Thermal Resistance and Thermal Response
The performance of an insulation material is measured by its 'Thermal Resistance' or 'R-value'. Uninsulated building sections, such as walls or roofs, also have an R-value. The greater the R-value, the more effective is the insulation or building section at resisting heat flow, and therefore the greater the level of insulation provided. While the R-value is an important influence on the rate of heat loss and heat gain from a house, the thermal comfort inside a house is also affected by the 'Thermal Response' of the building materials used.
The comment is often made that "double brick is better" from the point of view of thermal comfort. However, when the R- values of the common wall types are compared, they are quite similar uninsulated double brick (RO.50); uninsulated brick veneer (RO.46); and, uninsulated weatherboard (0.46, not shown on the diagram).
The difference in thermal comfort between these building materials can be explained by the difference in the thermal response of the materials.
Measurements of the R-value of materials are made in a laboratory using steady state conditions, that is, with a constant temperature difference across the material. In the real world, these steady state conditions are not usually present as the outside temperature changes throughout the day.
Under these changing conditions, the rate of heat flow through a building section is determined by both the R-value and the thermal response of the building materials.
The outside or ambient temperature may easily vary by as much as 7 to 25C during a night and day cycle. As the outside temperature cools down or heats up, so does the building structure, and it is this behaviour which we call the "Thermal Response'. A building of heavy material (ie brick or mud brick) responds slower than a building of lighter material (ie timber). This is known as 'thermal lag'.
If the thermal response is slow enough, the outside temperature changes before the temperature inside the building has fully responded, thus giving a low amplitude or flatter thermal response curve.
This indicates a building of more stable thermal conditions which does not require as much heating or cooling to maintain comfort conditions. It is for this reason, 'brick is better', but brick walls still need to be insulated as the outside temperatures in winter are mostly lower than the inside temperatures.
The R-value of the insulation and the R-value of the uninsulated building section can be combined to form the total thermal resistance (Rr) of the insulated building section. Total thermal resistances for common roof sections are shown in the diagrams below.
Note that the thermal resistance of an airspace is affected by directions of heat flow and the surfaces bounding the space.
For these reasons roofs have a different R-value for winter (heat flow up) and summer (heat flow down).
When quoting R-values, suppliers should use the metric unit for R-value which is
For bulk insulations, suppliers should quote the added R-value of their product, which is the value of the insulation by itself. For insulations which involve reflective foil, the added R-value of the product will be dependent on how the insulation is installed, that is, the size of the air gaps, and whether or not the insulation is installed vertically, or installed horizontally (or sloping) so that it can gather dust.
- m2= square metres
- K = degrees Kelvin (or Celsius)
- W = Watts of heat
How Much Is Enough?
The insulation level, or R-value, chosen should be appropriate for the climate.
For example, while an R-value of R1.5 - R3.0 would be appropriate for insulating a roof space in the Sydney region, an R- value of R3.5 - R4.0 would be appropriate for Armidale. Australian Standard AS 2627.1 - 1993 contains recommended R-values for roof/ceilings and walls for Australian dwellings. The recommendations are based, in general, on the cost of installing insulation, the cost of heating and cooling, and the climatic conditions in particular locations.
As mentioned above, care should be taken when interpreting the R-values for reflective foil insulations, as these have their best performance in summer. For locations which require winter heating it is important that the winter R-value is adequate.