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Community and Q&A

Replacing Insulated Glass Units (IGUs)

rliebrecht | Posted in Energy Efficiency and Durability on

Hey there,

A number of the glass units have failed in our 1994 home – all original windows, mix of dual pane vinyl sliders, single hung and fixed. Interior BC (climate zone 6)

We have gone all electric with a best pump, and want a clear view of the outdoors, so we are looking at replacing those IGUs. We considered window replacements to get triple glass, but the economics don’t make sense.

Low-e 180 seems to make sense for the south and west facing windows. Two face north, and 1 faces northwest, so perhaps 366 on those?

Then there’s i89. I like the u-value improvement, but reported concerns around interior condensation give me pause. I also read that the lower centre of glass temp of i89 causes more convection cooling. I’m weighing skipping i89 and sinking money into dual cell cellular shades to improve u value and reduce radiate heat loss/improve comfort. Given our budget isn’t unlimited, what’s the smartest call here?

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  1. kyle_r | | #1

    I would take a look at Cardinal’s technical glass guide. There is a table that gives you at what indoor relative humidity the glass will see condensation at different outdoor temperatures. This should help weigh the impact of i89 condensation resistance vs u value.

    Keep in mind that even with triple panes if you keep your cellular shades down all of the time you will still have condensation on cold nights.

    1. rliebrecht | | #2

      True about keeping the shades down. Since it's a tradeoff for i89 (which would almost certainly produce condensation given the chart you referenced) which mainly helps reflect radiative heat, I figure I'd skip the side tracks on the shades to get a bit of air exchange but keep the room "feeling" warmer.

  2. [email protected] | | #3

    Are you considering LoE-180 for it's solar heat gain properties as well as VT and 366 because it has the best U-factor?

    If that's the case, you might consider that Cardinal does not recommend LoE-180 for west-facing windows because of the high solar heat gain properties that can result in overly warm rooms, even in winter unless the windows are shaded in some way.

    No coating "produces condensation", what I89 or any surface 4 coating does is lower glass temperature by reflecting heat back into the room rather than allowing the glass to absorb that heat. Condensation occurs when the glass temp goes below dew point, which is much more likely to occur with closed insulating shades or curtains than with a surface 4 coating even if you skip side tracks.

    1. rliebrecht | | #4

      Thanks for the reply. We are indeed trying to use 180 vs 366 the way you describe. We have some large west windows and I'm leaning toward 366 on those.

      My thought re:condensation was if both i89 and shades lead to condensation, stick with the shades alone because we can at least control for condensation by leaving them open. I don't know how much to make of the condensation risk with i89.

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