A critical aspect of the design process, the thermal performance of a building depends on a broad range of factors from the layout to the materiality. When building a new home, it’s important to consider how well different elements such as external walls, roofs, windows, doors and floors resist the flow of heat – in and out – during different times of the year.
Heat always moves from a hot area to a cool area. In summertime, one should look to reduce the heat coming into the house. In winter it’s the reverse when the warmth should be retained inside including heat that comes in through windows during the day.
While the flow of heat can’t be stopped completely, one could reduce the rate at which it flows by building with materials that have a high resistance to heat flow. Therefore, indoor comfort depends on good insulation between the internal and external environments. This will also help minimise reliance on heating and cooling, and keep your energy costs and greenhouse emissions down.
Measuring the R-value of building materials
A building material’s R-value indicates its ability to withstand heat moving through it, which is measured in terms of thermal resistance. The greater the resistance to heat flow, the higher the R-value. It’s calculated on the thickness and bulk density (kg/m³) of the material and the rate at which it conducts heat – the greater the bulk density of a material, the lower its thermal resistance. Materials that are poor conductors of heat are better insulators because heat moves through them at a slower rate.
The table below shows the R-values of brick, Hebel PowerPanelXL and fibre cement, which are used in external walls. Even though a standard 110mm brick is thicker than the 75mm Hebel PowerPanelXL panel, brick has poorer thermal resistance due to its higher bulk density compared to Hebel. This indicates the role of thermal conductivity or the transfer of heat through a material and is influenced by different factors including its microstructure, composition, bulk density and size and structure of pores*.
R-values provided for brick veneer by James Fricker calculation 107.50 dated July 2010.
Difference between the R-value of materials and the building system
The brick veneer or rendered Hebel PowerPanelXL wall cladding on an external wall plays a vital role in resisting heat flow. However, there are multiple components that contribute to the thermal resistance of the wall such as the outside air surface, internal air spaces, insulation materials as well as fixings and the wall frame itself.
Collectively they form a building system or more specifically, a wall system and they’re used to calculate the total system R-value. The same approach applies to floors, roofs, windows and doors, which are also building systems.
Below are examples of total R-values of two external wall systems – Hebel and brick veneer – both using double reflective foil insulation.
How R-values are used for home energy efficiency assessments
Under the National Construction Code, building materials and systems need to achieve minimum levels of thermal performance – expressed in R-values – according to the climate zone and elevation above or below 300 metres. There are eight climate zones ranging from Zone 1 for the hottest to Zone 8 the coldest. Although each zone will have seasonal variations, there will be a predominant condition that needs to be addressed. For the Zone 1 rating, the concern is how much energy is likely to be used to cool the home in summer and for Zone 8, it’s heating in winter. There will also be state and territory variations covered in the National Construction Code.
These R-values are important in the design of a new home, which needs to achieve a minimum 6-star energy efficiency rating. This rating is an indicator of how much heating or cooling might be needed for comfortable living and is determined by an energy efficiency assessor. The assessment includes R-values of the building fabric (such as the roof, walls, floors, windows and doors), location and orientation of the home, air movement and external colours.
CSR Hebel provides summer and winter total R-values for its external wall systems, which designers use as reference to determine the best Hebel system for the zone, and assessors, to determine the energy efficiency rating.
Higher R-values for long term comfort
Building materials and systems with high R-values are more efficient in insulating your home than those with low R-values. By making the correct selection of materials and systems at the design stage, you can ensure comfortable living and energy efficiency in your home for the long term.
*Thermal Conductivity of Clay Bricks, M. Dondi et al, Journal of Materials in Civil Engineering, 16  (2004) 8-14 by American Society of Civil Engineers.