Lecture 7 thermal insulation

THERMAL INSULATION

June 5th, 2017 Lecture 7 Ephrem M.

The polar bear has a thick

body fur to insulate himself

from the chilly environment

around. Eskimo lends this fur

to keep his body warm, which

will be in stark contrast with

the surrounding.

A casual observation around clearly shows why women are less panicky

when it comes to handling hot things; it is because of their insulating

subcutaneous fat. The equivalent analogy on this biological systems is

the insulation of buildings from their natural setting.

THERMAL INSULATION

The sheathing often placed around steam

and hot-water pipes, for instance, reduces

heat loss to the surroundings, and

insulation placed in the walls of a

refrigerator reduces heat flow into the unit

and permits it to stay cold.

THERMAL INSULATION

Apart from geometrically responding to a harsh climate by manipulating the

form, insulation of buildings from the environment is a wise insight in energy

conscious world of the day.

Thermal insulating materials are used to reduce the flow of heat between hot

and cold regions.

What is Thermal Insulation?

Where there is difference in temperature of inside of a building and

outside atmosphere, heat transfer takes place from areas of higher

temperature to lower temperature. When the external temperature is

low, the internal temperature should be high so as to produce

thermal comfort.

Thermal insulation indicates the construction or provisions by way of

which transmission of heat from or in the room is retarded.

Thermal insulation can be attained during the design process from

providing thermal insulating materials on the “shell” of the building.

General methods of Thermal Insulation

1. heat insulation by orientation

2. Heat insulation by shading

3. Heat insulation by proper height of

ceiling




3. Heat insulation by proper height of ceiling

1. internal building

surfaces which can

receive direct incident

solar radiation from

winter to summer

2. Optimum inclination

of solar cells for global

radiation throughout the

year

the orientation of the building with respect to

the sun has a very important bearing on its

thermal behaviour. For optimum orientation,

there are usually conflicting requirements.

Minimum transfer of solar heat desired during

the day in summer, while maximum heating of

rooms by solar heat is required during winter.




Shading of roof brings down the surface temperature,

but it is very difficult to achieve this effect in practice,

especially when the altitude angle of the sun is high

during the peak heat gains in the afternoon, between

1100h and 1500h.

Raising the parapet walls can help only when the altitude

angle of the sun is low.





While the surface temperature of the ceiling does not

vary with its height, the intensity of the long wave

radiation, emitted by the ceiling decreases as it

travels downwards. The effect of vertical gradient of

radiation intensity is not significant beyond 1 to 3m.

Hence it should be adequate to provide ceiling at a

height of about 1 to 1.3m above the occupant.


Advantages of Thermal Insulation

✓It provides comfort as it keeps room:

▪ cool in summer and

▪ hot in winter.

✓It saves fuel by minimizing heat transfer,

✓prevents condensation or moisture deposition on interior

walls and ceilings and

✓it prevents freezing of water taps in extreme winter, and

heat loss in case of hot water system.

Thermal Insulation functions

1. reduce thermal conduction in the material in which heat is

transferred by electrons;

2. reduce thermal convection current, which can be set up

in air- or liquid-filled spaces

3. reduce radiation heat transfer in which thermal energy is

transported by electromagnetic waves.


Conduction and convection are suppressed in a vacuum, in which

radiation is the only method of transferring heat.

If the surfaces are made highly reflective, radiation can also be

reduced.

Thus, thin aluminium foil can be used in building walls, and

reflecting metal on roofs minimizes the heating effect of the Sun.


Air offers about 15,000 times as much resistance to heat flow

as a good thermal conductor such as silver does, and about 30

times as much as glass.

Typical insulating materials, therefore, are usually made of non-

metallic materials and are filled with small air pockets.

They include magnesium carbonate, cork, felt, cotton batting,

glass wool, and diatomaceous earth.



In building materials, air pockets provide additional insulation in

hollow glass bricks, double-glazed windows (consisting of two or

three sealed glass panes with a thin air space between them),

and partially hollow concrete tile.

Insulating properties become poorer if the air space becomes

large enough to allow thermal convection, or if moisture seeps in

and acts as a conductor.


Home-heating and air-conditioning costs can be reduced by proper

building insulation.

In cold climates about 8cm (about 3in) of wall insulation and about 15

to 23cm (about 6 to 9in) of ceiling insulation are recommended.

Super insulation has recently been developed, primarily for use in

space, where protection is needed against external temperatures near

absolute zero.

Super insulation fabric consists of multiple sheets of aluminized Mylar,

each about 0.005cm (0.002in) thick, separated by thin spacers, with

about 20 to 40 layers per cm (50 to 100 layers per in).



1. Low temperatures: insulation for vessels containing cryogenic

materials, such as liquefied natural gas

2. Ambient temperature: insulation for building structures

3. Medium temperature: insulation for tanks, pipes, and equipment in

industrial process heat applications

4. High temperatures: refectory or other specialized insulation materials

used in foundry work, nuclear power facilities, the aerospace industry

Thermal insulation controls heat flow under temperatures ranging from absolute zero to 3000oF

(1600oC). This broad range can be subdivided into four general temperature regimes that classify

applications for various types of insulation;

Three points to consider in using Thermal Insulating Materials

Where is thermal insulation used?

How does insulation work?

What is the R-value (Thermal resistivity)?

Use of thermal insulation

If the air outside is cold, you may want to protect your skin by wearing clothes

that keep the cold out and the body warmth in.

If your house has cool air inside during the summer, you may want to prevent

the temperature from becoming the same as the hot air outside by having the

house well insulated.

You may want to prevent hot drink from becoming room temperature by putting

it in a thermos bottle.


How Does Insulation Work?


Insulation is a barrier that minimizes the transfer of heat energy from

one material to another by reducing the conduction, convection and/or

radiation effects.

Most insulation is used to prevent the conduction of heat.

Less dense materials are better insulators.

Thus, gases insulate better than liquids.


Conduction occurs when materials--especially solids--are in

direct contact with each other.

To slow down the transfer of heat by conduction from one

solid to another, fiberglass and vacuum are mostly used.

Insulation from conduction



Radiation is transfer of heat from warm objects by

radiating infrared electromagnetic waves which can be

prevented by a shiny material to prevent radiation heat

transfer.

Insulation from Radiation


R-Value

The R-value of a material is its resistance to heat flow and is an indication of its

ability to insulate.

• It is used as a standard way of telling how good a material will insulate.

• The higher the R-value, the better the insulation.

Its units of measurement:

▪ (square feet x hour x degree F)/BTU in the English system

▪ (square meters x degrees C)/watts in the metric system


R-Value


Material R-Value Material R-Value

Hardwood siding (1 in. Thick) 0.91 Cellulose fibre (1 in. Thick) 3.7

Wood Shingles (lapped) 0.87 Flat glass (0.125 in. Thick) 0.89

Brick (4 in. Thick) 4 Insulating glass (0.25 in space) 1.54

Concrete block (filled cores) 1.93 Air space (3.5 in. thick) 1.01

Fiberglass batting (3.5 in. Thick) 10.9 Free tagnant air layer 0.17

Fiberglass batting (6 in. Thick) 18.8 Drywall (0.5 in. Thick) 0.45

Fiberglass board (1 in. Thick) 4.35 Sheathing (0.5 in. Thick) 1.32

Thermal Insulating Materials

1. slab or block insulation—known as blocks or boards, 2.5cm thick and 60cm x

120cm or more in area. These may be made of cork board, mineral wool,

vermiculite, cellular glass, cellular rubber, saw dust, asbestos cement, etc. these

are fixed to walls, slabs or roofs.

slab or block insulation


slab or block insulation


2. Blanket insulation—these are flexible fibrous rolls made from mineral wool,

processed wood fibres, cotton, animal hair, etc. available in thickness of 12 to

80mm. These are directly spread on the wall or ceiling surfaces.


3. Loose fill insulation—may

consist of fibrous materials like

rock wool, slag wool, cellulose

or wood fiber wool, etc. filled

loosely in the studding space.


4. Batt insulation—similar to blanket insulations except that these are small in size

but of greater thickness. These are also spread on surface of walls and ceiling.

Batt insulation


5. Insulating board—used

for interior lining of walls

and also for partition

walls, available in

different sizes and

thickness

6. Light weight aggregate—heat resistance of concrete can

be greatly increased by using blast furnace slag, burnt

clay, vermiculite


7. Reflective sheet—have high

reflectivity and low emissivity, thus

offering high heat resistance. It

consist of gypsum boards, steel

sheet, aluminium foils, etc.

Unlike others, reflective sheets rely on

their surface characteristics, thickness

of air space & temperature differences

for their insulating value.


8. Air barrier—effectively retard or

stop the flow of warm, moisture-

laden air from inside a building

outward through walls, ceiling, and

floors to the colder, dryer outside

atmosphere.

Application of Insulation materials

1. Flexible insulation – provided in open spaces

in the wall, ceiling, or roof construction

where it will not be subjected to loads, as it

is easily compressible.

Quilts or blankets are put between furring strips

on masonry wall. Blanket insulation divides the

space into two or more air spaces.

For greater efficiency, the insulation is fastened at the sides and ends so that

there will be no air leakage between spaces. Batts are placed between studs

2. Fill insulation - installed by pouring the loose granulated material into

open spaces between the studs or between ceiling joists. The material

may be blown into place through large flexible tubes employing low air

pressure. Care is taken to insure the complete filling of space between

stud.


3. Reflective insulation - fixed

between studs, joists or rafter.

It is held more tightly by using

nailing strips. This type of

insulation divides the space into

two or more air spaces. It is

essential that the insulation is

fastened at the sides, top,

bottom and all laps to restrict

the air circulation between the

two spaces.


Choice of Insulating materials

The choice of insulating material depends upon:

➢the cost of material,

➢area to be covered,

➢standard of insulation required and

➢coat of heating and cooling.

Properties of Insulating materials

✓high thermal resistance,

✓reasonably fireproof,

✓insect proof,

✓durable,

✓non-absorbent of moisture,

✓cheaper and readily available.

The insulating material Should have properties of:

It should be noted that low

density insulating material give

better insulations than high

density materials. Also the

presence of air spaces in the

insulating material increases

thermal insulation while presence

of moisture decreases thermal

insulation.

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Lecture 7 thermal insulation

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