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Windows are incredibly important to the livability of a home. The provide light, enhance aspect and allow good, controlled ventilation when you want it. But they also can account for up to 25% of your heat gain in summer and up to 20% of your heat loss in winter. According to Sustainability Victoria, a single glazed, 3mm deep pane of glass can lose from ten to 15 times more heat than an insulated wall of the same area. On the positive side windows allow winter sun in, which can be harnessed to warm your home.

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As heat flows through glazed elements in two ways, different window treatments are more effective at reducing the movement of heat across windows as follows:

Conduction – this is the movement of ambient heat across glass and is measured as a U value. As an example, a single glazed aluminium framed window typically has a U value of 10. Treatments like double glazing and internal blinds are most effective at reducing the movement of radiant heat through windows.

Radiation – when sunlight strikes a sheet of glass, most of the solar radiation is transmitted straight through, some is reflected and some is absorbed by the glass. As an example, for 3mm clear glass, 83% of solar radiation is transmitted. You feel this as warmth sitting behind glass on a sunny winters days. External shading of windows is the most effective way of reducing heat gain from the sun in summer.  Good passive solar design is the best way of maximising heat gain in winter through allowing the sun to hit the windows.

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In winter you obviously want to keep the warmth inside your house, not lose it through the window.  According to SV, a singleglazed, three-millimetre-deep pane of glass can lose from ten to 15 times more heat than an insulated wall of the same area. In winter, all windows require protection from heat loss.  There are a number of ways to achieve this including

Double Glazing

Double glazing is helpful to reduce heat loss through your windows as it prevents heat transfer via conduction.  In winter this works well as it will will still allow sun in penetrate and warm your home in winter, at the same time reducing heat loss from the inside.  In summer double glazed windows will still need need to be shaded to prevent heat gain.

How does it work?

A double glazed window assembly creates a layer of still air or gas between two layers of glass. This provides increased resistance (U value could be lower than 3) to the conduction of heat (in and out depending on the temperature difference) while allowing a large proportion of the solar radiated heat to be transmitted.

Don’t forget the frames

There is not much point installing a state of the art double or triple glazed window if the frame holding it all together allows heat to move easily through. Aluminium frames for example are a great conductor of heat, and will allow the warm air you are so diligently trying to keep within your home, to simply flow out. There are a range of solutions, some of which are outlined in the Your Home Technical Manual.

Choosing what is right for you

When it comes to double glazing there is a wide range of options, for professional to DIY – and all within a wide range of budgets.  Choosing which is right for you can be confusing so why not:

Maximising solar gain

Ideally you want to harness the sun’s rays in winter to warm your home.  This can be done by:

  1. Maximising the amount of sun enter your house in winter. Check out the Solar Passive Design section for information on what to consider and how to achieve this;
  2. Capturing the sun’s heat and storing it to be released back into the room via a thermal mass

Keeping Cool in summer

Achieving this differs somewhat depending on whether your are building or whether you are trying to modify an existing home.   For those building you have a great opportunity to make your home more comfortable in summer by incorporating passive solar design features into your home design.  For those of us who don’t have the luxury of moving our existing home so that you no longer have a bank of windows facing west, it is a little more challenging.  But don’t despair, there are a range of things for all budgets, that you can do to make your home more comfortable in summer, as outlined below.
External Shading
By far the most effective way to reduce heat gain in summer is to keep the sun off the window in the first place!  External shading can reduce heat gain by up to 75% and can include fixed building elements such as pergolas, verandahs, eaves as well as external shutters and awnings.
1. Verandahs & eaves
Because the sun tracks at different heights in summer and winter, it is easy to design a home that, on the north side, allows sun in in winter, when the sun is lower in the sky, whilst shading the windows in summer, when the sun is higher.  Sustainability Victoria’s Windows Fact Sheet is a great guide to help you design your windows (size and orientation) and the eaves (overhang) to maximise heat gain in winter and reduce it in summer.
Internal options
1. Internal blinds & curtains
Once you have done all you can to stop the sun hitting the windows in the first place, the next line of defence against the summer sun are your internal blinds and curtains.
Check out the following options:
  1. Not your typical blind, Renshade is a product that reflects the heat back from your windows.  Check out this simple demonstration from Green it Yourself on how to install it

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The Window Energy Rating Scheme (WERS) rates the energy and energy-related performance of residential windows, skylights and glazed doors, both in terms of the impact the glazing treatment has on reducing heat gain and reducing heat loss.