Eco Priority Guide: Windows (glass)

 

Overview

Glass selection is one of the most important issues in building design as windows are one of largest heat gain and loss pathways in buildings. However, with over 286,000 different varieties of glass¹ (although not all of these are suitable for building use), choosing the right glass for any task can be quite daunting. While some types of glass are available in different formats, e.g. float, laminated or toughened, some types are limited to specific formats, e.g. spectrally selective ‘sputter coat’ metallised coatings are only available within sealed insulated glass units (IGUs).

The various types of glass available have expanded rapidly in recent years, but generally fall into a number of key categories;

Windows

• Clear
• Tinted (reflective or heat absorbing)
• Spectrally selective (‘clear’ or tinted)
• Specialist glasses (e.g. ‘self cleaning’ or ‘electrically switched’)

Cladding

• Coloured Ceramic Glasses

The types of glass that provide specific environmental benefits such as thermal energy efficiency or operational energy benefits are tinted, spectrally selective and self cleaning. These benefits can be further enhanced through the appropriate selection of window frames, however this guide deals only with window glass. See Eco Priority Guide: Windows (frames) for further information on framing considerations.

All glass is recyclable as a commercially valuable product. CSR’s Viridian patterned glass for example is made from 95% recycled feed stock. Generally however, window glass and alkali glass is recycled into fibreglass insulation or powdered for use as filler in paint and roads. Some recycled glass may have contamination considerations such as imported mirrors which can have lead based paint backing.

 

Eco-Priorities

The following issues relate to both potential positive and negative issues associated with each product class:

Priority Order	Clear	Toned	Patterned Glass	Spectrally Selective (inc. Low E)	Self Cleaning	Double Glazed
1	GHG	GHG (Operational)	GHG + (embodied)	GHG + (Operational)	Life-Cycle +	GHG +
2	Resources	Resources	Resources	Resources +	Resources	Resources
Issues of concern/Red Lights?*	No	No	No	Minor issues due to fluorine based compounds	No	No

Table Key

GHG - Production of greenhouse gases, ozone-depleting chemicals

Life-Cycle Issues - Durability and maintenance

Resources - The use of raw resources, e.g. oil, metal ores.

+  Indicates an overall positive outcome.

*   Issues that are of high concern and are a potential eco-design basis for not using the product.

 

Making a Decision

Commentary

Common formats of architectural glass include;

• Float – Standard commercial glass
• Laminated Glass – Safety glass with thin sheets of float glass held together by an interlayer, typically Polyvinyl Butyral (PVB). Has much higher sound insulation, blocks 99% of transmitted UV light, but is only suitable for limited structural applications and does not provide significant thermal benefits2.
• Toughened Glass – Safety glass suitable for structural purposes
• Double Glazing – Two panes of glass with a sealed space between them which is filled with air or an inert gas such as argon. Double glazing is also referred to as an Insulated Glazing Unit or Insulating Glass Unit (IGU).

 

Considerations and benefits for various types of glass include;

• Clear Glass – The most basic form of glass. Clear glass can be used in varied formats or have specialised coatings added to improve its thermal performance.
• Toned Glass – Can either be tinted in the body of the glass or have a tinted coating applied to it. Toned glass reduces that amount of solar radiation transmitted through the glass. However, it tends to absorb rather than reflect heat energy and can cause heat re-radiation and conduction into the building.
• Reflective Coatings – Can be applied to new and existing windows. They tend to stop greater amounts of heat gain than some toned glass and increase daytime privacy. However, at night the direction of reflectivity reverses and is reflective on the inside. Use of reflective coatings needs to be integrated with solar design principles to avoid detrimental reflection of heat in winter months. Reflective windows should not be used where the reflected light can cause nuisance glare.
• Spectrally Selective Glazing – Maximise light transmission while simultaneously reflect unwanted solar radiation (UV and near infared).Spectrally selective coatings can also have low emissivity. Low-emissivity (Low E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from warm surfaces to cooler surfaces. Low E coatings also reduce light transmittance by about 10% compared to clear glass. 
• Low E Coating – Generally referred to as coatings with an emissivity less than 0.2. Radiation heat flow is reduced by approximately 75% and consequently the U-value is also reduced.
• Double Glazing - Offers a much better insulation than single glazing.  The space can be filled with air, or an inert gas such as argon, with both have much better insulating properties than glass. The best thermal performance for air-filled units occurs when the space between the panes is about 12mm.
• Self Cleaning - Available on a variety of glass types and is comprised of a 50nm coating of titanium dioxide on the outer surface of glass. The coating creates a photo-catalytic effect in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface. The coating also creates a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds².

 

Specific considerations for treated high performance glass include;

• All treated glass must be used with caution and properly considered in building thermal design as it can reduce heat gain and daylight in winter as well as summer. The higher the summer performance of the glass, the more marked this effect can be in winter, depending on whether the heat loads in the building under consideration are internal load dominated or climate dominated.
• Treated glass can dramatically reduce heat loads in buildings. However, the reality of exposed glazing, particularly heat absorbing glazing, is that without consideration of integrating external shading is that as efficient as it is, when the sun shines directly onto glass, it heats up and can become a major heat source in its own right. This causes re-radiation of the absorbed heat into the internal spaces, increasing the mean radiant temperature.

 

Once performance requirements are established for the design and climate of a project a variety of indicators exist to determine the most efficient glass to use. These include;

• U-Value – Is the overall heat transmittance or heat transfer coefficient of a window (it is also used for other materials/products) and measures how well a product prevents heat from escaping. Hence, it measures the rate of non solar heat loss or gain. When considered in the context of windows, the whole-window U-value (Uw) is used, accounting for the performance of the frame, edge-of-glass and centre-of-glass components (as used by WERS see below)³.
• Solar Heat Gain Coefficient (SHGC) – Represents the performance of a window with respect to solar radiation driven heat flow. The SHGC is a measure of how much radiation will enter the space through the glazing compared to the total amount of radiation striking the glazing at an angle of incidence of 0. Whole-window solar heat gain coefficient (SHGCw) as used by WERS (see below) accounts for the performance of the frame and glass components. With SHGC, there is no distinction between centre-of-glass and edge-of-glass³.
• Glass Visible Light Transmittance – is the factor derived from the percentage of visible light transmitted through the glazing under a standard sky, with 100 being clear glass and 0 being obscure.The whole-window visible transmittance used in WERS (see also below) (Tvis) is a product of glazing and glazing/frame area ratio.
• Window Energy Rating Scheme (WERS) – Developed by the Australian Window Association and the Australian Greenhouse Office, WERS rates the energy performance of windows, skylights and applied window films on a ten star scale based on heating and cooling performance. For further information see WERS website.
• ARUP & Australian Glass & Glazing Manufacturers Association (AGGA) – Tool that enables comparison of the energy performance of diverse range of glass and framing combinations to determine the right glass type to achieve a specified performance level for the façade or window system. For further information see The Glass and Window Toolbox website.

Decision-Making Checklist

1. Does a thing have to be made or used? If so, does it create a net benefit?
2. Fate: Start with the end in mind. If the product is not reusable, fully biodegradable or highly recyclable at the end of life, or facilitating these activities, its not sustainable.
3. Energy: What will the product’s likely net energy balance be over its life? Will it save more energy than it uses?
4. Durability: Does the product embody an appropriate level of durability for its accessibility, criticality and maintenance profile?
5. Biodiversity: Is there a chance that the product has had a negative impact on biodiversity? Erosion of biodiversity is a one-way street.
6. Toxicity: Is the product toxic and or persistent in the environment at any stage in its life cycle? If so, don’t use it.
7. Resources: Does the product use rare resources/ create a net negative flow of resources (e.g. poor maintainability/ high maintenance requirements)
8. Is the product socially sustainable?
9. Does the product, or its use, contribute to delivering synergy benefits in other building systems?

Source: Adapted from Andrew Walker Morison

 

Quick Guide

Clear Float
For High optical clarity High visible light transmission High solar transmission (for direct gain systems Recyclable	Against High thermal and solar transmission
Reflective
For High optical clarity Reduced solar transmission Recyclable	Against Moderate visible light transmission Potential for re-radiation of absorbed heat
Toned & Super Toned
For High optical clarity Moderate to low solar transmission Recyclable	Against Moderate to low visible light transmission Potential for re-radiation of absorbed heat
Low-E
For High optical clarity Moderate to low heat loss High daylight transmission Hardcoat pyrolitic is relatively durable and can be used with single sheet (with specific cleaning requirements. Recyclable	Against Softcoat must be used internally in IGUs –subject to damage Pyrolitic can be damaged by ‘Windex Blue’
Double Glazed - IGUs
For High optical clarity Low to very low heat loss High daylight transmission depending on glass types used Lowest possible heat loss characteristics besides triple glazing etc Can vary glass and gas infill types for increased performance Higher embodied energy offset quickly by energy savings Potentially reusable depending on age and exposure Recyclable	Against Higher embodied energy Less recycling efficiency because of sealed junctions at edges.
Self Cleaning
For High optical clarity Major energy and cost savings	Against Needs to be exposed to sunlight for catalytic action to occur
Patterned
For Uses 95% recycled glass Fully recyclable	Against Same thermal performance as single float glass

 

Further Information

For more detailed information on this topic contact subscribers@ecospecifier.org

 

References

1. Accessed 30 July 2007, http://glassproperties.com/density/room-temperature

2. Accessed 30 July 2007, http://en.wikipedia.org/wiki/Architectural_glass

3. Accessed 30 July 2007, http://www.wers.net