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Can a Window Be as Thermally Efficient as a Wall?
Adding insulation to walls in new construction or upgrading insulation in existing residential buildings has gradually become common sense in Australia.
It has been seen as a relatively cost-effective way of improving the star energy efficiency rating of new builds and renovations. However, we also know that holes and thermal bridges in the building fabric compromise thermal performance and a major source of these are the windows.
Windows are traditionally the weakest link in residential buildings when it comes to energy efficiency but here, we love our windows! So, can a window be as thermally efficient as a wall and help improve the energy star rating of our homes?
We normally measure the thermal performance of walls with R-values, the measure of resistance of a building fabric to heat flow, where the higher the better. On the other hand, the thermal performance of windows is usually expressed in U-values, the measure of heat loss through the window system, the lower the better.
For the purpose of comparison, using the U-value measure, an uninsulated cavity wall might have a U-value of approximately 1.6 W/m2K, while a solid wall approximately 2.0 W/m2K. Adding insulation will improve the U-values even further.
The Building Code of Australia suggests that for Melbourne, the minimum total R-value for walls should be 2.8, which is equivalent to a U-value of less than 0.4 W/m2K.
But window systems currently offered in Australia commonly have U-values of well over 4.0 W/m2K. In fact, of the WERS rated residential window products offered in Victoria for example, almost half have U-values at or higher than 4.0 W/m2K. Less than three per cent of products offered have U values under 2.0 W/m2K.
A 2010 report prepared for the government investigating the pathway to 2020 for low energy, low carbon buildings found that there are diminishing returns to adding or upgrading insulation in walls and roofs. Once insulation has been optimised, windows become the next most important element in residential energy efficiency.
Good building design optimises solar passive design through window distribution, glazing ratios as well as orientation. Good window design means considering the glazing, sealing, frame material and installation of the window system.
Glazing options are growing in Australia as supply of insulated glass units (IGUs) and low emissivity coatings improves. Single glazed clear glass should be a thing of the past, as it is in American residential architecture. The standard in the US for some time has been an insulated double glazed low emissivity window.
Better performance is achievable with hermetically sealed IGUs with at the very least a 12-millimetre gap between panes. Less than a 12-millimetre gap and the insulation effect starts to drop exponentially. Increasing the gap to 16 millimetres can reduce U-values closer to, or below two where low conductivity frames are used.
To complicate things, it is not necessarily the same glazing system for every window in a building – the choice of glazing depends on the building’s design, the window’s design and orientation as well as climate. Visible light transmission and solar heat gain coefficient are necessary considerations.
Higher performing windows have continuous, built-in double sealing systems and multiple locking points to provide a tight seal between the sash frame and the house frame reducing air infiltration. It is also worth looking at whether there are pressure seals on sliding glazed doors and windows to reduce the progressive wear and tear that otherwise occurs with regular seals.
It has been reported that a fairly common practice in Australia is to rely on the architrave to provide a barrier to the wall cavity instead of sealing windows between the window frame and house frame.
The material used in frames plays a significant role, not just for structural strength but also for thermal performance of the window system. Frames should overall be poor thermal conductors. Aluminium, however, is the fourth most conductive material (after silver, copper and gold) and about 1,000 times more conductive than uPVC. It therefore needs thermal breaks to improve performance.
Thermal resistance of a window system can be improved by choosing insulating materials like uPVC or timber and/or frames with insulated cavities and the thermal bridges eliminated. Highly thermally conductive frames have become virtually extinct in Europe, yet remain a standard offering in Australia.
To improve thermal performance, windows need to be installed airtight to the building. This can be achieved by using expanding foams or tapes, which together with good sealing systems can dramatically improve airtightness. Under the Building Code, all windows have to meet Australian Standard air leakage rates. The UK and the USA, for example, go further by mandating physical air tightness testing of new buildings.
There is no doubt that windows have a hugely important role to play in improving the energy efficiency of residential buildings. What we install today, whether for new builds or upgrades, will potentially lock in greenhouse gas emission impacts for decades.
A valid concern is the initial purchasing cost. High performance windows are more expensive in Australia than the incumbent standard, single glazed, thermally bridged window. And no doubt a function of supply-demand, they are costlier here than in major overseas markets where they are mainstream.
However, an Australian Window Association capacity/capability study found there was capacity locally to switch to majority fabrication of IGUs within 12 months. Based on overseas experience, the cost of IGUs can be expected to drop substantially when codes and policies shift the market to higher performing windows. With the right policy transition pathways and market pull, there is no reason not to expect a move to efficiently insulating windows similar to that of the US or European window markets.
So can windows be as thermally efficient as a wall? In theory, yes, evidenced by the emergence of ‘Super Windows’ with U-values less than 1.0 W/m2K in overseas markets. But here, we must first acknowledge that window systems are an important element for achieving residential energy efficiency.
This article was first published in Sourceable.net September 1st, 2016