Energy Efficient Windows

Good Window Design & The Importance of Double Glazing

Is Double Glazing Worth The Money?

Simone Schenkel of Gruen Eco Design

IS double glazing worth the money you invest? The answer is YES, but ONLY if it is installed correctly, without a cold bridge (thermal bridge). A window or a door is essentially a hole in the wall and responsible for most of the unwanted heat loss or gain.

Windows are essential for a house and the comfort and well-being of its inhabitants, as they let natural light and fresh air into the building and enable views. Appropriate window design, size, location and glazing treatment, combined with shading and internal covers, can significantly reduce the energy required for heating and cooling. Maximum solar access for north-facing windows can reduce winter heating bills up to 25%. External shading can block up to 80% of summer heat gain through windows. Double glazing and internal coverings can reduce heat loss in winter up to 40%.

Glass is the potential weak point of a building in terms of energy efficiency. A single glazed window can gain or lose up to ten times more heat than an insulated wall.

The main heat gain through windows is due to thermal radiation. Windows receive direct solar radiation when the sun strikes the glass, but also diffuse radiation reflected from the sky and the ground. Between 30-40% of total radiation to north windows is diffuse, depending on the weather conditions. Radiation from the sun travels through glass to the inside of a house. This radiant heat is absorbed by thermal mass, building elements and furniture, which when warmed up, re-radiates heat to the room air. This re-radiated heat is trapped inside, resulting in convective heat build-up within the room. This process is called ‘glasshouse effect’. In order to hinder direct rays from the sun entering the building in summer, glass needs to be shaded appropriately. On the other hand it is also important to ensure valuable winter sun can shine into the house, as heat gains in winter can reduce the requirements for mechanical heating.

Energy Efficient Window Design

The total radiation received per window varies according to the time of the year and the orientation. In summer, all windows receive heat gains, in particular those facing east and west. Whereas in winter, only windows facing north, north-west and north-east have a net heat gain, with heat gains outweighing heat losses. Windows facing all other directions will effectively lose more heat than they can gain. However, in the absence of northern solar access, windows to the east and west can provide some winter heat gains.

The most appropriate size of windows in terms of energy efficiency depends on many factors, such as glazing type, orientation of a building and thermal mass located inside the building materials. It is important to consider every room separately, as each room may have different acceptable limits and therefore may need different sized windows. Thinking about the windows early in the design process can save time and money otherwise needed later in the progress, to chase after the required stars to obtain a valid energy rating. We can help determine the effect of variations to window orientations, window sizes, internal glazing, double glazing versus single glazing, shading and internal coverings by using the FirstRate House Energy Rating software. Below are some clues on how and where to place windows.

How to orient and size windows

Windows should be oriented to the north where possible. If solar access is good, north-facing windows should be large, but the size also depends on the amount of thermal mass in the building. South and east-facing windows should be kept pretty small, and windows to the south need to be positioned to enable cooling summer breezes to pass easily through the rooms. Whereas west-facing windows should be avoided where possible, if needed they should be relatively small and well shaded.

Appropriate window sizing, combined with double glazing, and/or close-fitting internal coverings such as drapes with pelmets, can minimise heat loss in winter. Furthermore, it is important not to overshadow windows in winter by the structure of the building itself, as it will reduce the solar access.

How to respond to poor solar access

Innovative design can overcome problems of poor solar access and overshadowing,  especially in renovations, infill developments, higher density or small allotments with bad orientation, which can cause problems. In these cases, it’s important to use better performing insulation, protect windows, minimise overshadowing and courtyards, and reduce air leakage as much as possible. To compensate for poor solar access, the total window area of a building should be reduced.

Where solar access to north-facing windows is obstructed, clerestory windows are a good option to get solar energy into the building. Another option in responding to bad solar access is raising the sill height, as it will minimise permanent shaded glass areas, as these aren’t able to gain heat in winter and will lose heat instead.

Skylights and roof lights are also a good way to bring light into rooms, if obstructions from other buildings and structures prevent good solar access. Furthermore it’s a great opportunity to overcome overlooking into neighbouring properties, as windows above 1.7m don’t need to be screened. However, it is vital to protect the windows against harsh summer sun. Double glazing is mandatory as well as shading (a combination of external as well as internal shading would be the ideal solution).

How To Reduce Unwanted Heat Transfer

Summer heat gain

It is important to protect windows with external shading devices, through appropriate window sizing and location, in order to minimise heat gain in summer.

Comparison of heat gains through different treatments for windows in summer
(According to Sustainable Energy Authority Victoria 2002)

• Unshaded single-glazed window: 100%
• Standard double glazing as available in Australia: 90%
• Vertical blinds/open weave drapes: 76%
• Internal venetian blinds: 55-85% (Effectiveness is reduced as the colour darkens)
• Internal drapes or Holland blinds: 55-65%
• Tinted glass: 46-65%
• Solar control film/reflective glass: 20-60% (Available in different kind of configuration with varying effectiveness)
• Trees, full shade: 20-60%
• 1 metre eave over north wall: 30%
• Roller shutters: 30%
• External awnings: 25-30%
• 2m pergola over north wall covered with deciduous vines or shade cloth: 20%
• Outside metal blind or miniature louvers, parallel and close to window: 15-20%

External shading devices are an effective way to minimise heat gain through glass in summer and keep a building cool. They provide far better protection from heat gain than internal window covering. However, if external shading is not possible, internal coverings can at least reduce the unwanted heat gains. Shading devices should always enable ventilation outside the window, as shading fitted too closely to a window can trap warm air which can be conducted into the house.

Eaves, verandas or pergolas are commonly a part of the building structure, they are durable and do not require ongoing adjustments. It is essential to have a certain distance between the underside of the shading device and the top of the window. But these fixed shading devises should only be used over north-facing windows, as they lack flexibility and aren’t adjustable. East and west-facing windows need a flexible shading devise that can be completely retracted in order to let the valuable sun through in winter, but to protect from the harsh summer sun. Adjustable shading includes amongst other things canvas blinds, different types of shutters, angled metal slats, louvers or shadecloth over pergolas. Adjustable shading requires action from the occupants, as they have to respond to climatic conditions.

Winter heat loss

Unprotected  glazing  and single glazing in particular means the surface of the glass is noticeable colder than the warm air in the room. This lowers the room temperature and produces draughts. The Relative Air Velocity ends up too high and occupants will feel winter discomfort. For this reason, all windows require protection from heat loss in winter. To minimise winter heat loss, it is important to trap a layer of insulation still air between the window and the room. This can be achieved for instance by using internal coverings, such as drapes, Holland blinds, Roman blinds or Australian blinds, and thin or lace curtains combined with pelmets.

Effect of window treatments on winter heat loss
(According to Sustainable Energy Authority Victoria 2002)

• Unprotected single glazing: 100%
• Vertical or venetian blinds: 100%
• Unlined drapes or Holland blinds, no pelmet: 92%
• Heavy, lined drapes, no pelmet: 87%
• Unlined drapes or Holland blinds, pelmet: 79%
• Standard double glazing: 67% (the higher the U-value the less the heat loss can be)
• Heavy, lined drapes, pelmet: 63%
• Double glazing with Low-E coating: 57%
• Double glazing, heavy drapes, pelmet: 46%

Double glazing cuts winter heat loss

The most effective way to protect windows against heat loss in winter is a combination of double glazing and internal window coverings. However, if internal coverings are inappropriate or not desired, for instance in highlight or clerestory windows, in kitchens or simply where unobstructed views are wanted, double glazing is an indispensable measurement in order to prevent heat loss in winter. Yet double glazing won’t prevent sun coming into the building, which means that the windows need to be protected from harsh summer sun by means of external shading.
 

Window Frames

Another, often underestimated role in the energy efficiency of a window, is the frame itself, as it can affect negatively on the overall performance. As we talked about in the blog “Adequate Insulation”, some materials, such as metal, glass or aluminium, allow heat to pass through them more easily, therefore they shouldn’t be used for windows frames if at all possible. If metal frames are used, such as aluminium, they should have thermal breaks to reduce the heat transfer. Generally speaking, PVC and timber frames perform better than metal frames.

Comparison of heat loss through different window frames
(According to Sustainable Energy Authority Victoria 2002)

• Single-glazed industry typical aluminium: 100%
• Single-glazed thermally improved aluminium: 87%
• Single-glazed timber or PVC: 82%
• Double-glazed industry typical aluminium: 72%
• Double-glazed thermally improved aluminium: 60%
• Double-glazed timber or PVC: 54%

Sealing and weather-stripping

A good U-value is no guarantee for a well performing window. The installation of doors and windows needs to be done according to the manufactures guidelines. All gaps must be sealed and weather-stripped carefully in order to perform to the specified U-value. Unfortunately, the energy rating just states the material U-value of the window and not the end product and common practice often shows incorrect installation leading to thermal bridges around the windows.

Window Energy Rating Scheme

The Window Energy Rating Scheme (WERS) is a program implemented by the Australian Window Association (AWA) with the support of the Australian Greenhouse Office. The windows are evaluated with stars, the more stars, the better the performance. If buying windows, always check the label before making a decision.

A single-glazed window with a typical aluminium frame has U-values ranging from 7.9 W/m²K to 5.5 W/m²K (according to the indicative ranges of whole glazing element performance values in the BCA). These U-values will make it hard to reach a good energy rating for a building. Keep in mind, the lower the U-value the better performing a window. Double glazing windows with timber framing in Australia usually range between a U-value of 3.8 W/m²K and 2.5 W/m²K.

Windows And Double Glazing Overseas

Whereas most countries in Europe require double glazing and even recommend triple glazing, it is not standard in Australia yet. Unfortunately, double glazing is still more expensive than single glazing in Australia, in Europe it’s actually the other way around. Due to the fact that single glazing is not allowed any more, no one is producing it on a large scale making it quite expensive. Double-glazing on the other hand is a standard, and although better performing than common double-glazed windows in Australia, they are available for about a quarter of the price. For instance, the minimum required U-value for windows in Germany is currently 1.3 W/m²K. I trust that with time, double glazing will become more affordable and will become mandatory in Australia to achieve good passive solar design.
 

Conclusion

YES, double glazing is worth its money. It is the best method to reduce heat loss in winter, as long as it is applied, installed and used properly.  The window size should respond to the location and the climate, the insulation around the window needs to be snug fit, in order to prevent thermal bridges. Appropriate window frames need to be used and furthermore, adequate internal and or external covers needs to be applied. All these measurements need to work together, otherwise a window is nothing more than a hole in the wall and will be the major contributor for unwanted heat gain and loss. Energy efficient windows incorporating double glazing will deliver comfort , energy savings and reduced bills.

Simone Schekter is Director at Gruen Eco Design in Melbourne.

At Gruen, we are passionate about sustainable design; because we believe sustainable design can and must be affordable for everyone. As a matter of course, it should be integrated naturally throughout the whole design process, rather than designing something and trying to make it sustainable later.

Our mission is to help others to live a more sustainable life!

We pride ourselves on providing a unique design service to create a truly energy efficient and sustainable houses in cooperation with our clients.