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Specifying glass sustainably

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Despite defining the aesthetic of much unsustainable architecture, glass as a material is crucial for buildings. Fran Williams presents tips to help architects specify the material more sustainably

RetroFirst Logos 2019 1

RetroFirst Logos 2019 1

The modernist push for more transparency in building design has brought a profusion of totally glazed buildings all around the world. They appear in every nation regardless of their differing climates, urban contexts and building programs. 

Why do architects love glass so much? Perhaps the transparent material’s promise of spatial continuity between inside and outside has never quite worn off.

It is also a relatively low-cost material with fairly good durability and is manufactured easily through off-site production. Having transparency within a building’s façade not only brings abundant natural light into spaces but gives a greater connection to the outdoors and our immediate surroundings. For the nosey amongst us, it allows a peek into building interiors too!

The rise of the all-glass building – particularly those with curtain wall envelopes – has a history strongly related to that of air-conditioning, which is what made glassy buildings habitable.

However, in the drive to reduce operational energy use and to design more sustainably, we must reduce our emphasis on mechanical systems and re-assess the merits of all-glass envelopes.

On the whole, glass as a material generates minimal environmental impact if used correctly. For example, the total carbon emissions emitted by the manufacturing of an ‘energy-efficient double-glazing unit’ is offset on average within only three to 10 months by the energy savings realised, compared with the same building mass equipped with inefficient glazing. 

Glass is also made up of non-polluting raw materials: its manufacturing process is highly efficient, requires low levels of water and generates little waste. In addition, most glass products for buildings are recyclable at the end of their lives, contributing to their lower environmental impact. 

Glass is made from sand, soda ash and salt cake: the basic formula comprises 75 per cent silica sand, then soda ash, salt cake, dolomite and rouge/iron oxide are added. Metal oxides may also be added to create a variety of colours. Glass for commercial and residential construction is typically manufactured as sheets, created using a float process. 

Glazed envelopes often perform worse in terms of energy use, carbon emissions, thermal comfort and visual comfort

However, despite everything it has going for it and even with today’s high-performance products, glazed envelopes often perform worse in terms of energy use, carbon emissions, thermal comfort and visual comfort. Glass is a poor insulator, leading to higher energy use and thermal discomfort in winter. In summer, it admits solar gain, leading to high cooling energy use and, again, thermal discomfort.

Excessive use of glass creates a stark contrast between bright building perimeters and darker internal spaces. There are sometimes even issues with too much glass causing intensive levels of reflective sunlight in public spaces: in 2013, Rafael Viñoly’s ‘Walkie Talkie’ – 20 Fenchurch Street –was blamed for melting parts of a car parked on a nearby street.

Shutterstock 629913041

Shutterstock 629913041

Source: Shutterstock

20 Fenchurch Street in the City of London (commonly known as the ‘Walkie Talkie’) by Rafael Viñoly features a distinctive top-heavy form which acts as a concave parabolic mirror and focuses light onto the streets to the south

Careful façade design can help mitigate these problems with strategies calibrated to climate, orientation and program – most of the solutions below are contextual, with a combination of approaches tailored to each project.

1. Careful façade design

By fine-tuning façades to their site in terms of climate, orientation and by thinking about the program within, we can optimise the amount of glazing used and its impact on the performance of the spaces within.

Glazed façades and window openings must be designed with various site criteria in mind: temperature, humidity, isolation, wind speed and direction, psychometrics, sky conditions, landscape and topography. 

On top of this, the relationship between façade and the building’s program and interior space planning must be thought through carefully. For example, larger openings and greater expanses of glazing can be placed in areas where more light is required for the activity within.

Where no light is required – in plant rooms, for example, there is no need for windows and therefore these spaces can be placed where it is harder to achieve natural light anyway.

Careful positioning of window openings also helps create a natural airflow across the building – naturally ventilating the spaces and reducing the need for mechanical ventilation systems.

In projects requiring a fabric-first approach to achieve zero-carbon emissions – such as Passivhaus projects – the ideal is to optimise internal climates by providing a highly insulated, airtight structural shell to ensure consistent, comfortable internal temperatures year-round. Therefore, sufficient heat should be collected via solar gain, appliances and from the building’s inhabitants. 

Glazing and fenestration have a critical part to play in this: the positioning and orientation of windows is essential. The design should have lots of south-facing glazing – up to 50 per cent of the wall surface – whereas on the northern elevations, windows should be kept to a minimum.

Waugh thistleton architects cambridge heath jim stephenson 27

Waugh thistleton architects cambridge heath jim stephenson 27

Source: Jim Stephenson

The Green House, an environmentally friendly office building in Bethnal Green by Waugh Thistleton Architects, features a predominantly glazed western façade which provides passive regulation of noise, heat, sunlight and ventilation. The windows are set back from the external skin of the façade to provide solar shading and acoustic protection

2. Design to reduce glare

Another key aspect of Passivhaus requirements is the integration of solar shading to allow daylight to flood in during winter while cutting out the high summer sun. Brise soleil and integrated blinds are two popular options. Spandrels, fritting and shading panels also help with this – in addition to creating patterning on the exterior and adding privacy if needed.

Ga bath 008 bath school of art and design (c) paul raftery

Ga bath 008 bath school of art and design (c) paul raftery

Source: Paul Raftery

Grimshaw’s adaptation of its 1970s Herman Miller furniture factory into Bath Spa University’s Locksbrook Campus for its Schools of Art and Design features slats across the ground-floor glazing to the southern façade to prevent solar glare. The slats don’t disrupt views out but are angled to prevent too much light coming into the building

3. Highly insulated windows

The Passivhaus standard sets a number of essential criteria for glazed products. The entire unit must be highly insulating, with a U-value below 0.8 W/m²K (Building Regulations allow up to 2.0 W/m²K on new build homes). To achieve this, windows have to be triple-glazed, with an insulating frame, airtightness and optimised detailing (such as composite edging compounds) to avoid thermal bridging.

Good detailing is also essential – there is little point in specifying high-spec units that are poorly fitted, as they will compromise the performance of a building’s structural shell.

Camden mews 7120

Camden mews 7120

Source: bere:architects

bere:architects’ Max Fordham House is an ultra-low energy, all-electric home designed to work as a zero-heat house on even the coldest day of a typical London year. Its main technical innovation is the use of insulated sliding shutters, which reduce winter heat demand in areas where windows would be losing valuable heat

4. Optimised window opening-to-wall and surface-to-floor ratios

Optimum surface-to-floor ratios for windows and openings should be increased to an extent that the maximum amount of light can be achieved without creating overheating or glare issues. The ratio can be determined through energy and thermal simulations and by calculating the percentage ratios for a building.

Consideration of thermal heat transfer when specifying glass is important – thermal heat transfer happens when sunlight enters a building through the envelope. The amount of heat allowed into the building is measured as solar heat gain coefficient (SHGC).

The rate of heat flow is measured as the rate of heat transfer, or U-value. The resistance to the heat flow is measured by the reciprocal, or the R-value. Building Regulations set standards as to what U-values should be achieved on various project types.

Thehalle1310

Thehalle1310

Source: stephenson STUDIO

Hallé St Peter’s in Manchester by local practice stephenson STUDIO has triple-glazed windows to the main rehearsal space on the first floor to allow light in without compromising on acoustic quality

5. High-performance and technical glass products

Specifying double and triple-glazed units with inert glass filling and invisible low-emissivity coatings can significantly improve the insulation properties of windows and façades. Such products allow maximum natural daylight into buildings and can maximise or limit solar heat gains, depending on the desired thermal objectives and energy balance.

Other products use technology to control light transmittance and energy performance.

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