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Understanding Windows

Updated: Dec 12, 2019

Windows are vital to the performance of any building. They are a critical design element that allow natural light and ventilation into a building. They also provide the opportunity for those inside to see outside. This may be to look at a beautiful vista or into a small courtyard. Being able to see the outside world has a strong psychological impact as it allows us to maintain our internal clocks and it allows us to mentally extend beyond the walls of the space we are in.

In the past, window openings were not the dominant feature of an external wall. Think of a terrace house as an example. This was due in part to manufacturing techniques as well as a way of maintaining control of a building’s temperature. The modern approach is to provide a lot of glazing – to promote an indoor / outdoor connection and to bring the outdoors inside.

The downside of a lot of glazing is that it is harder to control the indoor temperature and energy usage with up to 40% of heating energy lost and 87% of heat gained through windows. Passive solar design at the design stage is the best way to have an efficient and cost effective solution. We consider both the macro and the micro issues. The macro issues include the building’s location, orientation, layout of the building, the building’s materials and the size of the windows and shading systems. The micro issues revolve around the design and selection of the window systems.

Choosing the right window system is often determined by two values – the U-Value and the Solar Heat Gain Coefficient (SHGC). The U-Value is a measure of how much heat energy is transmitted through the window system – this includes the glass, frame, seals and any spacers. The U-Value is determined via a formula. By example, a home with 70sqm of windows (aluminium frames and clear glass), on a cold night where it is 15o colder outside, would lose 6,510 watts of energy. That is equivalent to running a 2hp air conditioner at full capacity.

The SHGC measures how readily heat from direct sunlight flows through a window system. That feeling when you’re next to a window in winter and the sun is on you, warming you – that’s the SHGC. If the setting sun is low and coming directly in through western windows, there will be a higher SHGC than if the sun is high in the sky and hitting the window at an incidental angle.

In Sydney, northern windows should have a high SHGC value as these are the home’s solar collectors. Eastern and western windows should have an SHGC value as low as possible because they present an energy efficiency problem, with the use of external louvres to protect the glazing.

There is a wide range of glazing available. The thickness of glass has little impact on U-value or SHGC but will reduce noise and be safer. The cheapest and most basic glazing is single clear glass. The next step up is low emissivity glass (low-e). This glass has a vacuum deposited thin metal film coating or a pyrolytic coating baked onto the glass surface. The pyrolytic coating is more durable. Low-e coatings allow the sun to pass through into the house but reduce the amount of long wavelength infra-red heat that escapes through the window.

Insulated glass units (IGUs) are a combination of two or more layers of glass sealed with a gap between the layers, also known as double glazing or triple glazing. IGUs work in all climates, not just cold climates. The gap between the glazing layers serves as an insulator. The sealed gap can be air or an inert, low conductive gas like argon. By using different combinations of standard and low-e glass, you can configure the U-value and SHGC value to best suit your location and house.

When designing your building, we need to consider the big design decisions about orientation, layout and use, but we also have to have an understanding of the science of how your building is going to function and how much energy it is going to use and then factor that knowledge into the decisions we make.

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