The transfer of heat energy from one substance to another substance can generally take place by any of the following three means:
- By conduction
- By convection
- By radiation
Thermal insulation normally acts as insulating barrier to avoid the transfer of heat energy by any of the above effects. “Thermal insulation is the method of preventing heat from escaping a container or from entering the container. In other words, thermal insulation can keep an enclosed area such as a building warm, or it can keep the inside of a container cold.”1
It is mainly employed for the insulation of buildings or structural elements.
“The R-value is the reciprocal of the amount of heat energy per area of material per degree difference between the outside and inside. The R-value is proportional to the thickness of the material. For example, if you doubled the thickness, the R-value doubles.”2
It is also referred to as the thermal resistance or thermal rating of an insulating material. This is the parameter which is used to rate the thermal ability of an insulation. It determines the amount of resistance offered by the material to heat flow or transfer of heat energy. A higher R-value is usually preferred for better insulation. The factors upon which the R-value of any insulating material would depend include:
- Kind of material
- Thickness of material
- Density of material
In cases, where multiple layers of insulating materials are installed, R-value is generally calculated by summing up R-values of the individual layers. R-value is usually represented via following measurement units:
In English system units, R-value is (square feet x hour x degree F)/BTU whereas
In Metric system units, it is (square meters x degrees C)/watts.
All insulating materials should satisfy following major requirements:
- They need to maintain their shape and strength under all conditions.
- They should have adequate durability and robustness.
- Also, they should be impervious to the attack of mould and parasites.
- They should be able to meet the safety criteria for protection against fire.
Some important facts concerning insulation materials are mentioned below:
- The less dense the material, the better insulator it is. This is due to the fact that atoms in highly dense materials are bound closer to each other which results in an effectual transfer of energy from one atom to another.
- Liquids are considered to be better insulators than solids but not better than gases.
- A perfect insulating material which provides poor conduction of heat always happens to be a poor conductor of electricity also. For example, wood which is a non conductor of electricity provides better insulation than copper.
Following benefits are associated with the use of insulation within a building:
- It maintains the temperature of walls, ceilings, and floors hotter in winter season and sufficiently cooler in summer season.
- It creates a very relaxed and comfortable atmosphere within our house by keeping a consistent temperature all over the house.
- It helps in saving money and scarce energy resources of our country.
Types of Insulation
Following are the most commonly used thermal insulations:
Suitability of an insulation type usually depends upon:
- Kind of construction
- Amount of rehabilitation proposed for a building
- Relevant code requirements
The above mentioned different insulation types can be employed jointly within a building. For example, one can easily put in batt or roll insulation above loose-fill insulation and vice-versa. In general, it is always ensured that high density insulation materials don’t get installed over lower density materials. This is required because thickness of less dense materials tends to decrease after getting compressed by the weight of high density materials. Reduction in thickness will diminish their R-value or thermal performance. However, an exception exists to this elementary rule. In cases where temperature of attics falls under 0°F, some of the low-density insulation materials may possibly permit the circulation of air between the crest of the ceiling and the floor. This flow of air tremendously affects the performance of the insulation. One can exterminate this air movement by simply applying a layer of high density insulation materials over low density insulation. Batts, rolls, blankets, loose fills and low-density foams, all these forms of insulation usually work by restraining flow of air inside the building because the air when not in movement will act as a perfect insulator. Also, some exceptional gases exist which when added to foams like polyisocyanurate, polyurethane and extruded polystyrene result in better thermal resistance.
Selection of Insulation Type
While selecting an insulation type for a particular application, one must consider following points:
- Decide on the amount of insulation required for the purpose.
- Identify the ease of access of the insulation location.
- Consider the space availability for the insulation.
- Keep in view the availability and cost of insulation type.
However, there are few more considerations which are exclusive to each buyer. While making choice among different insulation materials, one must make sure that equal R-values are considered for comparison purpose. The effective performance of insulation largely depends upon the proper installation procedure.
Other Types of Insulation
Some other types of thermal insulations include:
- Opaque thermal insulation: These types of thermal insulations include either evacuated or non-evacuated insulating components. In evacuated parts, the transfer of heat takes place by means of radiation only. Hence, it is likely to obtain better heat transmission coefficients i.e. U-values considering the thickness of the insulating layer. The thermal conductivity value of evacuated insulating materials is found to be very much lesser than non-evacuated materials. Besides, high production costs are involved with the use of evacuated panels. Thus, their use is restricted to only few applications.
- Transparent thermal insulation: This type of thermal insulation makes passive use of sun energy on outer walls. The sun radiations which are shorter in wavelength are capable enough to pass through the transparent insulating material. After passing through the transparent insulation, they get to hit the wall partition at the back and warm it up. Now the long wave radiations gets emitted from the partition i.e. infrared radiations which do not have the potential to pass through the insulating layer since the transparent thermal insulation layer behaves as an opaque layer for such types of radiations. “Depending on the ambient temperatures, the heated up exterior surface of the wall leads to a reduced transmission loss or even to a transmission gain through the wall. In both cases the heating energy demand is reduced by the use of the transparent insulation construction.”3
- Green roof thermal insulation: It is the type of thermal insulation which basically employs the thermal insulating properties of plants. The placement of plant layers creates an insulating barrier which acts as a shield against wind, minimizes circulation of air and decreases heat loss by convection. Green roof insulations mainly find their application in reducing the overheating effect in a building produced during summer season.
Three possible ways for installing insulation layer inside a building are mentioned below:
- External thermal insulation: This is the most preferred implementation type amongst all. In this, an integrated insulation system i.e. thermal skin, devoid of any gaps is formed which encloses the entire building. Perimeter insulation is one particular type of external thermal insulation.
- Internal thermal insulation: This type of implementation is mainly appropriate for renovations
- Core thermal insulation: This implementation works suitably for both renovations of old buildings and newly constructed buildings. Core insulation is generally installed in the cavity of a wall.