Films to Improve Office Comfort

Window films can cut solar gain and thus cooling bills and the carbon footprint of glass-dominated commercial buildings. Yet they are little used. EVORA EDGE’s Andrew Cooper investigates.

Window film has traditionally been overlooked as a cost-effective method to improve office comfort and energy efficiency during retrofits of commercial buildings. The reasons are not immediately clear. Even among building physicists and energy modellers, such as myself, this technology remains on the periphery despite the importance of passive technologies in our work.

But as the predictions around climate change gain greater urgency, the pressure from governments and corporate social responsibility policies will be to decarbonise buildings faster and further than before.

Building engineers will also find themselves having to improve the resilience of buildings to climate change scenarios such as hotter summers. There is increasing evidence that the 2°C global target for this century, set at the Paris Climate Conference in 2015, will be exceeded.

We at Evora Edge have already been asked to model the impacts of climate change scenarios on large building portfolios owned by a major pension fund and we expect this type of work to increase over the next few years. To meet such challenges all aspects of a building will need to be examined and, as window film technology gets increasingly sophisticated, it will be harder to overlook the contribution it can make to both energy efficiency and occupier comfort and wellbeing.

The Physics of Windows

This largely comes down to the properties of visible light and thermal radiation. Visible light reduces the need for artificial light, and research has shown that natural light can improve wellbeing. However, visible light can also lead to glare with a negative effect on productivity and comfort. Meanwhile, thermal radiation transfers heat. It can replace or supplement heat generated by building services but can also lead to overheating and additional need for cooling.

Both visible light and thermal radiation have similar properties and during transmission they can be reflected (ie the direction of the wave changes as it moves away from a medium), refracted (ie the wave direction changes as it passes through a medium) and/or absorbed (ie the wave does not reflect or refract, it is absorbed).

Figure 1 summarises the solar thermal heat flow through a single glazed window. In this example the drawing shows that, where the value of solar gain is 100%, 82% is directly transmitted to the space, while 8% is reflected, and 10% absorbed (5% each side). This would place the ‘g’ value at 0.87 (87% total energy transmission) where a g-value of 1 = 100% total energy transmission.

In practice the amount of solar energy absorbed would not be transferred ‘uniformly’ into each side of the space since environmental and local factors affected this heat transfer; including the temperature heat gradient and the weather conditions on the outside face.

The Role of Window Film

Simply put, window film changes the properties of a window pane by reducing the amount of ‘energy rich’ shortwave radiated solar heat and the amount of visible light transmitted into a space. It can also reflect longwave radiated heat back into a space to reduce winter heating loads. Although window film is not itself a new concept, the technology has evolved through complex manufacturing processes including the use of precious metals.

This means the application of film can change the U-value of a window, its g-value and reduce visible light and glare. It can be specified to deliver these benefits independently, or through a process of layering – where they can be combined, with each layer having different values depending on the specific purpose it is required for.

All these layers serve very specific functions and can total as little as 45 micron thickness for internal films or 75 micron for external and to a maximum of 325 micron for safety films. Dependent upon the film type, they can be applied to the outer surface of a pane, the inner surface of a pane – or even to one of the two middle surfaces of a double-glazed unit if specified by the manufacturer. This makes them highly versatile for architects, designers and engineers alike.

Building Modelling

I believe part of the reason window film has been overlooked in the past is the inability to get accurate data on how it contributes to the improved sustainability credentials of a building in a virtual environment. Modelling is being increasingly used for all building projects both as part of the initial design process, but also because it is required when ensuring compliance with Building Regulations and providing Energy Performance Certificates.

Yet it is currently difficult to accurately model the benefits of window film and to ensure a consistent approach to data inputs and the subsequent results (the modelled outputs). We know this because at EVORA EDGE we have recently been engaged in a research project using LLumar window film, manufactured by Eastman Performance Films.

The idea is to create a Building Information Modelling (BIM) library which would allow engineers, architects and modellers to simply download a type of window film and easily add it into a model along with user guides on how to correctly install and achieve benefits.

The work was challenging because the differences between the various types of modelling software can produce different results depending on their purpose and default setting. This is not new, the challenges around building models have been argued for years, but with the completion of the LLumar BIM Library we are one step closer to making building models more accurately replicate the real world.

Window Film and Energy

EVORA EDGE has modelled the benefit of window film, but there is also ‘real world’ evidence of its benefits. For example, a study by Professor Michele De Carli and his research team at the University of Padua looked at energy consumption and savings in the MG Tower building in Padua, Italy both before and after the application of a LLumar Helios solar control exterior window film.

In addition to the quantitative analysis techniques used, surveys were undertaken to identify the perceived impact of the film on the building occupants. The results showed the building reduced its annual carbon dioxide emissions by 46 tonnes following the application of the window film and there was a return on investment within four years, largely through reduced need for air conditioning.

The window film reduced indoor temperatures by up to 5°C when HVAC cooling had been switched off. The reduced need for air-conditioning and significant glare reduction significantly increased occupant satisfaction and the research team concluded this increased satisfaction improved productivity by €40 per employee per cooling month.

The application of the film was shown to be a better investment option than increasing HVAC capacity, and with no negative impacts (either through occupant satisfaction or increased lighting use) recorded during lower lighting levels of the year.

Summary

The last two decades have seen an explosion in the amount of glass used in commercial buildings with less thought given to the environmental and thermal impact of these shimmery, reflective, architectural dreams – with many windows not openable.

Architects and planners need to consider the environmental cost of such designs and glass manufacturers are starting to look for different technologies to reduce this impact. This means that, for some buildings, finding retrofit solutions will be the only option to improve the carbon footprint and thermal comfort of so much glass.

It’s hard to see how building engineers will be able to ignore the use of window film for much longer.