Generally, polystyrene is a synthetic aromatic polymer made from the monomer styrene, derived from benzene and ethylene, both petroleum products. Polystyrene can be solid or foamed. Polystyrene is a colorless, transparent thermoplastic that is commonly used to make foam board or beadboard insulation and a type of loose-fill insulation consisting of small polystyrene beads. Polystyrene foams are 95-98% air. Polystyrene foams are good thermal insulators and are often used as building insulation materials, such as insulating concrete forms and structural insulated panel building systems. Expanded polystyrene and extruded polystyrene are both made from polystyrene. Still, EPS is composed of small plastic beads fused together, and XPS begins as a molten material that is pressed out of a form into sheets. XPS is most commonly used as foam board insulation.
Extruded polystyrene (XPS) is also a thermoplastic polymer. Extruded polystyrene has a closed-cell structure and is often stronger, with higher mechanical performance, and is, in principle, often more expensive than EPS. Its density range is about 28–45 kg/m3. XPS is produced from the same base materials like EPS and has crude oil as its basis. The production process of extruded polystyrene is only slightly different from expanded polystyrene.
Similar to EPS, XPS has a wide variety of applications. It can be used to insulate buildings, roofs, and concrete floors. Extruded polystyrene material can also be used in crafts and model building, particularly architectural models.
Although both expanded and extruded polystyrene have a closed-cell structure, they are permeable by water molecules and can not be considered a vapor barrier. There are interstitial gaps between the expanded closed-cell pellets in expanded polystyrene that form an open network of channels between the bonded pellets. If the water freezes into ice, it expands and can cause polystyrene pellets to break off from the foam.
In further reading, there is a brief description of these types of insulation materials.
Inorganic Insulation Materials
As can be seen from the figure, inorganic materials can be classified accordingly:
- Fibrous materials
- Glass wool
- Rock wool
- Cellular materials
- Calcium silicate
- Cellular glass
Organic Insulation Materials
The organic insulation materials treated in this section are all derived from a petrochemical or renewable feedstock (bio-based). Almost all of the petrochemical insulation materials are in the form of polymers. As can be seen from the figure, all petrochemical insulation materials are cellular, and material is cellular when the structure of the material consists of pores or cells. On the other hand, many plants contain fibers for their strength. Therefore almost all the bio-based insulation materials are fibrous (except expanded cork, which is cellular).
Organic insulation materials can be classified accordingly:
- Petrochemical materials (oil/coal derived)
- Expanded polystyrene (EPS)
- Extruded polystyrene (XPS)
- Polyurethane (PUR)
- Phenolic foam
- Polyisocyanurate foam (PIR)
- Renewable materials (plant/animal derived)
- Cellulose
- Cork
- Woodfibre
- Hemp fibre
- Flax wool
- Sheeps wool
- Cotton insulation
Other Insulation Materials
- Cellular Glass
- Aerogel
- Vacuum Panels
Thermal Conductivity of Extruded Polystyrene
Thermal conductivity is defined as the amount of heat (in watts) transferred through a square area of material of a given thickness (in meters) due to a difference in temperature. The lower the thermal conductivity of the material, the greater the material’s ability to resist heat transfer, and hence the greater the insulation’s effectiveness. Typical thermal conductivity values for extruded polystyrene are between 0.025 and 0.040W/m∙K.
In general, thermal insulation is primarily based on the very low thermal conductivity of gases. Gases possess poor thermal conduction properties compared to liquids and solids and thus make a good insulation material if they can be trapped (e.g., 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., extruded polystyrene) function simply by having a large number of gas-filled pockets which prevent large-scale convection.
Alternation of gas pocket and solid material causes that the heat must be transferred through many interfaces causing rapid decrease in heat transfer coefficient.
Example – Extruded Polystyrene Insulation
A major source of heat loss from a house is through walls. Calculate the rate of heat flux through a wall 3 m x 10 m in the area (A = 30 m2). The wall is 15 cm thick (L1), and it is made of bricks with thermal conductivity of k1 = 1.0 W/m.K (poor thermal insulator). Assume that the indoor and the outdoor temperatures are 22°C and -8°C, and the convection heat transfer coefficients on the inner and the outer sides are h1 = 10 W/m2K and h2 = 30 W/m2K, respectively. Note that these convection coefficients strongly depend on ambient and interior conditions (wind, humidity, etc.).
- Calculate the heat flux (heat loss) through this non-insulated wall.
- Now assume thermal insulation on the outer side of this wall. Use extruded polystyrene insulation 10 cm thick (L2) with the thermal conductivity of k2 = 0.028 W/m.K and calculate the heat flux (heat loss) through this composite wall.
Solution:
As was written, many heat transfer processes involve composite systems and even involve a combination of conduction and convection. It is often convenient to work with an overall heat transfer coefficient, known as a U-factor with these composite systems. The U-factor is defined by an expression analogous to Newton’s law of cooling:
The overall heat transfer coefficient is related to the total thermal resistance and depends on the geometry of the problem.
- bare wall
Assuming one-dimensional heat transfer through the plane wall and disregarding radiation, the overall heat transfer coefficient can be calculated as:
The overall heat transfer coefficient is then:
U = 1 / (1/10 + 0.15/1 + 1/30) = 3.53 W/m2K
The heat flux can be then calculated simply as:
q = 3.53 [W/m2K] x 30 [K] = 105.9 W/m2
The total heat loss through this wall will be:
qloss = q . A = 105.9 [W/m2] x 30 [m2] = 3177W
- composite wall with thermal insulation
Assuming one-dimensional heat transfer through the plane composite wall, no thermal contact resistance, and disregarding radiation, the overall heat transfer coefficient can be calculated as:
The overall heat transfer coefficient is then:
U = 1 / (1/10 + 0.15/1 + 0.1/0.028 + 1/30) = 0.259 W/m2K
The heat flux can be then calculated simply as:
q = 0.259 [W/m2K] x 30 [K] = 7.78 W/m2
The total heat loss through this wall will be:
qloss = q . A = 7.78 [W/m2] x 30 [m2] = 233 W
As can be seen, adding a thermal insulator causes a significant decrease in heat losses. It must be added that adding the next layer of the thermal insulator does not cause such high savings. This can be better seen from the thermal resistance method, which can be used to calculate the heat transfer through composite walls. The rate of steady heat transfer between two surfaces is equal to the temperature difference divided by the total thermal resistance between those two surfaces.
