EPS as Thermal Insulation

General

The excellent insulating properties of EPS, derived from its microcellular closed cell construction, provide one of its most important and widely used properties.

From sub zero temperatures as low as -40°C experienced in freezer insulation, to the high temperatures around 60°C occurring on hot water pipes, EPS provides efficient, cost effective insulation. In countless everyday situations EPS is widely used for its insulating ability. In the construction and food industries EPS is the first choice for insulation.

Insulation Fundamentals

Heat always flows from warmer to colder areas. This movement or transfer of heat occurs by one or any combination of three following methods:

• Conduction
  Heat energy is transferred directly through materials in contact with each other, where a temperature difference exists. Heat transfer along a metal rod is a simple example of conduction.
   
• Convection
  Air, when heated, becomes less dense than the surrounding air and rises upwards. The denser and cooler air flows downwards. These air movements, known as convection currents, can occur in spaces between the framing members of ceilings or walls of buildings causing a significant amount of heat loss.
   
• Radiation
  Heat energy may be radiated across the air space and then be absorbed by another body. Radiant energy from the sun is an example, where this energy may be absorbed as heat by the human body.

An example of all three methods of heat flow occurs in the wall space of buildings. The following graph demonstrates the effects of adding a reflective surface and of filling the air space with an insulation material such as EPS. Clearly, heat transfer by convection, a major component of heat flow, can be almost eliminated by the use of insulation.
 

Heatflow Terms and Measurement

Heat, as has been stated, will flow from a higher to a lower temperature through one, or any combination of all three heat transfer methods - conduction, convection and radiation. The rate at which heat will flow through a material is dependent not only on the nature of the material, but also upon the difference in temperature between the hot and cold sides. Comparisons of the effectiveness of insulation must be made on a basis which excludes the influence of variable factors such as thickness and the temperature differences.

k Value (W/mK)

The comparison of thermal conductivity can be measured by the 'k' value. The k value, or Thermal Conductivity, specifies the rate of heat transfer in any homogeneous material. If a material has a k value of 1, it means a 1m cube of material will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. The k value is expressed as 1 W/mK. The lower this value is, the less heat the material will transfer. The following chart shows the k value of a number of common materials, and demonstrates that EPS has a low thermal conductivity compared with other materials.

Typical k Values (W/mK)

The lower the value, the higher the insulation ability

r Value (1/k)

The 'r' value, or Thermal Resistivity, is the symbol that refers to unit thickness and is defined as the reciprocal of thermal conductivity (k value).

C Value (W/m2K)

The 'C' value, or Thermal Conductance, refers to a particular thickness of a material or structural component such as a wall or floor. Thermal conductance is the amount of heat energy transmitted through the unit area of a structural component (or of a structure) per unit temperature difference between the hot and cold faces. The value of C is expressed in W/m2K.

Where a thickness of material other than 1 m is used the 'C' value must apply and the thickness of the material, or structural component, must be stated.

R Value (m2K/W)

The 'R' value, or Thermal Resistance of a material, expresses the ability of a particular thickness of that material to resist heat flow. The definition of R value is the reciprocal of the material's thermal conductance (C value). The R value refers to the thermal resistance of a material, or assembly of materials such as the wall of a building, and is used to find the overall thermal resistance of an assembly of materials by simply adding individual component R values. The following calculations are practical examples of how the R value is used:

Problem 1
Find the R value of a 100mm thick piece of SL class EPS.

d (thickness) = 100mm = 0.10m
k (thermal conductivity) = 0.039

R        =    _d (m)_
                 k (w/mk)

R        =    0.100(m)
                 0.039 (w/mk)

R        = 2.56m2 K/W

Problem 2
Find the thickness of SL class EPS required to achieve an R value of 2.5

Required R Value = 2.5m2 K/W
K (Thermal conductivity) of SL Class EPS  =

d   =   thickness
d   =   R x k
d   =   2.5m2KkW x 0.039 W/mK
d   =   0.098
or d = 98mm

Typical R values for 50 mm the EPS are:

(at mean temperature + 15°c)

U Value (W/m2K)

The 'U' value, or heat transfer coefficient, is quite similar to the 'k' value in that it is a measure of the quantity of heat which will flow through a specific section one square metre in area during one hour when there is a hot to cold side temperature difference of 1K. The U value is used when a section is made up of a number of elements and it depends on the 'k' or 'R' value of the products which make up that section, because it measures heat transmission or conductivity, the lower the 'U' the better the insulation value.

The 'U' value is the reciprocal of R value. It is easily determined by simply adding the total of the R values of all the elements in the section, and then taking the reciprocal of the total R value. The 'U' value is used by heating and air conditioning engineers when designing equipment. The following example shows how the 'U' value is calculated.

Problem 3
Find the total 'U' value of a wall consisting of double cavity brick with 50mm of SL class EPS between 100mm thick bricks.

k value brick = 1.15
d (brick) = 100mm
k value SL Class EPS = 0.039
d (EPS) = 50mm

Rt = R Total
Ut = U Total

R (Brick)     = 100mm           = 0.087m2k/w
                        1.15W/mk

R (EPS)      = 0.50                 = 1.28m2k/W
                        0.039 w/mk
                       

R (Total)    = R (Brick) 0.087 + R (EPS) 1.28 = R (Brick) 0.087
Rt                = 1.454m2 K/W

U (Total)     = 1/Rt
Ut                 = 1/1.454   = 0.69 W/m2k

Ut                 = 0.69 W/m2K 

The Effect of Moisture on Insulating Materials

The moisture content of an insulating material at the time of testing can have a considerable effect on the value of thermal conductivity obtained, and is probably responsible for some of the variation in published 'k' values.

Of all materials used for insulation applications, EPS is one of the most resistant to the adverse effects of moisture. Condensation, which may build up within any insulation material under critical vapour flow conditions, only marginally affects the thermal performance of EPS. Even if condensation develops through improper use EPS will retain its dimensional stability and superior insulation values. The " Water content " chart demonstrates the effect of moisture on k values of several commonly used insulation materials.

Variation in Mean Temperature

The thermal conductivity of EPS varies with the mean or average, temperature on each side of the EPS. As the mean temperature decreases, so the thermal conductivity of EPS decreases, making it more efficient as an insulation material. Thermal conductivity in Europe is usually tested with a mean temperature of + 10ºC, but in Australia it is tested at a mean temperature of +25ºC. For New Zealand conditions, figures quoted in this publication are measured at +15ºC.

It is important when selecting EPS to ensure that when material is selected the mean temperature is known.