GLASS WOOL USED IN INSULATION

GLASS WOOL USED IN INSULATION

Glass wool (originally also known as fiberglass) is an insulating material made from fiberglass arranged using adhesives into a texture similar to wool.Glass wool and stone wool are produced from mineral fibers and are therefore often referred to as “mineral wool”. Mineral wool is the common name for fiber materials formed by spinning or drawing molten minerals. Glass wool is a molten glass furnace product at a temperature of about 1450 ° C. From molten glass, fibers are spun into fibers.

                                                                                                                                                            Nanoflex sheet glass wool

The process is based on spinning molten glass fiber in a high-speed spinning head just like the process used to produce cotton candy. During fiberglass spinning, a bonding agent is injected. Glass wool is then produced in rolls or sheets, with different mechanical and thermal properties. It can also be produced as a material that can be sprayed or applied topically, on insulated surfaces.

Applications of glass wool include structural insulation, pipe insulation, filtration and sound insulation. Glass wool is a versatile material that can be used to insulate walls, roofs and floors. It can be a liquid filling material, blown into the attic, or, together with an active adhesive is sprayed on the underside of the structures. During the installation of glass wool, it needs to be kept dry at all times, since an increase in humidity causes a significant increase in thermal conductivity.

Thermal conductivity of glass wool.

Thermal conductivity is defined as the amount of heat (in watts) that is transmitted over an area of a material of a certain thickness (in meters) due to temperature differences . The lower the thermal conductivity of the material, the greater the resistance to heat transfer of the material and therefore the greater the efficiency of the insulation. 

The characteristic thermal conductivity value for glass wool fibers is between 0.023 and 0.040W/m ∙ K.

 In general, insulation is mainly based on the very low thermal conductivity of gases . Gases have poor thermal conductivity properties compared to liquids and solids, and therefore produce good insulation if they can be trapped (for example in a foam-like structure). Air and other gases are generally good insulators. But the main benefit is in the absence of convection. 
 Therefore, many insulating materials (e.g. glass wool) work simply by having a large number of gas-filled bags, preventing large-scale convection.

 The alternation of airbags and solid materials causes heat to be transmitted through multiple interfaces, causing the heat transfer coefficient to drop rapidly.

 Example – Glass insulation:

 A major source of heat loss from a house is through walls. Calculate the speed from thermal passage through a wall with an area of 3 mx 10 m (A = 30 m 2 ). The wall is 15 cm (L 1 ) thick and it is made of bricks with thermal conductivity of k 1 = 1.0 W / mK (poor heat insulator). Suppose that, indoor and outdoor temperatures are 22 ° C and -8 ° C, and the coefficients of convective heat transfer inside and outside are h 1 = 10 W / m 2 K and h 2 = 30 W / m 2K, respectively.   Note that, these convection coefficients are highly dependent on ambient and internal conditions (wind, humidity, etc.).

   Calculate the heat flux (heat loss) through this non-insulated wall.

Now suppose insulation on the outer side of this wall. Use 10 cm thick insulated glass wool (L2 ) with a thermal conductivity of k2 = 0.023 W / mK and calculate the heat flux ( heat loss ) through this composite wall.

   Solution:

As has already been written, many heat transfer processes involve mixed systems and even involve a combination of both conduction and convection . With composite systems, it is usually convenient to work with an overall heat transfer coefficient, called a U-factor. The element U is defined by an expression similar to Newton’s law of cooling: 

   The overall heat transfer coefficients are related to the total heat resistance and depend on the shape of the problem.

Ceiling wall:

Assuming one-way heat transfer through a flat wall and ignoring radiation, the overall heat transfer coefficient can be calculated as follows: 

  The overall heat transfer coefficients are then:

U = 1 / (1/10 + 0.15 / 1 + 1/30) = 3.53 W / m 2 K

Thermal flux can be calculated simply as follows:

q = 3.53 [W/m2 K] x 30 [K] = 105.9 W/m2

The total heat loss through this wall will be:

q loss = q. A = 105.9 [W/m2 ] x 30 [m2 ] = 3177W

  Composite walls with insulation

Assuming one-way heat transfer through a composite wall, without thermal contact resistance and disregard for radiation, the overall heat transfer coefficient can be calculated as follows:

The overall heat transfer coefficients are then:

U = 1 / (1/10 + 0.15 / 1 + 0.1 / 0.023 + 1/30) = 0.216 W / m 2 K

  Thermal flux can be calculated simply as follows:

q = 0.216 [W/m2 K] x 30 [K] = 6.48 W/m2

The total heat loss through this wall will be:

q loss = q. A = 6.48 [W/m2 ] x 30 [m2 ] = 194 W

  As can be seen, the addition of thermal insulators significantly reduces heat loss. It must be added, the subsequent addition of insulation does not cause such high savings. This can be seen more clearly from the heat resistance method, which can be used to calculate the heat transfer through composite walls. The steady rate of heat transfer between two surfaces is equal to the temperature difference divided by the total thermal resistance between those two surfaces.