It’s been an exciting two weeks with the window installation making our house feel more like a home. Thermally broken double (or sometimes triple) glazed windows are the third pillar of passive house currently being incorporated into our house.
Window selection and installation in passive house combines consideration of thermal properties and air tightness. The thermal resistance (the R value) of the windows needs to match that of the walls, so that the windows aren’t letting heat or coolth in, or out, of our house without our permission.
Heat and coolth can sneak through the glass directly, or via the frame, hence the need for windows to be ‘thermally broken’. The break in the thermal bridge is created by clever engineering. They are quite a contraption, but they make a difference! Take a look at this diagram from our suppliers website.
Two having two layers of glass means there is an insulating layer of air between them, and that the transfer of the external heat or coolth via the glass is slowed down by air being crap at conduction. Sometimes the air is replaced by a gas like argon, as ours are. Argon has even lower conductive ability. Some climate zones (liker Hobart or Dunedin), will require triple glazing, but we should be able to get away with double glazing here in Sydney, especially as triple glazing is more to do with controlling condensation (a bigger temperature difference between inside and outside).
The next thermal bridge in a window can be the frame, where the amount heat transfer ed via the frame will vary depending on the material. Wood and uPVC are the most thermally efficient. The fancy technical construction method you see in the diagram prevents the heat being conducted via the frame.
Another consideration with a window is air-tightness. Air can leak around the pane of glass and around the frame. Those babies need to be in tight, so that air can’t leak out around them taking precious thermal comfort (and your hard earned dollars in heating and cooling expenses) with it.
The thermal properties of a window are measured by the U-value which is the inverse of the R- value. R-value you want a higher number, U value you want a lower number. When to want a high number and when to want a low number is doing my head in a little.
To select our windows we sent the window schedule to two providers of passive house compliant products, and chose the option that best suited our budget in a colour that suits the style of our house.
The final piece of information in this post is to answer a question we get often when we talk about air-tightness. Can we open our windows? Oh yes we can! And we will! Opening windows just means the house isn’t ‘performing’ as a passive house while the windows are open. Close the windows, run the AC for 5 mins and then let the house (and HRV) do its thing.
Passive Not Massive 
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