Propertis of Thermal Insulation Materials.

Abhijit.S.Musale

www.abhijitsmusale.com | August.2025

---

---

Introduction

Insulation materials have been part of industrial development since the dawn of civilization. Prehistoric people have been using naturally available insulation materials such as animal furs, skins, wool, eel grass, clay and straw, dried manure, etc… to protect themselves from extreme weather conditions. As human civilization developed and industrial age begin, artificial insulation materials developed to meet various industrial requirements. Today mineral wool, calcium silicate, fiber glass, Perlite, cellular glass, foam glass, polyurethane foam etc… are widely used insulation materials in industry. Although there are many kinds of insulation materials, they have some common properties through which they are compared and selected.

This article shall mention the some of the important properties of insulation materials. The importance of each property depends on the use of insulation. Some properties might be irrelevant for certain applications and some might be very essential. Following table gives the list of insulation material properties.

Maximum Usage Temperature

It is the maximum temperature at which insulation material can be used safely without loosing its structural integrity, chemical composition, shape and strength. No exact value can be placed for temperature upto which use of the material is allowed. It depends on many factors like the purpose and application of insulation, the grade and material composition, heating rate, thickness, etc… Maximum usage temperature should be clearly mentioned by the manufacturer for their specific grade of product.

ASTM C447 (Standard Practice for Estimating the Maximum Use Temperature of Thermal Insulation) mentions the guidelines of how to determine and test the maximum temperature limit of insulation. This standard can be used for all type of insulation like blankets, lose wool, mineral wool, block, board type, molded pipe sections etc… there is no specific criteria defined in this standard through which specific temperature can be set for specific material. Material needs to be tested against the application temperature and it should show acceptable behaviour. If the material gets distorted, damaged or cannot withstand temperature for desired service duration then lower temperature can be determined as usage temperature. Usage temperature needs to be confirmed by the manufacturer and agreed with the user.

Thermal Conductivity

The thermal conductivity of the insulation material is important factor in calculating insulation thickness. It is a property of material which indicates its ability to transfer the heat. The more conductive the insulation material more thickness requires to contain the heat. There are multiple testing standards available to test the thermal conductivity of the material based on their shape, geometry. Some of the standards are mentioned below.

ASTM C177 (Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus) is uses basically for insulation materials shaped in a flat boards and mats as it uses guarded hot plate method. Numerous sources can be found on internet about how this guarded hot plate method works, its procedures, apparatus, preparation of specimen, etc… hence its details are not mentioned here. Reader is to refer these standards for exact details

ASTM C335 (Standard Test Method for Steady-State Heat Transfer Properties of Horizontal Pipe Insulation) is used for the insulation materials shaped in the form of pipe covers and used for pipe insulation. The apparatus used for this type of testing is modified version of guarded hot plate apparatus only but it has cylindrical shape.

ASTM C518 (Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus) is another standard that gives guidelines to determine thermal transmission properties.

Density

Density of the material is defined as the mass of the material per unit volume. Measured in kg/m3. In case of some insulation materials high density material also has high conductivity. Which in turn requires higher thickness to contain heat. So, high density materials increases overall weight of the insulation used. In such case less density is always preferable. However there is no rule that says less density means less thermal conductivity. It entirely depends on material.

ASTM C302 (Standard Test Method for Density and Dimensions of Preformed Pipe-Covering-Type Thermal Insulation) gives the test method to determine the density of insulation material. As the name suggests it is method for pipe insulation covers. Basically it mentions methods and procedures for how to take measurements of specimen (mainly dimensions to calculate volume) and what tolerance should be considered, measurements of weight (mass), formulas to determine volume of molded sections, how to produce reports and what precision and bias should be considered. It is rather easy, basically you are measuring volume and mass of the specimen and arriving at its density

Friability / Mass lose by tumbling

This is a property of solid insulation material which shows its tendency to break into smaller pieces under the tumbling, cyclic loads, rubbing, or expansion and contraction due to thermal strains. Material with high friability looses mass over the long service duration due to many cycles of dynamic thermal movements of hot surfaces and so insulation looses its effectiveness. Friability test most commonly used in the pharmaceutical industry to measure the Friability of tablets. Machine used for this test is shown in picture below.

ASTM C421 (Standard Test Method for Tumbling Friability of Preformed Block-Type and Preformed Pipe-covering-Type Thermal Insulation) mentions method to measure this property. Similar machines used for the friability test of insulation material, however instead of circular vessel shown in the machine above C421 mentions square box with specific dimensions. Specimen of insulation material of specific dimensions are kept in this box along with some wooden cubes of specific dimensions. This box is then rotated at certain specific numbers of rotation with specific RPMs. After that largest of the remaining specimens are taken out and measured for their mass. Lose of mass is recorded in percentages which gives us the reading of friability of material. For specific details, calculation, methods and procedure please refer the standard document.

Sag Resistance (Thickness Change)

Some of the insulation materials (particularly fibrous materials such as mineral wool, glass wool, etc) shows thickness change over the long duration of service. Their shapes distorts due to external factors such as changing loads, thermal expansion and contraction of insulation surface. Due to this phenomena the required thickness no longer remains available to content the heat. Which results in non effectiveness of the insulation material and lose of heat. The property of material that keeps the material in shape and resists sagging is called Sag resistance.

Figure below shows sagging of insulation material around pipe section. Initially when insulation is newly installed there is uniform thickness of insulation all around the pipe but after certain long duration of service this thickness does not remain uniform around the pipe.

ASTM C167 (Standard Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations) gives the test method to measure thickness of insulation. Simple depth gauge with sharp end point which can penetrate through insulation and touch the hard surface can be used to measure the insulation thickness at various locations on the surface. This standard gives information about methods, procedures, sampling techniques, gap between measurement locations, precision and tolerances that should be considered while measurement, calculation methods and so on. Please refer the standard document for detailed information.

Compressive Strength

Some insulation material such as calcium silicate and perlite are hard materials and they have compressive strength. At high forces these materials break. The ability of these insulation material to withstand compressive loads is called compressive strength.

ASTM C165 (Standard Test Method for Measuring Compressive Properties of Thermal Insulation) gives method to measure the compressive strength of insulation material. Basically test procedures are mentioned for two type of insulation materials. One is for the materials that shows almost linear relationship on load vs deformation diagram, means as you increase load the deformation increases in same proportions. And the other is for materials that become more stiff as you increase the load. Fibrous materials such as rock wool, glass wool show this kind of behaviour. Test is simple, specimen of specific dimensions and conditions are placed on compressive test machine that can apply load on specimen in controlled manner. It could be hydraulic machine or mechanical one. Load is applied on the specimen till desired deformation point up to which it can be used. Readings on machine can be used to make the load vs deformation curve for that material and its compressive strength is determined.

Flexural Resistance/ Breaking Load

It is an ability of insulation material to resist bending or breaking. Hard insulation materials such as calcium silicate, perlite, cellular glass etc… have this kind of property. Lose insulation material such as mineral wool and glass wool does not have this property.Below image shows the testing machine that is used to measure the Breaking strength of the insulation material. The dark gray slab on the machine is insulation material (Probably foam glass).

ASTM C203 (Standard Test Methods for Breaking Load and Flexural Properties of Block-Type Thermal Insulation) gives the test methods to measure this property. There are two test methods(Method I , Method II) and four procedures ( Procedure A, B, C, D) mentioned in this standard. Either of the two methods can be used with these four procedures. Difference between the two methods is the way specimen is being loaded on the bending test machine. And the difference between the four procedures is type of material is being tested. Procedure A is used for material that show very small deflection and break soon. Hard insulation materials such as calcium silicate and perlite should be tested with this procedure. Procedure B is used for material that very large deflections before breaking. Procedure C measures insulation material properties at the constant stress rate where as Procedure D measures material properties at constant strain rate. Please refer the standard for detailed procedures and calculations.

Linear Shrinkage

It is a property of material that causes dimensional change when brought under thermal loading. Preformed insulation material which are made into specific shapes in order to fit on the equipment and complicated surfaces shows this kind of behaviour. When exposed to heat this material shrink deform.

ASTM C356 (Standard Test Method for Linear Shrinkage of Preformed High-Temperature Thermal Insulation Subjected to Soaking Heat) gives the test method to measure the linear shrinkage of the insulation material. It involves preparation of specimen of the insulation material in specific dimension and characteristic and heat that specimen in to the furnace up to certain temperature. As the specimen soaks the heat linear shrinkage develops in the material. Different materials shows different degree of shrinkage. Once this material is brought out of the furnace and cooled, measurements are taken, and compared with the measurements before putting it into the furnace. To know specific details of how to prepare specimen, take measurement, arrangement of apparatus, test temperatures, calculation methods, precision and tolerances, please refer the standard document.

Hot surface performance: Warpage and Cracking

It is a property of material that causes material shape to twist and distort and/or develop cracks on the surface due to heat exposure. Warpage is different from the shrinkage. In shrinkage the dimensions of the material change and the volume of the material reduces. Where as in warpage the overall volume of the specimen remains same but its shape gets distorted. Figures below explains warpage in rectangle block. First figure shows normal specimen but remaining two figures shows warped specimen.

Cracking on the surface of insulation material can also be formed due to high exposure of to heat. Figure below shows surface cracking on the specimen.

ASTM C411 (Standard Test Method for Hot-Surface Performance of High-Temperature Thermal Insulation) gives specific information about how to carry out hot surface performance test, preparation of specimen, apparatus settings, method to take readings, tolerances and biases, etc… for details please refer standard document.

Surface Burning Characteristics: Flame Spread Index

It is a measurement of the burning behaviour of materials by observing the flame spread across the material surface.

ASTM E84 (Standard Test Method for Surface Burning Characteristics of Building Materials) is the standard that describe the test method to determine this property. It basically determines how fast and how far flame spreads once the material ignited. This test is also called tunnel test. It involves specimen placed in a ceiling of the box and then it is ignited at one end. Flame then spreads on the bottom surface of the specimen. Flame spread index is calculated based on in how much area the flame spreads in given time. For more details, formulas, calculations, setup, methods and procedure refer standard document.

Surface burning Characteristics : Smoke Density Index

Comparative measure derived from smoke obscuration data collected during the test for surface burning.

ASTM E84 (Standard Test Method for Surface Burning Characteristics of Building Materials) which gives the information about the flame spread index also gives the information about the smoke development index. As mentioned above the specimen is placed in a box ceiling for flame spread index measurement. This box also has light source and photo cells which can sense the intensity of light. Once the material is ignited and smoke is developed this light source gets obscured and photo cells registers time vs light intensity data. Based on this data the curve is created and area under that curve is compared with other reference material’s curve, and with use of some formulas it gives you the flame spread index. So basically smoke index gives you the the idea about how quickly you will lose light due to smoke if the material is ignited. It does not tell anything about the toxic nature of the smoke or how it will affect your lungs. That entirely depends on chemical properties of burning material.

Water Vapor Permeability

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 at specific temperature and humidity conditions is called water vapor permeability.

ASTM E96 (Standard Test Methods for Water Vapor Transmission of Materials) gives the detailed information on how to carry out the test to measure the permeability of material. Basically it involves two kinds of simple tests. Desiccant test and water test. Either of the two can be used to determine the permeability. It involves placing desiccant or water in a small vessel and cover the top of that vessel with the material you need to test and then keep observing that vessel’s weight for some hours or some dates. Weight is measured at certain intervals of time. In case where the desiccant is used in experiment the weight of the vessel will gradually increase with time if the test material is highly permeable of water vapor. In case where water is used, the weight of vessel will gradually decrease with time. Weight vs time data gives information about how much vapor has passed through the test material. For specific details such as how to prepare specimen, how to set up the apparatus, how to control temperature and humidity, formulas and calculations please refer the standard document.

Water Vapor Sorption / Capillarity / Absorbtivity

Water Vapor Sorption is a measure of the amount of water vapor sorbed by an insulation material under high-humidity conditions.

ASTM C1104 (Standard Test Method for Determining the Water Vapor Sorption of Unfaced Mineral Fiber Insulation) gives the test method to determine absorptivity of the material. This test method is only applicable for the unfaced fibrous insulation materials such as mineral wool. Hear one should keep in mind that the absorption is different than permeability. In absorption the material absorbs other material (gas or liquid in solid) and hold it inside it and their by its mass changes. Where as in permeability the gas or vapor passes through the material without affecting it weight. Highly absorbtive insulation material will become heavier after absorbing vapor where as permeable material will let the moisture pass through the material without getting heavier.

Test method mentioned in C1104 is simple. You prepare specimen as per the dimensions and procedure mentioned in the standard. Then place that specimen in highly humid environment for several hours or days. Periodically take the measurement of its weight and prepare time vs weight chart. After test duration you will notice that the specimen becomes heavy by absorbing vapor. Change in weight in % shows the absorbtivity of the material. For detail calculations and exact method thorough which observations should be made please refer the standard document.

Moisture content

Measure of a moisture content in insulation material. This is different from the absorbtivity. Moisture content is the property that shows how much moisture is present in the material it self. This kind of materials comes with some small % of their weight as moisture content or volatile matters which gets evaporated once you heat them till certain temperature.

ASTM C1616 (Test Method for determining the Moisture content of Organic and inorganic inslation materials by weight.) explain the details of the test to determine moisture content. Basically it is very simple test like absorbtivity test but only in opposite procedure. Here you prepare a specimen as specified in the standard and note its initial dimensions and weight, then place that specimen in oven and heat it up to certain temperature. After it has cooled down you again measure its weight and dimensions and the difference between the readings gives the moisture content value. For details refer the standard document.

Odor Emissions

Material’s property to prevent generation of foul odors due to chemical degradation or the microbial growth over long duration of service is very important property for applications where human health is of primary concern. Applications home and hospital building insulation should not develop odor as they get old. Odor can be generated inside insulation because of fugal or bacterial growth over time, it also could be because degradation of certain chemicals within insulation material.

ASTM C1304 (Standard Test Method for Assessing the Odor Emission of Thermal Insulation Materials) gives the procedure as to how this test should be carried out. Odor or smell is not a measurable property so it involves experienced judgement from more then one individuals. As per standard approx 3 to 5 individuals/ inspectors are appointed for this test. Specimen of specific size are made and placed in clean container. This container then passed to each inspectors one by one. They need to open the container, hold it at specified distance from nose for specified duration of time, and then close the container. And after that they need to give written or verbal observation or judgement. Easy… right? For more details refer the standard.

Chemical Reaction / Alkalinity / pH

Properties of the material to react with chemical, gases or hot surface. Where corrosion protection is stringent requirement this properties needs to be taken in to consideration. How the material reacts with the different chemicals is determined by this property. It basically describes whether material has acidic or basic tendency. When material comes in contact with water it makes water acidic or basic based on its chemical composition and this is measured in terms of its pH value. Now, when water that has come in contact with material and turns in to mild acidic, this water afterwards comes in contact with other metal of the equipment and pipe and produces corrosion. Hence it is very important to know chemical properties of insulation material.

ASTM C871 (Standard Test Methods for Chemical Analysis of Thermal Insulation Materials for Leachable Chloride, Fluoride, Silicate, and Sodium Ions) describes the methods determine this property of the insulation material. Multiple methods and procedures has been mentioned for each type of ions (fluorides, chlorides, silicates, and sodium) and description of each of those method is quite a lengthy and not necessary here. It will be sufficient to say the presence of each type of ions in the insulation is tested by electrochemical, chemical reactions, spectroscopy, flame photometer and other many methods. Reading through this standard is very enjoyable experience as you get to know about many chemical properties of these substances. For specific numbers, formulas and information please refer the standard document.

Resistance to sound or Sound Transmission Lose Value

Ability of insulation material to absorb sound waves. This properties are very useful while designing acoustic insulation for rooms, theater, enclosures to noisy plant equipment, etc… Design criteria of various equipment usually specifies noise limit around 70 to 100 dB. Where insulation also need to work as a sound barrier their this property becomes very important.

ASTM E90 (Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements) gives specifics of the test method to determine material’s sound resistance properties. This is very elaborate standard document mentioning how the room for the test should be arranged, where the sound source should be, where the microphones should be arranged, where and how the specimen should be placed, how the observations should be taken and calculations should be to measure material’s sound resistance property. For further details refer the standard document.

---