The energy efficiency of a building is influenced by different design aspects such as orientation, insulation, ventilation, shading, sealing and building size, all interacting with each other.
Double glazed windows are often recommended to increase the energy efficiency of buildings, using their thermal insulation, improved efficiency of heating and cooling systems and noise attenuation as selling points.
The relentless promotion of double glazed windows as the answer to all energy efficiency needs has created a number of myths.
Myth 1: Double glazing is required by the Building Code
The Building Code of Australia specifies requirements to ensure a reasonable level of energy efficiency. These requirements relate to the performance and usage of the windows, not to the type or design.
The Building Code may require a specific window to meet or exceed specific performance levels, but does not dictate how that performance level must be achieved. There are a range of options available to window designers to change the performance of a window including window type, glazing thickness, glazing coatings, double glazing, framing materials and thermal breaks within the framing.
Myth 2: Double glazing gives better thermal insulation than single glazing
The energy efficiency characteristics of Australian window systems are modelled to international standards and are publicly available on the Window Energy Rating Scheme (WERS) website. WERS contains ratings of around 3,000 window systems and is the recognised source for energy efficiency characteristics of Australian window systems.
The rate of non-solar heat loss or gain through a window is measured on a whole-of-window basis (framing and glazing combined) and is measured by the U-value. A lower U-value indicates that the window has greater resistance to heat flow and offers better insulation.
A review of aluminium-framed window ratings available from the WERS website shows that although double glazed windows can have very low (better) U-values, there is a wide range of performance. When compared to aluminium-framed Altair louvre windows, it is clear many double glazed windows actually have higher (worse) U-values than some configurations of single-glazed Altair louvre windows from Breezway .
Myth 3: Double glazing always improves the energy efficiency of a building
This myth might be true only if the building is sealed up tightly and the air conditioning system is run 24 hours per day and 7 days per week. However, given the choice, most people prefer to open their windows and doors and enjoy fresh, natural breezes and a connection to the world outside.
At identical sizes, Altair louvre windows deliver significantly more cooling and natural ventilation than any double glazed window. With more cooling ventilation, there is less need to run air-conditioning systems, leading to energy savings.
Myth 4: Double glazing is more environment-friendly than single glazing
While some (but not all) double glazed windows may improve the energy efficiency of a building, there are a number of other aspects of environmental friendliness where single glazed windows outperform double glazed windows:
Raw material usage and embodied energy.
Double glazed windows require more glass to manufacture. Additionally, the spacers that hold the two panes of glass apart also require raw materials that would not be needed for single glazed windows. Larger quantities of energy are required to produce larger quantities of raw materials.
Double glazed windows are heavier than single glazed windows, often requiring specialised machinery to handle them and more energy to transport them from the window fabricator to the building site.
Make an educated decision
In some circumstances double glazed windows may be the best option to achieve energy efficiency; however it is important to consider all the other factors that contribute towards naturally comfortable buildings.