Windows worth their mettle

James Keeling-Heane of Senior Architectural Systems explains how developments in aluminium window systems can shed new light on cost-effective ways of ensuring energy performance targets are met.

All properties lose heat through windows and doors, but the amount of energy that is lost can vary significantly depending on the type of system used. Replacing older, single-glazed windows with double or even triple-glazing is an obvious starting point, but choosing aluminium systems can open up even greater possibilities.

Aluminium is in itself a good conductor of heat and a poor insulator, but on its own it offers little protection from heat loss and solar gain. It wasn’t until manufacturers looked to make technological advances in the thermal performance of aluminium that this type of metal framed windows really came into their own.

The use of an effective insulating material between the inner and outer frame is key to avoiding thermal bridging which, put at its simplest, is a weak spot in the insulation of a building that allows heat to escape from the inside out, and for cold air to pass back into the room. As well as losing energy, a thermal bridge can also cause cold spots and drafts which can contribute to making a home harder to heat.

The development of thermally broken window frames provided a solution, but despite this innovation, the earliest examples were not without their issues. The use of polyurethane as an insulator to prevent heat loss between the inner and outer frame provided only limited thermal performance, and was prone to shrink over time and create leaks in the frame. The next generation of aluminium-framed windows used a much stronger polyamide to create a thermal break and provide increased protection against heat loss.

The best products on the market have embraced further innovation, such as the incorporation of expanded polyurethane foam, a material that is more commonly used in insulation and cladding products and that has long been recognised for its excellent thermal properties.

Understanding u-values

When it comes to U-value ratings, less is more – therefore, the lower the figure, the more heat that is retained. The U-value of a window system is dependent on a number of factors, including the frame material, the type of glazing and the use of a warm edge spacer bar, which provides the space and insulation between the two or three panes of glass. As legislation and guidelines can (and often do) change, it is well worth ‘future-proofing’ housing stock by specifying a window system that not only meets current targets, but actually exceeds them.

By achieving U-values that are far lower than stipulated, the thermal performance of a building is not only improved but, significantly, major improvements can be made to the overall carbon footprint.

Perfect planning

As well as careful specification, care must also be given to the positioning of windows within a property. During the installation process, details such as ensuring the continuity of insulation by wrapping it around the frame, and paying attention to the interfaces with the walls to achieve the maximum levels of airtightness, are essential if the energy efficient system is to perform to its full potential.

Here, the use of Building Information Modelling (BIM) can reduce risk and provide detailed information on the size, finish and positioning of a window, such as if it is top or bottom hung. The BIM model can also contain information on the life expectancy of the window system, making it significantly easier for those working in the rental market to formulate an ongoing maintenance strategy, calculate the required U-values, as well as assess and monitor the lifecycle costs.

Style with substance

Aesthetic appeal is a major advantage of aluminium windows. The material’s inherent strength means larger glazing panels can be supported in much narrower frames, resulting in sleek and slim profile windows which maximise the flow of light. Offering an almost unlimited selection of colours and finishes, and an extensive choice of styles and configurations, there’s flexibility to match any design scheme.

As well as keeping heat in, windows also need to keep the noise out. Although this is another area where PVCu systems have traditionally been seen to have the edge, technological advancements in the development of the aluminium systems means this is no longer the case – with double and triple glazing helping boost both thermal and acoustic performance.

James Keeling-Heane is Sales director at Senior Architectural Systems