Methods of Thermal Insulation

Based on the causes of heat transfer, the areas to be concentrated for heat insulation accordingly the methods to be adopted are:

1. Thermal Insulation of roofs
2. Thermal Insulation of exposed walls
3. Thermal Insulation of external windows, and doors

1. Thermal Insulation of Roofs

The insulation can be done on outside or inside of a roof. Generally it is better to put a thermal insulation above the slab. Following methods may be adopted:

1. Heat insulating materials may be installed over the roof but below a water proof course. If it is an internal application of the material, it may be fixed by adhesives or otherwise on the underside of the roofs.
2. False ceiling may be provided inside by insulating material with certain air gaps.
3. On the top of the roof reflecting insulating material may be laid.
4. Air spaces may be provided on the top of the flat roof by using asbestos sheets over the bricks.
5. Spraying water regularly on the top of the roof may reduce the heat flow.
6. White-washing of roof before onset of each summer also prevents heat transfer.
7. Providing adequate shading on the exposed roof surface also reduces the heat flow.

2. Thermal Insulation of Exposed Walls

1. Increase in thickness of walls to some extent forms a thermal insulator. But it is expensive.
2. Providing cavity walls effectively acts as a thermal insulator.
3. Heat insulating materials may be used along with other building materials during the construction of walls.
4. Heat insulators explained earlier may be installed or fixed on the inside and outside of the exposed wall in order to reduce the thermal transmittance.
5. For partition walls air spacing may be created by fixing sheathing of hard boards or battens on either side of the wall.
6. Exposed surfaces may be white-washed or light colour distemper may be used on the exposed surface.

3. Thermal Insulations of External Windows and Doors

Heat is considerably transmitted through windows and doors which are exposed. This can be reduced by adopting the following method:

1. The incidence of solar heat on exposed doors and windows have to be reduced. This can be done by use of external shading such as louvered shutters, sun breakers, chajjas, etc. Internal shading such as curtains in heavy folds and venition blinds may be used. In the glazed windows, the heat reduction may be achieved by insulating glass or double glass with air space may be used.

28.4 ACOUSTICS AND SOUND INSULATION

Sound is produced when part of the atmosphere is compressed suddenly. It is transmitted in the form of waves. The waves are a series of compressions and rare factions created in the air medium. The average sound travels at a speed of 340 m/s at ordinary temperature. It depends on the medium through which it travels.

28.4.1 Effects of Audible Sound

Sound can travel through some medium like air. It can not travel in vacuum. Thus for the sound to be audible to the ears, the sound source and ear must be connected by an elastic medium like air. The characteristics of audible sound are discussed below.

1. Frequency of Sound

Frequency of pitch of sound is defined as the number of cycles or vibrations per second. The highest audible sound (e.g., whisle) has a frequency of 20000 cps (cycles or vibrations per second). The lowest audible sound has a frequency of 20 cps (e.g., whispering). The frequency is a measure of the quality of sound.

2. Intensity of Sound

The intensity of sound is defined as the flow of sound energy per second through unit area. The intensity of sound is the strength of the sensation received by the human ear. Intensity of sound is a purely physical quantity. But loudness of sound depends on the characteristics of ear.

3. Measurement of Sound

The range of intensity of sound is very large. The loudest sound is about 10¹³ times the sound which is just audible by the human ear. There is a wide range of sound levels, it is realised that a scale has to be adopted as a guidance. The intensity of sound is measured on a logarithmic scale due to wide range of variation of the intensity of sound. Bel is the measure of intensity of sound named after Graham Bell, the inverter of telephone.

As the unit of bel is comparatively large, hence a shorter practical unit decibel (db) equal to 1/10th of a bel unit. The range of audible sound to painful noise varies from 1 to 10¹³ which is covered on logarithmic scale between 1 and 130 db units. Thus one db unit is approximately the smallest change of sound intensity which the human ear can hear. Table 28.1 (Sharma, 1988) shows the rating to sound generally experienced.

Table 28.1 Intensity of sound and comfort

28.4.2 Principles of Acoustics

The behaviour of sound plays an effective role in the acoustical design of different types of buildings and in the sound insulation process.

A sound originating from a source, such as music or operation of machine, is transmitted through the medium in all directions. The transmitted sound strikes on some surface, like wall, ceiling, floor or any other barrier. Depending on the type of surface part of it is reflected back, and a part being absorbed by the surface. If the sound is not absorbed by the material, it will be transmitted in part to another side of barrier.

If the ultimately reflected sound is not properly controlled the reflection may result in acoustical defects, viz., echoes and reverberations. This reflected sound is important in the acoustical design of buildings. The part of sound absorbed by the surface is represented by an absorption coefficient. This coefficient is the ratio of the energy absorbed by the area of the surface to the energy striking the area. This coefficient is a function of the frequency of sound. The values of the absorption coefficient for different surfaces for a normal frequency of 500 cps are presented in Table 28.2. It is assumed that the sound wave will completely pass through a window, then it is taken as 1.

Table 28.2 Absorption coefficient for different surfaces

The reduction of intensity of sound of a transmitted sound through a barrier is called as transmission loss. This transmission loss is a measure of the effectiveness of a surface as an insulating material.

Thus transmitted and absorbed sounds have important bearing on the acoustical condition of a building. However, both transmitted and absorbed sounds are inter related and influence the acoustic and sound insulation.

Highly porous materials have the quality to dissipate considerable energy and the absorption will be relatively high. An ideal material for sound absorption should possess the following qualities:

1. It should have a high coefficient of sound absorption of economy.
2. It should be fire resistant.
3. It should have high structural strength.
4. It should be unaffected by decay, attack by insects and vermin.
5. It should be easily workable to fit in any location.

28.4.3 Acoustical Defects

The acoustical design of an enclosed space is basically depend on the behaviour of the reflected sound. Due to the reflection of sound two main defects are developed, viz., echoes and reverberation.

1. Echoes

Echo is said to be produced when a reflected sound wave reaches the ear just when the original sound from the same source has already been reached. The sensation of sound persists for one-tenth of a second after the source has ceased. Thus an echo forms when the time lag between the two sounds is about 1/17th of the second.

Further considering the velocity of sound in the atmosphere air as 34.3 m/s, it is shown that when the distance of the reflecting surface is between 8 and 17 m, echoes are formed.

The defect of echoes also occurs when the shape of the reflected surface is curved with smooth character. Echoes are unpleasant to hear and cause disturbance to hearing.

2. Reverberation

When the surfaces of an indoor place are hard and smooth, very small energy is lost at each impact of sound and many reflections take place before the sound dies down. This repeated reflection of sound is called prolongation or reverberation. Then if sound exists too long, then successive words of a speech will overlap and confuse.

The remedy for this defect is to select a correct time of reverberation known as optimum time of reverberation. This is achieved by suitably selecting a proper absorbent or acoustical materials for different reflecting surfaces.

Acoustical level of a room and reverberation times are shown in Table 28.3, Table 28.4 shows the recommended values of the optimum reverberation time in India for halls after considering the indicated audience strengths.

28.4.4 Acoustical Materials

Common building materials are absorbents of different levels. Such materials are called as absorbent materials.

Table 28.3 Acoustics level and reverberation time

Table 28.4 Optimum reverberation time for closed area

Qualities of a good acoustical materials are as follows:

1. An absorbent material should be economical for construction and should be waterproof, fireproof, adequate strength and heat resistant.
2. It should be durable and easy for maintenance.
3. It should have high absorption capacity.
4. It should be easy to fix and should be self-supporting.

Some of the acoustical materials are discussed below.

1. Acoustic Plaster

It is a granulated insulation material mixed with cement. The quantity of cement used in such materials is to provide enough strength and have sufficient pores. The coefficient of absorption varies from 0.15 to 0.30.

2. Acoustic Tiles

These are commercially available materials with adequate absorbent qualities. The absorption of sound is uniform and most suitable for small rooms. But it is costly compared to other acoustical materials.

3. Porous Boards

Straw boards, pulp boards and compressed fibre boards have varied coefficient of absorption. These boards can be fixed by ordinary panelling. The thickness is about 17 cm and coefficient of absorption varies from 0.17 to 0.50.

4. Perforated Boards

The material can be formed by combination of mineral wool and cement asbestos or with mineral wool and hard board. The coefficient of absorption for the first combination is about 0.95 and for the latter is 0.20. Such boards are generally suspended from trusses.

5. Quilts and Mats

These materials are made out of mineral wool or glass wool. The coefficient of absorption varies which depend on the thickness of the board or mat, density, perforations, mode of fixing and frequency of sound.