The materials which control the transmission of heat and cold and offer resistance to reflection and transmission of sound and electricity are known as insulating materials.
While designing and constructing public and residential buildings; much importance needs to be given to heat, sound and electrical insulation.
In order to provide/promote comfortable living, safety, efficiency of work and auditory of the building; it is imperative to provide good thermal, acoustical and electrical conditions.
a. Heat/thermal insulation is provided to control the transmission of heat or cold so as to maintain the proper temperature inside the rooms of the building.
b. Sound insulation is provided to control the disturbances due to noise and to remedy the acoustical defects as to achieve the best sound effects.
c. Electrical insulation is provided to separate the electrical conductors from other bodies to prevent the escape of electricity.
1. Classification of insulating materials
The insulating materials may be classified as follows:
1. Heat insulating materials (or heat or thermal insulators)
2. Sound insulating materials (or sound insulators)
3. Electrical insulating materials (or electrical insulators).
1.1. Heat Insulating Materials
1. The function of a thermal or heat insulator is to resist the flow of heat through its body.
2. The heat-insulating materials are required to grant protection against heat and cold.
It may also be employed to prevent either the flow of heat from a heating furnace to the surrounding atmosphere or the ingress of heat from the environment to the plant operating at a lower temperature.
3. These materials are generally porous and their properties are governed not only by their porosity but also by nature of pores, their distribution, size and whether they are open or closed.
The materials with a great number of fine, closed and air-filled pores are the best heat-insulating materials.
4. The bulk density of heat-insulating materials is usually below 7000 N/m3 and their coefficient of thermal conductivity does not exceed 0.75 KJ per in hr°C.
5. The heat-insulating materials should be protected against moisture (since the coefficient of thermal conductivity of water is about 25 times higher than that of air).
Requirements of a Heat Insulating Materials
The main requirements of good heat-insulating materials are:
1. Thermal stability
2. Chemical stability
3. Physical stability
4. Low thermal conductivity
5. Resistance to moisture
6. Low specific heat
7. Low specific gravity
9. Resistance to vibration and shock
11. Porous and fibrous texture
12. Economical in its initial cost.
Classification of Heat Insulating Materials
The heat-insulating materials may be classified as follows:
1. Organic heat insulators
2. Inorganic heat insulators
1. Organic insulators
Wool, Cattle hair, Eelgrass, Cotton wool, Corkboard, Silk, Wood pulp, Sugarcane fibre, Sawdust, Cardboard (corrugated), paper, etc.
2. Inorganic insulators
Air(steel), Slag wool, Mineral wool, Glass wool, Aluminum foil, Diatomaceous earth(powder), Charcoal, Wood ashes, Gypsum (powder), Slag, Asbestos, etc.
Some of the important heat-insulating materials
It is derived from the bark of oak trees. It is ground, sized and baked in moulds. When ground and baked, the natural resin in the cork binds the material into a homogeneous mass which can be pressed into flexible sheets or boards etc.
It is available in the form of granulated Cork, slab cork, and re-granulated baked cork.
The structure of cork consists of an aggregation of minute air vessel, provided with a thin strong wall so that if the material is compressed it behaves more like a gas than an elastic solid; unlike the behaviour of spring, which exerts a pressure proportionate to the line of the amount of compression.
Cork, when compressed, exerts a pressure which increases in a more rapid manner and varies, approximately, inversely as the volume.
Following are the properties of Cork:
a. Light in colour.
b. Porous in structure.
c. Not affected by moisture.
d. Thermal conductivity is low.
e. Can be easily compressed.
f. When dried it is resilient and reasonably elastic.
Following are the uses of Cork:
a. Cork sheets and boards are used for insulating walls and ceilings, both against heat and cold and also as a sound insulator.
b. Used as a non-conducting covering for pipes carrying steam or hot water.
c. Used in refrigeration and cold storage insulation.
d. Used as non-conducting material for scientific apparatuses.
e. . Also used for bottle stoppers, vibration pads and floats for rafts and fishnets.
b. Glass wool:
Glass wool is produced by blowing high-pressure jets of steam or air on molten streams of glass at a high temperature.
Molten glass is violently scattered in all directions, to give this product.
Glass wool is a form of fibrous glass with short and fine fibres, scattered in various directions.
It is available in the form of loose fibres, mats, rigid quilts, or semi-rigid slabs or blocks etc.
a. Fibrous in structure.
b. Light in weight.
c. Has good tensile and dielectric strength.
d. Low thermal conductivity.
e. Quite durable.
f. Acts as an excellent insulating material because of the presence of large pockets of air in it.
g. Not affected by low temperatures and has been used successfully at temperatures as low as –212°C.
Glass fibres have the following characteristics:
a. Do not catch fire.
b. Not easily affected by heat.
c. Not spoiled by insects and moisture.
a. Mainly used for insulation of pipes, bends, valves etc.
b. Used for panel insulation for all types of industrial equipment.
c. Can be used for thermal and sound insulation of aircraft.
d. Glass wool blocks can be used in the construction of partition walls for thermal insulation purposes.
e. It is mostly used in boilers, ovens, cylinder or pipe insulation.
c. Rock wool
It is produced from flint rock containing some calcareous matter. In the absence of such a natural rock, flint and lime are mixed in the requisite proportions and melted in a furnace at a temperature of about 1700°C.
This molten material is then formed into small globules by means of a steam jet. These globules are then drawn into very fine fibres by hurling them in a large container.
These fibres of wool are then formed into boards or blankets (to be used as insulators). It can also be pressed, rolled and secured between the fabric of wire -netting of brass or copper.
It is available in the following forms:
Loose fibres, mattresses, mats, boards or felts, rigid or semi-rigid slabs, quilts.
a. Soft and flexible
b. Resilient and woody consistency
c. Heat and soundproof (due to the presence of millions of minutes of minute dead air cells)
d. Specific gravity is about 0.48.
a. Employed for heat and sound insulation purposes.
d. Slag wool
An aggregate of fine filaments of slag obtained by blowing air through a stream of blast furnace slag known as slag wool.
It is available in the form of loose fibres.
It is used for heat insulation in high-temperature furnaces.
Asbestos is a mineral fibre composed of hydrous silicate of magnesia with a small amount of iron oxide and alumina.
Asbestos sheets or boards consist of natural asbestos fibres mixed with a binding agent(usually cement) and then rolled in the form of sheets or boards. They are available in the market under the trade name ‘Salamander‘.
a. White, grey or brown in colour.
b. Flexible and can resist high temperature.
d. Unaffected by acids and fumes.
e. Resistant to corrosion and vermin effect.
d. Excellently resists heat and electricity.
Employed for heat and sound insulation in buildings. Also used for insulation of furnaces.
Thermacole is one of the trade names of polystyrene. This product was developed (in the U.S.A) during the Second World War.
It was made by direct extrusion of the foam form raw materials.
a. It has a very attractive, natural, snow-white colour.
b. It is extensively light in weight (density:150 to 300 N/m3 ). The foam is very light because it contains over 98% (by volume) air, trapped in 3 to 6 million closed cells per litre.
c. Compressive strength= 0.07 to 0.1 MN/m2 ; cross breaking strength = 0.14 to 0.18 MN/m2 .
d. Very low value of thermal conductivity.
e. Highly resistant to moisture.
f. Odorless, chemically stable and resistant to fungus attack.
g. Fully resistant to water, salt, soaps, bleaching agents and HCl(upto 35%), HNO3 (up to 50%), H2SO4 (up to 95%), caustic soda, caustic potash, strong ammonia, alcohols and silicon oil.
h. Not resistant to organic solvents like benzene, paint thinners and saturated aliphatic hydrocarbons like petroleum and gasoline.
I. Very good shock-protecting properties.
J. Ability to be moulded into well-fitting contoured cases.
a. Thermocole (with an operational range of -200°C to 80°C) is an excellent material for cold insulation in refrigerators, and cold storage, air conditioning, chilled pipelines and chemical processes.
b. It is used for industrial insulation and insulation for buildings against extremes of climate.
c. In the form of specially made flexible sheets, thermacole can be used on intermediate concrete floors in multi-storey buildings, to reduce impact sound transmission.
d. It is used for packing electronic goods like transistors, radios, tape recorders and calculating machines, clocks, medicine bottles, cameras, etc.
e. It is also used for air-dropped packaging, decorative and gifts packaging and edge protecting packaging.
g. Reflecting paper
It is a strong tough paper that is lined with aluminium or copper foil on the exposed side, which reflects back heat waves coming from a source and this keeps the walls and the enclosed rooms cool.
Sometimes, reflecting coatings of varnishes, paraffin, gums or synthetic resins are applied to various grades of paper of fibrous materials.
a. Strong and tough in nature.
c. Possesses adequate dielectric strength.
Used for heat insulation purposes.
It is a hydrated sulphate of calcium (CaSO4 .2H2O) occurring in monoclinic crystals.
It seldom occurs in nature in a pure state; contains impurities such as alumina, calcium carbonate, magnesium carbonate and silica up to 6 %.
When it is burnt in kilns, ‘Plaster of Paris’ is obtained.
After mixing with asphalt and casting into slabs, it is burnt in a kiln to form very strong sheets which possess very good insulating properties.
a. Crystalline and fibrous in structure.
b. Controls the setting time of Cement.
c. Gypsum boards are good insulators of heat.
It is mostly employed for heat insulation purposes.
Ceiling panels made of gypsum are used for suspended ceilings.
i. Aluminum foils
These are very thin foils or sheets of aluminium and are also known as ‘Alfoils’.
These are available in the form of paper-backed foils, separated layers of foils and some rigid materials faced with foils.
a. Light in weight.
b. Low thermal conductivity.
c. Possess smooth and shining surface.
d. Low emissivity (which decreases the radiation losses).
e. Resistant to ordinary atmospheric gases.
Used as a heat insulator in refrigerators.
j. Expanded blast furnace slag
It is obtained during the production of iron steel. It is collected in the form of liquid slag collects on top of the molten iron.
It is highly resistant to corrosion and attack by insects and microscopic organisms.
It has good resistance to fire. But it has high water absorption properties and can be used only in a situation where there is no risk of moisture penetration.
It can be used in the roofs and in floors above the level of damp proof course.
k. Lightweight concrete
Lightweight concrete, also known as cellular concrete, consists of crushed slag or grits and cement mixed with aluminium powder. Bubbles of hydrogen gas are then liberated through the concrete, known as lightweight concrete.
Employed for lining walls and roofs for heat and sound insulation of buildings.
It is the geological name given to a group of hydrated, laminar minerals which are aluminium-iron-magnesium silicates and resemble mica in appearance.
It possesses excellent fire-resisting properties.
By varying the density, different strength and thermal properties can be obtained.
a. Low-density vermiculite is used for insulating roofs and wall cavities as a loose filler material.
b. Vermiculite can be used for protection of steel girders and stanchions. It can be bonded with bitumen and used as composite thermal insulation and waterproofing material.
c. Vermiculite concrete can be used for in situ roofing and also for making blocks, tiles and slabs.
m. Coconut fibres
The fibres obtained from the outer layers of coconut are hard and elastic. The felted fibres are sandwiched between paper and covered on both sides with a layer of bitumen. Such material is completely water repellent.
a. Employed for underfloor insulation and floating floors.
b. Also used for internal insulation of walls.
It is made by converting paper waste or other wood to fibre form by the addition of some chemicals like borax, boric acid, aluminium sulphate, etc.
By shredding and pulverizing waste paper and mixing it with dry chemicals, most of the cellulose insulating materials.
It is used as loose-fill in the insulation of ceilings and walls of residential and commercial buildings, both for new construction and retrofit purposes.
1.2. Sound Insulating Materials
The materials which offer resistance to reflection and transmission of sound are called sound insulating materials.
The sound-absorbent materials can be incorporated in building structures either in a compressed state or in a suspended state or in a free state.
Requirements of sound insulating materials
A good sound-insulating material should fulfil the following requirements:
1. It should be able to absorb noise to the desired extent.
2. It should be such that it can be cleaned, washed or painted easily to maintain a clean appearance.
3. It should be resistant to attacks of vermin, insects, termite and dry rot.
4. It should be fire-resistant.
5. It should be able to withstand weathering effects.
6. It should be lighter so that it can be handled and fixed easily.
7. It should be economical in its initial cost.
8. In its finished form, it should offer a pleasant appearance.
Classification of Sound Insulating Materials
The sound insulating/absorbent materials may be classified as follows:
These materials have sufficient porosity and are good sound absorbers. Examples: Asbestos, rock wool, glass, silk, Han felt.
b. Semi-hard materials
They are stiff enough to stand rough handling and can also serve as building panels. Examples: Mineral wool boards, cane fibres.
c. Hard Materials
These are the hard materials which have been made porous during manufacture. They also serve as protective surfaces. Examples: Porous tiles of masonry
Common Sound Absorbent Materials
1. Acoustical plaster
a. It is made from the mineral called perlite.
b. It is also called fibrous plaster.
c. It possesses an absorbent coefficient of 0.30 at 500 cycles per second.
d. It is available in different colours and is usually applied in two coats to a finished thickness of about 12 mm. The rough finish of such a surface has slightly better sound absorption effect as compared to a smooth finish.
e. The acoustic plasterboards are also available. They can be fixed on the wall and the coefficient of absorption varies from 0.15 to 0.30.
2. Acoustic pulp
a. It is mainly composed of asbestos and cellulose fibres mixed with certain binders and preserving chemicals. When water is added it becomes plastic and can be applied to wall and ceiling surfaces to a thickness of up to 20mm.
b. It is applied in layers of 6mm thickness.
c. It can be easily shaped and finished (being plastic).
e. Unifil acoustical plaster
It is manufactured from vermiculite, other constituents being gypsum and lime or portland cement. Water is added to make it plastic for application.
It is an inert, featherweight and a granular substance.
It is adopted to an energy type of architectural treatment and is used mainly for interior finishes.
f. Acoustical boards or tiles:
a. They are usually made of either compressed cane or wood fibre or mineral wool.
b. They are pre-finished at the factory and can be painted or coloured to give desirable decorative appearance and light reflection characteristics.
c. They can be made in different sizes and shapes to suit the requirements.
d. The acoustical tiles are, however, relatively costly than other absorbent materials.
e. Pre-fabricated boards may be of unperforated or perforated type.
Acoustic tiles are most suitable for rooms in which small area is available for acoustical treatment.
Some of the common types of boards and tiles made of a material other than mineral fibre are:
b. Celotex building board
e. Econacoustic tiles
f. Unifil acoustical tiles
h. Muffletone tiles.
g. Glass fibres
a. Glass fibres of 0.015 mm diameter are used for the purpose of sound insulation.
b. When faced on one or both sides with a non-metallic flexible material such as paper, muslin, or glass cloth, it is known as fibreglass blanket. Such blankets are extensively used in broadcasting, television and film studios (available from 12 mm to 100 mm in thickness).
c. Fibreglass boards and tiles can be manufactured into a lightly decorative material having a uniform texture and excellent light properties.
h. Quilts and mats
a. Quilts and mats are prepared from glass wool or mineral wool and are fixed in the form of acoustic blankets.
b. Their absorption coefficients depend on the density, thickness, perforation, nature of backing, frequency of sound and mode of fixing.
1.3. Electrical Insulating Materials
The materials which offer a very large resistance to flow of current and for that reason they are used to keep the current in its proper path along the conductor.
Characteristics of Good Insulating Materials
a. Large insulation resistance.
b. High dielectric strength.
c. Uniform viscosity – it gives uniform electrical and thermal properties.
d. Should be uniform throughout
e. Least thermal expansion
f. When exposed to arcing should be non-ignitable.
g. Should be resistive towards oils or liquids, gas fumes, acids and alkalies.
h. Should have no deteriorating effect on the material, in contact with it.
i. Low dissipation factor.
j. High thermal conductivity.
k. High mechanical strength.
l. Low permittivity.
m. Free from gaseous insulation to avoid discharges.
n. Should be homogeneous to avoid local stress concentration.
o. Should be resistive towards thermal and chemical deterioration.
Classification of Electrical Insulating Materials
1. Based on Materials
Mica, wood, porcelain, rayon, silk, paper, glass, terelyne, cotton, rubber and cellulose materials.
Linseed oil, spirit, synthetic varnishes, refined hydrocarbon mineral oils, etc.
Dry air, nitrogen, argon, carbon dioxide, etc.
2. Based on Temperature
|Class||Insulating Materials Included||Assigned Limiting Insulating Temperature|
|Y ( Formely O)||Cotton, silk, wood, paper, cellulose, etc neither impregnated nor immersed in oil.|
Materials of Y class are unsuitable for electrical machines and apparatus as they deteriorate rapidly and are extremely hygroscopic.
|A||Materials of class Y impregnated with natural resins, insulating oils, cellulose esters, etc. Also included in this list are laminated wool, varnished paper.||105º C|
|E||Synthetic resin enamels, cotton and paper laminates with formaldehyde bonding, etc.||120º C|
|B||Mica, glass fibres, asbestos with suitable bonding substances, built-up mica, glass fibre and asbestos laminates.||130º C|
|F||Materials of class B with bonding materials of higher thermal stability.||155º C|
|H||Glass fibre and asbestos materials, and built-up mica, with silicon resins.||180º C|
|C||Mica, ceramics, glass, quartz without binders or with silicon resins of high thermal stability.||above 180º C|
|Read Also: Concrete Admixtures|