At present, ASTM does not provide a consensus test procedure for pipe insulation at below-ambient temperatures (i.e., heat flow in). Conductivity data for below-ambient applications is therefore obtained either by extrapolation from above ambient tests or via tests on flat material. In some cases, tests on flat materials have yielded lower conductivity values than tests on equivalent cylindrical materials.

A number of other terms related to thermal conductivity are sometimes used. These are not material properties, but are used to describe the thermal performance of specific products or systems. Common descriptive terms include:

  • Thermal Conductance, or C-value, is 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. For a flat board or blanket insulation, C is calculated as the thermal conductivity divided by the thickness (C=k/t).
  • Thermal Resistance, or R-value, is 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. For a flat board or blanket insulation, R is calculated as the thickness divided by the thermal conductivity (R=t/k). Thermal resistance is the inverse of thermal conductance.
  • Thermal Transmittance, or U-factor, is the heat transmission rate through unit area of a material or construction and the boundary air films, induced by a unit temperature difference between the environments on each side. Units of U are typically Btu/(h·ft²·°F).

Density is the mass per unit volume of a material. For insulation, we are normally concerned with the “bulk” or the “apparent” density of the product. Bulk density is the mass of the product divided by the overall volume occupied and is an average of the densities of the individual materials making up the product. Density is denoted by the symbol ? and expressed in units of lb/ft³ or kg/m³. Historically, density was used as a proxy for other properties of insulation (e.g., compressive resistance) and is still found in various insulation specifications. It is useful in the design of support/hanger systems where the overall weight of the system must be considered. It also becomes important in transient heat flow situations.

Specific Heat is the amount of thermal energy required to raise the temperature of a unit mass of a material by one degree. It is normally expressed in units of Btu/lb·°F or kJ/kg·°K.

Thermal Diffusivity is the ratio of the conductivity of a material to the product of its density and specific heat. It is an important property in transient situations. Generally, the lower the diffusivity, the more “thermal flywheel” in the system. Units are ft²/h or m²/s.

Alkalinity or pH describes the tendency of a material to have a basic or acidic reaction. For insulation materials, it is measured on an extract of the material in distilled water. Results are reported on the pH scale, with readings above 7.0 indicating alkaline and below 7.0 indicating acidic.

Compressive Resistance is defined as the compressive load per unit of area at a specified deformation. When the specified deformation is the start of complete failure, the property is called compressive strength. Compressive strength is measured in lb/in.² or lb/ft² and is important where the insulation material must support a load without crushing (e.g., insulation inserts used in pipe hangers and supports). When insulation is used in an expansion or contraction joint to take up a dimensional change, lower values of compressive resistance are desirable. ASTM Test Method C165 is used to measure compressive resistance for fibrous materials and ASTM Test Method D1621 is used for foam plastic materials.

Flexural Resistance of a block or board insulation product is the ability to resist bending. It is determined by ASTM C203 and is measured in lb/in.² or lb/ft². The related term flexural strength is the flexural resistance at breaking.

Linear Shrinkage is a measure of the dimensional change that occurs in an insulation material under conditions of soaking heat. Most insulation materials will begin to shrink at some definite temperature. Usually the amount of shrinkage increases as the exposure temperature becomes higher. Eventually, a temperature will be reached at which the shrinkage becomes excessive and the material has exceeded its useful temperature limit. Linear shrinkage is determined by ASTM C356, which specifies soaking heat for 24 hours.

Water Vapor Permeability is defined as 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 conditions. For insulating materials, water vapor permeability is commonly expressed in units of perm-in. A related and often confused term is water vapor permeance (in perms), which describes the water vapor flux through a material of specific thickness and is generally used to define the performance of a vapor retarder. In below-ambient applications, it is important to minimize the rate of water vapor flow to the cold surface. This is normally accomplished by using vapor retarders with low permeance, insulation materials with low permeability, or both. ASTM Test Method E96 is used to measure the water vapor transmission properties of insulation materials.

Water Absorption is generally measured by immersing a sample of material under a specified head of water for a specified time period. It is a useful measure when considering the amount of liquid water that may be absorbed due to water leaks in weather barriers or during construction. Water absorption is measured by a number of different immersion methods (ASTM C209, ASTM C240, ASTM C272, and ASTM C610). These methods differ in length of immersion time (from 10 minutes to 48 hours), reported units (percent by weight or percent by volume), and requirements for both pre-conditioning (i.e., heat aging) and post-conditioning (specimen draining and pat-off). These differences make direct comparison of water absorption data difficult.

Water Vapor Sorption is a measure of the amount of water vapor sorbed (either by absorption or adsorption) by an insulation material under high-humidity conditions. The test procedure (ASTM C1104) involves drying a sample to constant weight and then exposing it to a high humidity atmosphere (120°F, 95 percent RH) for 96 hours.

Wicking is the infiltration of a wetting liquid into a material by capillary action. For insulation materials, wicking of liquid water is undesirable because it can degrade the properties of the insulation. Wicking is measured by ASTM C1559, which involves inserting insulation samples in a pan of liquid water and measuring the capillary rise after a 1-week period.

The MIDG offers the Performance Property Guide for Insulation Materials, which allows you to enter the desired operating temperature and compare various insulation materials that can be used at that temperature and their physical properties. The tool is at www.wbdg.org/design/midg_materials.php#ppgim.

Insulation Fabrication

Most mechanical insulation systems require some degree of fabrication, depending on the complexity of the job and the materials used. Some mechanical insulation products can be ordered directly from insulation manufacturers in standard sizes with factory-applied facings. These products still require some fabrication in the field to accommodate valves and fittings, etc. Other insulation materials (i.e., cellular glass, phenolic, polyisocyanurate, and polystyrene) are manufactured in relatively large blocks or buns and must be cut into the appropriate size and shape. While this fabrication is sometimes done in the field, the work is more often done by insulation fabricators and/or distributor/fabricators that specialize in this work.

Insulation fabrication standards and guidelines are generally developed by the insulation manufacturers based on experience with their products. These standards provide specifics on the dimensions to be used and the allowable tolerances. In some cases, industry standard specifications are available. They are:

  • ASTM C585 Standard Practice for Inner and Outer Diameters of Rigid Thermal Insulation for Nominal Sizes of Pipe and Tubing
  • ASTM C450 Standard Practice for Fabrication of Thermal Insulating Fitting Covers for NPS Piping, and Vessel Lagging
  • ASTM C1639-07 Standard Specification for Fabrication of Cellular Glass Pipe and Tubing Insulation.

Specifiers are encouraged to use these standards when specifying fabricated products, or to specify fabrication to the insulation manufacturers’ instructions. This can be particularly important in regard to the type of adhesives used.


A wide variety of insulation materials, facings, and accessory products are available for use on mechanical systems. The list changes continuously as existing products are modified, new products are developed, and other products are phased out. The task for the insulation system designer is to select the products or combination of products that will satisfy the design requirements at the lowest total cost over the life of the project. In most cases, the designer will find there are a number of products or systems that will work, and the final choice will depend on cost, availability, or other considerations.