How to decide which window to get?

At Good Architect, we understand that choosing the right glazing for your home is crucial. To help you make an informed decision, let's explore some key properties of glass.

Visible Light Transmittance (VLT): The amount of light that passes through glazing is known as Visible Light Transmittance (VLT) or Visible Transmittance (VT). Opting for a low VLT can reduce heat gain from the sun, but be cautious as extremely low VLT can darken your interiors, leading to increased energy costs due to additional artificial lighting.

Conduction and U Value: Conduction, represented as the U value, measures how efficiently a material conducts heat. For windows (Uw), it describes the conduction of the entire window unit, including the glass and frame. A lower U value indicates greater resistance to heat flow and improved insulation.

To calculate the heat conducted through a glazed unit (in watts, W), you can use the formula Uw × T × A, where:

• Uw is the U value of the window,

• T is the temperature difference between the inside and outside,

• A is the area of the glazing unit.

Solar Heat Gain and SHGC: Solar Heat Gain Coefficient (SHGC) for windows (SHGCw) measures how effectively a window, including both glass and frame, allows solar heat to enter the house interior. SHGC values range from 0 to 1, with lower SHGC indicating less solar heat transmission.

Manufacturers provide SHGC values for each window type and design, but the actual SHGC can be affected by the angle of incidence. This angle depends on the glazing's orientation and the position of the sun concerning location, season, and time of day.

As the angle of incidence increases, more solar radiation is reflected and less is transmitted. Once the angle exceeds 55°, the solar heat gain falls sharply. Additionally, a higher angle of incidence reduces the effective area of exposure to solar radiation.

Please note that the solar gain can vary depending on a window's position and the season. For instance, a north-facing window in summer may have an angle of incidence of 82°, while in winter, the angle can decrease to about 35°, resulting in different solar heat transmission levels.

At Good Architect, we are committed to helping you choose the perfect glazing for your specific needs, taking into account all these vital glass properties. Feel free to reach out to our experts for personalized guidance and solutions. 

Glass

There are many different types of glass products to choose from.

Single glazing

Single glazing uses a single pane of glass. Single glazing with clear glass is not very efficient when it comes to heat loss or gain. To improve performance, you can use single glazing with a more energy-efficient type of glass such as low emissivity (low-e) glass.

Double or triple glazing

Double or triple glazing (also known as insulated glass units or IGUs), is the combination of 2 or more layers of glass sealed into a frame with a gap between the layers. Multiple layers can be assembled with sealed cavities between each sheet of glass.

IGUs generally offer better energy performance than single glazing, because they transmit less energy. However, the energy performance of IGUs also depends on:

• the properties of each layer of glass. Different glass types (for example, clear and low-e glass) can be put together in an IGU. Different combinations of clear, toned and low-e glass can deliver a wide range of SHGC and VLT values to suit your performance needs the contents of the cavity. IGU cavities can be filled with air or a more inert, low-conductivity gas such as argon the width of the cavity. Cavity thickness is usually 6 to 18mm. Wider cavities provide lower (better) U values, with 12mm normally accepted as the preferred gap

• how well the cavity is sealed. Cavities must be dry and well sealed to prevent moisture getting in. Manufacturers have special dry rooms for IGU assembly. If argon is installed to the cavity in place of air, moisture is reliably excluded

• the level of desiccant (drying agent). The spacer (metal or polymer strip) that separates the glass layers contains a desiccant to absorb any moisture. Inadequate desiccant may cause moisture to condense on the glass surface in cold conditions, reducing thermal performance.

It is sometimes wrongly assumed that insulated glazing is only for cold climates. In fact, IGUs can deliver better energy performance for all climates, especially in heated and air-conditioned homes.

Low emissivity glass

Low emissivity glass (commonly known as low-e glass) reduces heat transfer. Low-e glass may be either high or low transmission:

High transmission low-e glass has a coating that allows daylight from the sun to pass into the house to achieve good solar heat gain, but reduces the amount of the long wavelength infrared heat that can escape back through the window.

Low transmission low-e glass has a coating that reduces the amount of solar heat gain while still maintaining good levels of visible light transmission.

Low-e glass has either a pyrolytic coating or a vacuum-deposited thin film metal coating. Pyrolytic coatings are durable and can be used for any glazing; vacuum-deposited coatings are soft and are only used within IGUs.

Low-e coatings can significantly improve both U value and SHGC; however, they must be used correctly or they will either deteriorate or fail to perform as required. They are often more susceptible to surface damage than standard glass. Low-e coatings can be used in combination with clear, toned or reflective glass.

Toned glass

Toned glass has colouring additives included during manufacture. It is available in various colours, usually bronze, grey, blue and green. Different colours will change the amount of visible light transmitted (VLT) and the SHGC; however, the colours do not change the conduction (U value) of the glass.

Toned glass options include ‘supertoned’ glass, which has heavier colouration that transmits visible wavelengths while filtering out solar near-infrared wavelengths. This provides improved energy performance by lowering solar heat gain but does not affect light levels.

Laminated glass

Standard glass will readily break into long shards and small sharp slivers. Laminated glass has a plastic glazing layer, called an interlayer, which is adhered permanently between 2 sheets of standard glass. This reduces the danger of the glass breaking, and if it does break, keeps all shards in place so they do not form loose dangerous shards.

Laminated glass is often used in areas in the home most prone to injury from human impact such as bathrooms, doors, around staircases and in areas close to the floor (it meets the requirements of ‘safety glass’ that is mandated for use in these areas by Australian Standard AS 1288 Glass in buildings).

Careful selection of different interlayer types can also address noise concerns and energy efficiency requirements to some extent, but it is not a substitute for double glazing.

Toughened glass

Toughened glass has been ‘tempered’ by being reheated and quickly cooled again. This process makes it much stronger than standard glass – it can resist higher impact loads before breaking. It also makes it safer because, when it does shatter, it breaks into many small cubic pieces rather than dangerous shards. It can be used as ‘safety glass’ as mandated in Australian Standard AS1288, and may be mandated in bushfire-prone regions. However, toughened glass has no thermal or acoustic benefits over other glass of the same toning or thickness.

Secondary glazing

Secondary glazing is where single-glazed windows are retrofitted with a transparent acrylic or glass sheet attached to the inside of the frame or openable sash with a secondary frame or with magnetic strips. This creates an air space between the 2 layers, which reduces the U value and air infiltration. Secondary glazing will not perform as well thermally as a manufactured IGU, since it is impossible to totally seal the perimeter, but it can provide good noise control.

Window Films:

Window films are thin polymer films with absorbing dyes or reflective metal layers, backed by adhesive. They adhere to your glazing, offering the ability to change its color or make it reflective. Window films present a cost-effective way to improve the thermal performance of existing windows and doors.

By applying certain window films to existing glass, you can reduce the Solar Heat Gain Coefficient (SHGC) by absorbing and reflecting solar radiation. This proves particularly beneficial in hotter climates or on east and west-facing elevations exposed to prolonged sunshine.

However, some window films may also reduce visible light transmittance. Additionally, glass with applied films exposed to direct sunlight can become hotter than untreated glass. For this reason, it's essential to follow industry guidelines and use accredited installers for window film application.

Frames:

Frames significantly impact the thermal performance of windows and doors, as energy can be gained or lost through them, not just the glass. The choice of frame is crucial for effective passive design.

Here are some common frame types and their energy performance values:

Aluminium Frames:

Aluminium frames are lightweight, strong, and durable, available in various powder-coated and anodized finishes. However, aluminum is an excellent conductor of heat, which can decrease the insulating value of the glazing unit unless engineered to reduce thermal conductivity.

Thermally Broken Aluminium Frames:

Thermally broken frames consist of two aluminum sections connected by a structural insulator, reducing heat flow through the frame. They are among the highest-performing frames suitable for most climates, even though they may be slightly more expensive.

Timber Frames:

Timber frames are natural insulators that suit many home designs. They should be made from durable species or treated to prevent decay and deformation. Good draught sealing is essential to prevent air infiltration due to natural swelling and shrinking of timber with atmospheric humidity changes.

uPVC Frames:

uPVC frames provide excellent thermal performance, often superior to timber or thermally broken aluminum. They require minimal maintenance, offer excellent air sealing, and are becoming increasingly popular in Australia.

Composite Frames:

Composite frames combine aluminum profiles on the outer sections with timber or uPVC inner sections, offering a balance of low maintenance, durability, and improved thermal performance.

Fibreglass and Other Materials:

Materials like fibreglass and steel (especially for buildings requiring high fire ratings) are available but may have limited availability in Australia.

Frame Styles and Sash Design:

Window and door styles and configurations can influence energy performance in various ways, affecting cross-ventilation, opening areas, and rain protection. For instance:

Casement windows can effectively encourage cross-ventilation, especially when oriented to welcome breezes.

Sliding and double-hung windows have limitations in the area they can open and are not as effective in preventing rain entry.

European-style 'tilt and turn' windows offer good U values and air tightness, providing versatile performance.

At Good Architect, we strive to help you make informed decisions about window films and frames, ensuring optimal energy efficiency and comfort for your home. Contact us for expert advice and personalized solutions tailored to your needs.

At Good Architect, we understand the vital role glazing plays in determining the thermal performance, comfort, and energy efficiency of your home. Several factors interact to influence the impact of glazing on your building's thermal performance:

Climatic Conditions: Temperature, humidity, sunshine, and wind in your location affect the glazing's efficiency.

Building Design: The orientation, form, and layout of your building influence the glazing's performance.

Building Materials: The amount of thermal mass and insulation in the construction affects the overall thermal efficiency.

Size and Location of Glazing and Shading: Properly locating and sizing glazing and shading can optimize solar gains and prevent unwanted heat gain.

Thermal Properties of the Glazing System: The U value measures the ability to retain heat in winter and keep the interior cool in summer, while the SHGC indicates how much solar heat the glazing transmits.

To enhance the thermal performance of your home, we recommend following these principles when choosing glazing:

Optimize Sunshine: Position and size glazing and shading to allow sunlight in during cold weather and exclude it during hot weather.

Utilize Thermal Mass: Use thermal mass to store the sun's heat and provide night-time warmth in colder conditions.

Cross-Ventilation: Place window and door openings strategically to facilitate natural cooling through cross-ventilation.

Effective Seals: Ensure your windows and doors have good seals to minimize unwanted draughts, improving energy efficiency.

In temperate climates, like the Central Coast, north-facing glazing with a high SHGC is recommended, paired with effective shading for winter and summer conditions. East- and west-facing glazing requires deep overhangs or active shading devices to protect from the sun's heat.

Consider the orientation of your windows carefully, as optimizing glazing by orientation can significantly improve overall home performance. Different types of glass may be needed for different orientations, and thermal modeling can be helpful in determining the best glazing options.

Ensure glazing is properly sealed to prevent air leakage and energy loss. Noise control can be achieved with increased thickness or multiple layers of glass.

To avoid condensation, select energy-efficient windows and doors that keep the interior surfaces at room temperature. Glazing can also impact fading of interior furnishings, and appropriate glass can block some wavelengths responsible for fading.

When evaluating glazing products, refer to the Window Energy Rating Scheme (WERS), which rates the energy performance of windows, skylights, and glazed doors according to AFRC procedures.

Be aware of local regulations, such as the National Construction Code (NCC), which sets glazing performance requirements and safety standards. Expert professional advice can help navigate these regulations.

Proper specification and documentation of windows in your building plans are crucial to ensure products meet intended performance standards and comply with regulations.

At Good Architect, we are dedicated to helping you make informed decisions about glazing and thermal performance, ensuring your home is energy-efficient, comfortable, and sustainable. Contact us for expert guidance and tailored solutions for your unique needs.

Here are some more great resources for determining what windows to use:

https://www.yourhome.gov.au/passive-design/glazing

https://www.agwa.com.au/

https://renew.org.au/renew-magazine/buyers-guides/window-buyers-guide/

https://www.agwa.com.au/WERS/WERS/Default.aspx