ENERGY EFFICIENT GLASS AND GLAZING
Double Glazing (also referred to as Insulating Glass Units)
Double glazed units consist of two panes of glass separated by a spacer around the edges and sealed to the perimeter, in factory controlled conditions. The spacer contains a desiccant which eliminates moisture vapor in the cavity. Double glazed units are available in many glass combinations, and the space between the glass panes can be filled with air or gas.
The trapped air/gas in the gap is an insulator – it does not stop heat being conducted from the inner pane to the outer pane – but it does slow heat transfer by conduction down a lot! The gap is too narrow to allow the air to circulate easily. This therefore also reduces the rate of heat transfer by convection.
Low Emissivity or ‘Low E Glass’
Low-E glass, which uses a special coating on the internal surface, has a low rate of emission, it has a lower rate than normal glass of allowing heat to pass through. In other words, if there is a heat source inside your house (or outside), the glass bounces the heat from that object back away from the glass. So, in the winter months, if you have Low-E glass in your home, much of the warmth (heat) given off by heaters and all the objects which they have heated, is bounced back into the room. In the summer, the same thing happens but in reverse. The sun heats things up (the air, driveways, neighbors’ brick wall, etc.) outside of your house. This heat radiates from those objects and tries to get into your house. With Low-E glass much of this heat bounces off the glass and stays outside.
How double glaze & ‘low e’ works
Glazing has a major impact on the energy efficiency of the building envelope. Poorly designed windows, can make your home too hot or too cold. If designed correctly, they’ll help maintain year-round comfort, reducing or eliminating the need for artificial heating and cooling.
Windows in a typical insulated office or home can account for more heat gain or loss than any other element in the building fabric. Double glazing or ‘low e‘, can reduce overall heat loss by up to 50%.
nergy efficient windows will make your home or office more comfortable, dramatically reduce your energy costs and help to create a brighter, cleaner, healthier environment.
Double glazing and some ‘low e’ products can also reduce interior noise levels considerably (up to 55%).
Our Climate (cool temperate)
In a predominately mild climate like Melbourne’s (it’s zoned as a cool temperate) the best results are obtained from windows that insulate well (low U-value) and admit plenty of free solar energy (high solar heat gain coefficient – SHGC). Tinted or reflective surface types of glass will reduce the SHCG values.
Cool temperate or heating climates are those in which a typical house uses more than 70 percent of its total space-conditioning energy for heating in winter and less than 30 percent for cooling in summer.
It is better to install efficient windows than to rely on heating, ventilation and air conditioning systems to solve thermal comfort problems. There are some considerable energy savings that can be made by using better window and glazing selections.
Energy Reports specify materials and design characteristics to be used in construction of the building.. With respect to windows, the report will specify any number of given performance levels to meet.
What is needed to comply?
The report will list various values for windows, the most important being the U-value and SHGC values.
In order to comply, the window system must perform to the set parameters.
It’s important to note, that these values are “whole window values”, and not glass alone.
When looking for a U-value, it’s important to find a window with a U-value Lower than or equal to specified. The SHGC must be within 10% of the set value.
Does the material or glazing specified on the report matter?
No, the material or glazing noted on the report is used as an indicator to show “typical” windows in that range. Many window manufacturers have alternative solutions which still achieve the same or better performance values. When looking for a window, search on U-value and SHGC to ensure you comply.
Star Rating Impact (for cool temperate)
The U-Value, has the potential to improve the rating up to approximately 0.3 stars for each unit of reduction in U-Value’s
Each star corresponds to reduction in heating/cooling requirements of approximately 20% to 30% from before the star rating system was introduced.
In general improved U-Values act to reduce heating load throughout majority of the year, though it may marginally increase cooling requirements in periods of hot weather.
Approximately 10,000 to 15,000MJ of energy saved per star, mostly heating – worth about $300 per year and approximately 1.0 tonne of green house gasses. (Based on Melbourne/Hobart, 240m2 house)
Examples of Performance Values (“whole window”)
Awning window – using aluminum improved (and thermally improved) frame.
- 4mm Clear – U-value 6.35 – SHGC .65
- 5mm Grey Tint – U-value 6.25 – SHGC .52
- 4mm Evantage – U-value 5.0 – SHGC .54
- 6.38mm Clear Comfort Plus – U-value 5.0 – SHGC .40
- 4mm Clear/6 Air Space/4mm Clear – U-value 4.2 – SHGC .58
- 4mm Evantage/6 Argon Space/4mm Clear – U-value 3.5 – SHGC .54
- 6.38mm Clear Comfort Plus/6 Air Space/4mm Clear – U-value 4.0 – SHGC .47
- 6.38mm Clear Comfort Plus/6 Argon Space/4mm Clear – U-value 3.6 – SHGC .34
Note: It is important for the builder to seal all gaps and cracks around window and door frames.
Blinds and curtains, can enhance performance and can be an effective way to overcome problems with new & existing windows.
Reducing solar heat gain can be achieved by blinds that reflect solar heat that was transmitted through the window, back out through the window. This is not as effective as preventing the solar heat from entering the window in the first place because only a portion of the heat is reflected back to outside.
To reflect solar heat the external surface of blinds should be white or near-white. Some offer a metallic, reflective film on the external surface, with a decorative fabric facing in. The space between the blind and window will trap a lot of heat – a ventilation opening in the window can allow that to escape.
Reducing convective heat transfer through windows can be achieved by snugly-fitted blinds and curtains with pelmets, which trap a layer of still air next to the window. Avoiding air gaps around all perimeters of the curtain and pelmet is key to improving performance.
Heavy fabrics and multiple layers of fabric help increase the insulation provided by curtains by reducing the amount of heat conducted between the air in the room and the air adjacent to the window. This benefit will be reduced if air-movement around the curtain is allowed.
Ref: AGGA – Australian Glass & Glazing Association. WERS – Window Energy Rating Scheme. (The Star Rating information is a guide only.)
LOW E GLASS – CLEANING GUIDE
ComfortPlus™, Sunergy™ & Energy Advantage Low E™ have a very thin coating on the interior glass surface which cleans differently to ordinary glass. The following guidelines are recommended for the best hand cleaning results.
Hand cleaning of the coated (interior) surface of ComfortPlus™ , Sunergy™ & Energy Advantage Low E™, to visibly remove accumulated dust or fingerprints can be accomplished by using a number of different glass cleaning products. The exterior surface of the glass is not coated, so can be cleaned in the same fashion as ordinary glass.
Recommended general cleaning products
Windex® Glass & Surface (clear liquid, not blue, available from supermarkets), manufactured by S.C. Johnson & Son.
- A mixture of one part vinegar with ten parts clean water.
- In addition to the above products, commercially available vinegar-based glass cleaners have generally demonstrated an ability to provide a clean, streak-free glass surface.
- Golden Windows Pty Ltd does not recommend the use of ammonia-based or alcohol-based glass cleaners, due to these products leaving visible streaks.