Thermal Insulation of Building

India is the third largest energy consuming country and also third in carbon di oxide emission releasing 2.65 Billion MT of carbon as on 2018.

Buildings consume 40% of global energy, 25% of water and 40% of other resources. The buildings and buildings construction sector combined are accountable for one third of the total global energy consumption and nearly 40% of total direct and indirect carbon di oxide emission.

History of Thermal Insulation Material

The history of thermal insulation is not very old. In the primitive age, human being started building up shelter for their protection from various attacks. They were using reed, flax and updated themselves with use of straw, clay, etc. as development progressed. A better comfort from weather pushed them further to search for improved and material with longer life. In the 19th century, the first insulated panel with organic material came in use and followed by more and more artificial materials

What is a Modern Thermal Insulation Material?

The group of materials which help to reduce the heat gain or heat loss in a building is known to be thermal insulation material.

The transfer of heat is caused by the temperature gradient at the outdoor and indoor. Heat is lost through ceilings, exterior walls, doors, windows and floors. Heat loss or gain in a building is not appreciated, since it increases the load on HVAC system. Insulation material helps to decrease the energy demand of heating and cooling systems.

There are three modes of heat transfer — conduction, convection and radiation.

Most of the insulation systems entrap air in large quantities in form a discontinuous cell. Some insulation material entraps inert gases like Freon, carbon di oxide, other inert gas., to show a lower thermal conductivity than materials having air within it. Now, in view of CFC free gases to be used cyclopentane is the latest material in use to foam insulation.

Key Performance Features Related to Thermal Insulation

a)Thermal Conductivity (k): Thermal conductivity of a material indicates how much of heat flows per unit area, unit time and unit thickness at a given temperature difference. The greater the value of thermal conductivity, the more is the heat transfer due to thermal conduction; this can help us to calculate the rate of heat flow: q= k.(T1-T2)/ l; where, q is the heat flow rate in watt, k is thermal conductivity in w/m K, l is the thickness of insulation material in m.

b) Thermal Resistance (R): It expresses the resistance to heat flow — gain/loss through the insulation material of a certain thickness. The unit is expressed as Sqm K/w: R = l/k, where, l is the thickness in m and k is the thermal conductivity in w m/K

c) Specific Heat (cp): The amount of heat required to increase the temperature of unit mass of material by one degree. Unit is J/kg

d) Density (d): It expresses the unit weight for unit volume and denoted as kg/cum

e) Thermal diffusivity (Y): It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy, Thermal Diffusivity = Thermal conductivity / Density x specific heat and unit is sqm/s

f) Vapour Permeability: It is the amount of water vapour / moisture transmitted through a unit area in a specified time under the specified condition of temperature, humidity and unit pressure difference. The unit is permeance — kg per sec per sqm per pascal

g) Thermal Envelope: Thermal envelope is defined as the skin of the building, which protects the thermal and acoustic comfort to its interior. This is made up of its opaque walls (walls, floor, ceiling), its operable elements (doors & windows) and its thermal bridges, which are the points that allow pass more easily (points with geometric variations or change of materials).The separate calculation for heat transmittance for different areas may be done in case of a nonhomogeneous material for a greater accuracy.

h) Thermal Transmittance (U): It defines the ability of an element of structure to transfer heat under steady state condition. It is a measure of the quantum of heat that would flow through unit area in unit time for a unit difference in temperature of the individual environment between which the structure intervenes. It is calculated as the reciprocal of sum of the resistance (R) of each component part or layer of the structure, including the resistance of air space of the inner and outer surface.

R total = R si + R1 + R2 + R3 + — — — + Rn + Rse

R1, R2, R3 to Rn: Thermal resistance offered by Individual Layer / Component of Material

Rsi & Rse: Interior and Exterior Surface Thermal Resistance according to norm of climate

U value: I / R total; U value is expressed in unit of w/ sqm K

U value also can be measured post construction by using a heat flux meter. This is consisting of thermopile sensor that is firmly fixed to the test area, to monitor the heat flow from outside to inside or vice versa over a continuous period of minimum two weeks; the accuracy of measurement is dependent on a number of factors such as — magnitude of temperature difference, weather conditions, good adhesion of thermopiles to the test area, duration of monitoring, no of test points considered. Two other variables play a role named as ambient temperature and air velocity related to convection current,

Significance of U Value

The U value is particularly important for the envelope of building. In buildings engineering, the fluid is air. All the building elements act as a heat transmitting objects, through each one offers certain resistance as shown in the calculation of U value. To improve the thermal transmittance, insulation material is installed. The aim

is always to achieve the lowest U value by using an efficient thermal insulation material, which offers the highest thermal resistance (R), since U=1/R; Also, the total heat load is calculated

Q= U. A. (T1- T2). Total time duration= Total heat load; U= Thermal transmittance, A= total area, T1 & T2= Out side and inside temperature. This heat load figure will act as input to HVAC designer to calculate the capacity of HVAC system.

j) Embodied Carbon: It covers the total energy spent in producing the material covering all the processes and bringing it up to the factory gate and additional energy involved towards the transportation of material to the site and energy incurred in installation of the material.

Modern Thermal Insulation Materia

Another critical use of thermal insulation material is cryogenic application, where operating temperature is 77 to 300 K. Materials used for this purpose is cellular glass, Perlite, vermiculite, certain types PU foam, vacuum, etc.,

Aerogels

Sharing about a unique thermal insulation material named Aerogel. It is a synthetic low-density material with magical physical properties. It has a density just three times higher than air (contains 99.5% air) in a foam form with mesoporous structure (pore structure 2 nm to 50 nm). It is also known as “frozen smoke” due to its peculiar shape. Aerogel is in use since 1960 for space suit of astronauts. Now, with further development of aerogel, it is being considered for wider uses. Incidentally, aerogel was the lightest material till other da. Thereafter microlattice became the lightest material and thereafter aero graphite came out to be the lightest material now.

Selection Criteria of A Good Thermal Insulation Material

Like all materials and based on material science, all insulation materials have its advantages and limitations. It is the role of the designer to select the best material suiting for the specific use considering its features, application easiness, life cycle cost and unique advantages.

The followings may be considered as selection criteria for a thermal insulation material:

a) Low thermal conductivity (stable form)

b) Dimensional stability

c) Negligible or Nil moisture/ water absorption

d) Low vapour permeability

e) Low thermal diffusivity

f) Good/ High Compressive Strength ( for over deck application)

g) Light weight

h) Resistance to ageing

i) Fire resistance/ retardant

j) Ecofriendly

Optimum Thickness of Thermal Insulation

Thermal insulation reduces the heat losses. The cost of insulation increases linearly with the increase in thickness and while the cost of heat loss decrease exponentially. The total cost, which is the sum of the insulation cost and lost heat cost decreases first; reaches a minimum and then increases. The thickness corresponding to the minimum total cost is the optimum thickness of insulation and thus, the recommended thickness. This is based on today’s energy cost

Thermal insulation system installed for a long life. Today’s energy cost would go high many folds in next 10/20 years, hence the optimum thickness of insulation based on today’s energy cost will not remain as optimum after some years. Therefore, it is wise to consider a higher thickness based on trend in the increase in energy cost and considering the life cycle of the insulation.

As a thumb rule , use a thickness of thermal insulation as 75 mm for terrace and 50mm for exterior wall in today’s context or based on actual calculation.

Application of Thermal Insulation in Building

Thermal insulation is largely used in building for HVAC system — duct, chillers, chilled water pipes, terrace, under deck, False ceiling and exterior walls.

It is always debatable whether thermal insulation material is to be applied over deck or under deck. There is a distinct advantage in using the insulation material over deck, the concrete/ metal deck shall be protected from the exposure to temperature variation resulting longer life and no deterioration.

Terrace may be insulated in a conventional manner or using inverted roof system. Inverted roof will have an edge over conventional system,

Thermal insulation on wall may be fixed with the help of adhesive & nylon anchor at a suitable pitch and finally protected with a cladding or Cement mortar plaster and finished with elastomeric coating.

Energy Conservation Building Code — ECBC

Bureau of energy efficiency has set a guide line for the energy efficient building through ECBC code in 2017. The purpose of this code is to give the requirement of an energy efficient design and construction of building to achieve an enhanced level of energy efficiency. Buildings fall under ECBC 2017 must have a connected load of minimum 100 kW or a contract demand of 120 kVA or more and are used for commercial purpose. Buildings intended for private residential purpose only are not covered by this code.

The important parameter to note:

Energy Performance Index (EPI) of a building is its annual energy consumption in kwh per sqm of the building. While calculating the EPI of a building, the area of unconditioned basements shall not be included. EPI can be determined as

EPI = Annual energy consumption in kWh/ total built up area excluding unconditioned basement area.

To comply with the code EPI shall be less than 1 or maximum equal to 1.

Building must meet all the mandatory requirements as stated in following clauses:

4.2- Building Envelope

5.2- Comfort system & Control

6.2- Lighting control

7.2- Electrical & Renewable Energy control

Conclusion

Energy is a scarce item and at the same time use of energy creates carbon foot print and global warming. Therefore, it is always strongly recommended to save energy. Use of thermal insulation in a building would be the easiest option to save energy and reduction in consumption. There are low number of building where the thermal insulation is used on the terrace and it is rarely used on the exterior walls.

Keeping the Paris agreement in mind and India’s nationality determined contribution (NDC) commit to reducing emission intensity of its GDP 33–35% below 2005 level by 2030, the energy saving must be stringently followed.

Currently ECBC code only includes commercial buildings, As there are large number of high rise and big buildings in residential sector, it may be worth to cover the same also suitably.

It would not be a bad idea to reward people towards a consistent reduction in energy consumption.