From the Window to the Wall

One of the big-ticket items in any build is the external windows and doors – often defining the character of the home, providing natural light and ventilation. Windows are typically the weakest link in a building from a thermal performance perspective and it’s common to see condensation on the inside of the window frame or water entering through the openings around old existing windows. Being one of the five key principles of Passive House, we thought we would dive a bit deeper into this topic as it’s something every build can relate to.  

The great thing about windows is that we can measure their performance and there are two important values that need to be taken into consideration:

Solar Heat Gain Coefficient (SHGC/G-Value) - This measures how much heat can be gained through your windows when there is direct solar contact. The lower the SHGC, the less solar heat it transmits.

U-Value - This tells us the conductivity of a whole window (window frame and glass). It measures how well the window will retain its heat in winter and reject heat during summer. The lower the U-Value, the slower heat will be able to move through it, resulting in better performance as an insulator. The U-Value is highly dependent on what material you build your window from and whether it is thermally broken or not, as certain materials conduct more heat and allow energy to move quickly through them. For a window to be considered high performing, it must have a maximum U value of 1.5 (w/m2K)

We have plenty of options when it comes to the material you can construct your window from.

Aluminium
Aluminium is the most commonly used frame system in Australia because of its low cost and maintenance, however, it does not perform well thermally as it’s extremely conductive. This issue can be solved by thermally breaking the windows using a PVC core between the windows and filling it with insulation foam, reducing the U-Value of the window.

Timber
Timber windows are very common in Australia as they can be crafted into any shape and have great structural stability. Timber is a very low conductor of heat, meaning that it can achieve a low U-Value. External aluminium covers are installed to protect from UV rays and water damage, however this does not affect the U-Value as timber is a natural thermal break.

uPVC
Relatively new in Australia, uPVC is used widely throughout Europe. It’s strong, lightweight, flexible, water-resistant and has an excellent thermal performance. uPVC does not contain any harmful chemicals, can be recycled, and won't rot, rust, warp, or even fade, making it a very low maintenance product. It comes in a range of colours including natural wood-like textures and is a great option for Passive House homes. There is a misconception that uPVC windows fade, however, this is not true as they are UV resistant.  

Timber/Aluminium Composite
This consists of a half and half approach, with aluminium on the outside to provide great weather resistance, but timber on the inside to provide you with excellent thermal and sound reductions. This is starting to become a popular choice for architects as the timber creates a natural thermal break and both materials come in a range of shapes and colours. 

For a home to be Passive House certified, windows must be triple glazed. These windows are very high performing and just like you may have opted to go double glazed over single glazed, the same reason can be said for going triple glazed. Triple glazing can improve your window’s thermal resistance by up to 94% depending on the glass selected. Glazing should have Low Emissivity Coating (low e) as this helps retain the warmth inside a room, as opposed to it being conducted out. This will reduce condensation and improve thermal control. The windows should also be filled with a gas between the glazing panels as it has a lower thermal conductivity than air (meaning energy passes through it at a slower rate). 

Every little detail counts and special attention should be paid to the seals on the windows as well as the spaces between each panel, choosing materials that conduct less heat (ie plastic or rubber spaces over aluminium). Triple glazed windows will also greatly reduce the amount of external noise entering the house. 

There is no point in selecting a high-quality window if it is installed poorly. Windows should be installed in the insulation layer of the frame and taped correctly to prevent air and water penetrating the frame. We have been using Proclima products recently, specifically the Contega IQ which allows us to create a complete seal from the window to the external membrane, meaning no water can penetrate through. It takes no time at all and we know this will hold up.

Another thing to consider is the airtightness of windows. I have said it before and I’ll say it again - you can have all the glazing in the world, but if the window is leaky, it won’t do much other than keep water out. Windows should have the correct rubber seals (not brush seals), allowing a tight vacuum seal when closed to prevent air and heat flow through the window sashes. 

It’s no secret a triple glazed window will cost you more, roughly 30-50% more than a double glazed window, however, this cost is decreasing due to more suppliers entering the market. We will start to see even more triple glazed manufacturers pop up in Australia and further developments in window technology as the demand for Passive Houses increase. The big plus for triple glazed windows is that they will create a higher level of comfort in your home, especially in the colder months. 

Insulated windows are just as important as an insulated wall. High performing windows are essential in a Passive House as they provide a high level of internal comfort. We need to stop just worrying about what a window will look like and factor in how well it will perform. Architects, Designers and Builders need to understand how windows work so they can correctly inform their clients on the best window to select for their home.