The level of living comfort is determined by the quantity of heat that must be added to, or removed from a building; and insulation is designed to control this factor.
Heat always flows from warmer to colder areas i.e. from a heated building interior towards colder air outside or from hot outside air to cooler interiors. This movement or transfer of heat occurs by one or any combination of the following methods:
Heat energy is transferred directly through a material, or between two materials, in contact with each other, where a temperature difference exists. Heat transfer along a metal rod is a simple example of conduction such as the heat conducted along a teaspoon in a hot drink.
Air when heated, becomes less dense than the surrounding air, and rises upwards. The denser and cooler air flows downwards. When these air movements (convection currents) occur in spaces between the framing members of ceilings or walls, a significant amount of heat may be lost (winter) or gained (summer) reducing energy efficency.
Heat energy may be radiated across an 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 or reflected by roofs and walls even on cool days. Consider the options for controlling horizontal heat flow across a 50mm vertical air space in a wall. This chart shows the reduction of horizontal heat flow in a typical 50mm wall space.
Thermal Conductivity of EPS
The low thermal conductivity of EPS is a result of the combination of
1. The thermal conductivity of air within the cellular structure
2. The conduction of the solid polystyrene
3. Radiation across the fine cellular structure of each bead that forms the sheet.
The smallest contribution to thermal conductivity of EPS is the Polystyrene itself, and this contribution is in the order of 10% of the total.
Air that is not moving is an excellent insulating material. Because of this, the permanently entrapped air within the cells of EPS ensures its very low conductivity.
It is this factor which gives EPS its outstanding thermal insulation properties.
How EPS Insulates
EPS is an excellent thermal insulation material.
Heat is slowed dramatically passing through EPS by trapped air. Temperatures are more easily controlled as heat does not readily escape or enter through the panel. This makes it ideal for building products where efficient insulation is needed.
Heat Flow Terms and Comparisons
Within a material or between materials in close proximity heat will flow from a higher temperature 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 or between materials is dependent not only on the nature of the materials, but also upon the difference in temperature between the hot and cold sides.
Comparison of the effectiveness of insulation must be made on a basis, which excludes the influence of variable factors such as thickness and temperature differences. This comparison of thermal conductivity can be measured by the Thermal Conductivity ('k' value).
Thermal Conductivity - k value measured in W/mK
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 1 metre cube of material will transfer heat at the rate of 1 Watt for every degree of temperature (Kelvin) difference between opposite faces. The k value is expressed as 1W/mK.
Comparison of different building materials
Note: The lower the k value the more effective the insulation material.
EPS has a remarkably low k value compared with most other insulating materials used in similar applications.
Thermal Conductance - C value (W/m2K)
The Thermal Conductance or C value refers to any thickness of a material or structural component such as a wall or floor.
Thermal conductance is the amount of heat energy transmitted through 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.
Thermal Resistance - 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 i.e. R = thickness of material/k value. It is the inverse of Thermal Conductance.
The R value is a measure of the ability of a particular material to resist heat flow.