Optimal argon cavity in triple pane windows?
Quite technical / nerdy, but wondering if anyone has any insights on this subject. From the limited research I have been able to do on the internet, it appears that the optimal thickness for the argon cavity is 14.5mm
It appears to be a tradeoff between reducing conduction with thicker spaces, but increasing risk of convection as the space gets larger.
I am wondering because I have seen many passive house insulated glass units (IGUs) with 18mm gaps, but one or two with 25mm gaps. I’ve been told by a manufacturer that the topic is quite complex, but 18mm seems to be an optimal compromise for window performance over a wide range of temperatures.
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Optimum is really more of a range than a fixed width.
When using an argon fill, performance isn't going to vary much between 11mm to 19mm width airspace. If the airspace is narrower or wider than that range then performance will start to degrade - albeit very little as airspace width increases above 19mm, a bit more loss as the airspace width decreases below 11mm.
Variation of airspace width between manufacturers is often more a factor of window style or sash construction than it is an energy performance factor.
The sash is not the most efficient part of the window system. Often the biggest single factor in improving window performance to passivhaus levels is achieved thru improving sash performance by incorporating insulation into the design of the sash.
In such cases the manufacturer will fit the IGU to the sash design. If it takes 18mm airspaces to do it, then they will use 18mm spacers to meet that requirement.
Of all the factors to consider when designing top-end energy performance windows, IG width is typically not considered to be one of the more significant as long as the airspace width falls somewhere within the optimum range.
Thanks Greg. I gathered that the 18mm would be ok if that many manufacturers are using it.
However, what do you think of the larger cavities? e.g. an IGU triple pane with 4-24-4-20-4 gaps (4mm glass, 24 and 20mm airspaces).
Jerry,
As I understand it, the main reason that the gaps between the panes of European argon-filled IGUs are larger than the gaps between the panes of American argon-filled IGUs is that Europeans test their windows at a different outdoor temperature than Americans.
Here's how Stephen Thwaites explained the issue in a comment (Comment #10) posted below my "Passivhaus Windows" article:
In Europe, U-factor is "calculated for smaller Delta-T [than in North America]. As I understand it, in Europe U is based a 0°C (32°F) outdoor temperature and a 20°C (68°F) indoor temperature. In North America U is based on -18°C (0°F) outdoor temperature and a 21°C (70°F) indoor temperature. While most materials don't change their insulating properties over this temperature range, gases can. As a result in Europe the optimal air/gas space is 16 mm, while in North America the optimal air/gas space is 12.5 mm. North American windows with optimal spacing between panes, when evaluated to European norms are sub-optimal. (The reverse is not true).
"(As an aside there are strong arguments for following Europe on this temperature issue. In North America the -18°C (0°F) outdoor temperature to calculate Uwindow was adopted to size heating equipment. Given that;
- heating systems are routinely obscenely over sized
- most homes in North America only rarely experience -18°C (0°F) wintertime temperatures
- Uwindow's role in predicting seasonal energy use is more important, than is role in sizing heating systems
there is a strong argument that the delta T used to calculate Uwindow should be decreased to something that represents typical rather than extreme conditions - more like what is used in Europe.)"