Tag Archives: Double Glazing

The 20/20 House – Part 1

The Gruen Eco Design blog about how to convert your dream from an energy efficient home into a reality.

My partner and I are planning to build an affordable energy efficient house for our family. So I thought I take this opportunity to write about all the things we will have to go through to get into our home. And hopefully this can give some guidance or help to others in the same position.

I do want to talk about the whole process. Starting on how we get the finance sorted, finding the right side. How to determine what rooms we need and how the house has to function for us. But also what things we have to consider during the design phase, how to find and deal with a builder. Through to the construction process and then hopefully a happy ever after move into our new dream home.

But for now, we are just at the start. So in my first blog I will talk about finance. And what you need in order to get a pre approval from the bank, so that you can start actively looking for the right site.

Stay tuned and follow our journey.

Infrared Thermal Imaging to detect Air-Leakage and Thermal Bridges

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Whereas a blower door test can test how air tight a building is, infrared cameras are not able to give you an actual performance or score of your house, but they can show you where thermal bridges occur.

In Europe, infrared cameras are often used to locate the misapplication of materials and resulting thermal bridges. The lighter the colour the warmer the materials, the darker the colour the colder the materials. Great differentiation between colours means great temperature difference.

The first picture below shows a typical German home. Although double glazing and thermally improved window frames are used, the windows have a lower U-value than the walls, as the required U-value for the external walls is 0.24 W/(m²K) and the U-value for the windows 1.10 W/(m²K). Expectedly, the windows present in a darker colour as they let more heat escape through them than the walls. Determining if a thermal bridge is within the allowed limits requires meticulous measurements and comparison of internal and external material and air temperatures, humidity levels and following calculations of heat transfer. In this particular case, the thermal bridges occurring due to different U-values are within the allowed limits.

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Stack-Effect and Clerestory Windows

stack effect

Stack-Effect

The term “stack-effect” goes back to the chimney. The heat source – in this scenario, the fire – heats up the air. Hot air rises and is discharged through the chimney, as it has a lower density than cold air. This effect can be used to replace air inside a house. For instance, when it’s colder outside the windows can be opened to let in cooler air. Warmer air inside the room will rise towards the ceiling, exiting via high openable windows, clerestory windows and skylights. Warm air inside is replaced by fresh and cooler outdoor air.

Clerestory Window

clerestory is a usually a high wall with a band of narrow windows along the very top. The clerestory wall usually rises above adjoining roofs.

Originally, the word clerestory referred to the upper level of a church or cathedral. The Middle English word clerestorie means “clear story,” which describes how an entire story of height was cleared to illuminate large interiors.

If you want to maintain wall space AND keep a room well-lighted, or if normal solar access is either not possible or restricted consider this type of window arrangement for your home. Clerestory windows are most often used to naturally illuminate large spaces such as sports arenas, transportation terminals, and gymnasiums. But can be a great addition to any home.

 

Window Energy Rating Scheme

 

Untitled1The Window Energy Rating Scheme (WERS) is a program implemented by the Australian Window Council Inc. (AWC) 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/ built an energy efficient home.

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.

Sealing and weather-stripping
However, 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.

 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 1.1 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.

Up until then, you, as the client, has to make informed decisions about what glazing you are buying. You can’t just trust a manufacturer stating their glazing is energy efficient. They have to prove the performance to you by showing you the actual u-value of the window system.

What to look for when buying windows?
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, therefore preventing energy efficiency.

Winter heat loss through Windows

Winter heat loss

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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
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 roll in the energy efficiency of a window, is the frame itself, as it can effect 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.

 

Summer heat gain through Windows

heat transfer

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 devise 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.