September 16, 2020

Author

Suzanne Grant

TYPE

Passive Design

The value of great windows

PART 3 - BUILDING A PASSIVE HOUSE

What to consider when choosing windows

Windows are one of the critical elements in a Passive House and is typically where the most heat and energy is lost in a home. There are three main performance areas for a window in a Passive House:

1. U-Value

2. SHGC Value (Solar Heat Gain Co-efficient)

3. Airtightness (unique to Passive House)

Once we had the first draft of plans and the Passive House assessment, we had a very specific performance target that the windows had to achieve in order to obtain Passive House certification.

  • U-Value = 1.31
  • SHGC average value of 0.51

U-Value

These values are often thrown around when looking at buildings and energy ratings, but myself included never really understood what they mean. The U-Value measures how well the 'whole window' (including frame, glass, seals and spacers) prevents heat from escaping. U-Values generally fall between 2.0 - 10.0 for Australian products. The lower the U-Value the greater the windows resistance to keeping out the heat or cold.

When we were making preliminary phone calls to some of the major window manufacturers and told them what u-value we needed to achieve they would almost have a little chuckle to themselves thinking we were joking! The problem most of the Australian manufactures we contacted had with achieving the U-Value, was that they could achieve the U-Value with the glass alone, but it was the frame that would let down their efficiency.

SHGC

The SHGC value measures how much solar radiation passes through the window, it will be a number between 0 and 1. The lower the SHGC, the less solar heat it transmits. In a cooler climate such as Melbourne, we want the windows to have a higher SHGC to allow for more heat to pass into the house essentially offering free solar heating for the home.

The designs

The original designs for the windows were primarily three panes of glass, with two openable sections as you can see from these elevations.

After contacting a couple of window manufacturers who were able to meet all three of the Passive House criteria, nearly all suggested limiting the amount of frame the windows had in the design. The frame is the major element of the manufacturing that was making the windows more expensive.

Taking this into account we decided to keep the front facade the same, as this was important to us to maintain the bay and office window design, but we adjusted most of the other windows to two panes of glass and one openable section.

Choosing a manufacturer

With the revised design, we received five quotes from different window manufacturers. The quotes ranged from $80,000 to $23,000!

Out of these five manufacturers, we were confident three of these were able to deliver to a Passive House standard from their previous experience and the documentation of the testing they have done to demonstrate the performance of their windows.

We decided to go with German manufacturers Unilux Windows, which we were able to source through a fellow certified Passive House tradesperson and supplier of Passive House products in Tasmania, Justin O'Connor from Passive House Construction and Products.

The Unilux windows came in two options, u-PVC/Aluminium (slightly cheaper) and Timber/Aluminium. Given the intensity of the UV in Australia we didn't want issues of the u-PVC discolouring or fading, therefore we went for the Timber/Aluminium frames. Performance wise, the windows are triple glazed with an average u-value of 0.8 for the home. If we were to imagine our house to have a standard 6 star energy rating, requiring double glazed windows (even this isn't always required) and aluminium frame, it would cost from a typical Australian manufacturer between $12,000 and $15,000. In comparison the Unilux windows, coming all the way from Germany were just under $38,000 delivered to site!

It just goes to demonstrate the lack of demand on the Australian manufactures to lift their game and supply better performing windows.

Another exciting element of the windows from Unilux, is that they don't just come in the standard Colourbond colours. We had to choose the colour from an international colour standard called RAL. We ordered the sample swatches specifically to chose the window colour, and were glad we did as there were so many choices.

The windows do have a three month lead time, so this was one of the first elements we ordered and locked in so that they should hopefully arrive onsite before Christmas.

Dreaming of a new home?