Window specification – time to take a new view?

Set against a backdrop of stringent sustainability requirements and costly energy bills, specifiers are increasingly looking at new ways of designing projects that can help save both money and the environment.  Mark Wadsworth, managing director of Senior Architectural Systems explains

All buildings lose heat through windows and doors but the amount of energy that is lost can vary significantly depending on the type of system used. Specifying windows with double or even triple-glazing is an obvious starting point but choosing a system that contains a thermal barrier is where the real energy savings can be made.

Traditionally the material of choice for thermal barriers has been the low thermal conductor polyamide. However new product innovations are offering even greater energy efficiency, such as the development of a new window system that is the first on the UK market to incorporate a thermal barrier made from 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.

When it comes to U-value ratings, less is more and the lower the figure, the more heat 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 spacer bar, which provides the space and insulation between the two or three panes of glass. Part L of the Building Regulations, BREEAM, Code for Sustainable Homes and Passivhaus standard can all have requirements for the whole window U-values of windows and doors but as such legislation and guidelines can and often do change, it is well worth ‘future-proofing’ projects by specifying a window system that not only meets current targets but 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. By cutting back on CO2 emissions through the specification of low U-value windows, the project team potentially have the flexibility of looking at making monetary savings in the overall build cost by reducing the need for other, often more expensive, sustainable features such as photovoltaic roof panels or under floor insulation. But it’s not just about how well a product performs. As well as helping to cut the operational carbon emissions of a building, specifiers must also look at how they can reduce the calculations of embodied carbon and it is vital that product manufacturers evaluate their own processes and the energy used in the manufacture, transportation, assembly and deconstruction of materials.

As well as careful specification, care must also be given to the positioning of doors and windows within a building. 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. By using BIM, the installation process can be dramatically improved as schedules are automatically created with the BIM model also able to 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 to formulate an ongoing maintenance strategy, calculate the required U-values as well as assess and monitor the lifecycle costs.