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Triple glazing or high-end double glazing?

I have decided on Milgard fiberglass windows for our house. I like the price point and warranty but I'm trying to buy their best window. The rep has tried to talk me out of the triple glaze but I think it's because he is not familiar with them in this area. He says the high end double pane with their best coatings have better u values and they can't or don't put that coating on triple pane. The cost is about the same. Triple seems to be better to me but I could use some advise. The house is in Missouri. Thanks!

Asked by user-7088022
Posted Jun 12, 2018 6:35 PM ET
Edited Jun 13, 2018 5:38 AM ET



User 708 etc.,
First of all, can you tell us your name?

You mention that "they can't or don't put that coating" on triple-pane glazing, but I'm not sure what coating you are talking about. Are you talking about low-e coating? Almost all manufacturers that offer triple glazing include at least one, and sometimes two, low-e coatings on their triple glazing.

That is true for Milgard, as you can determine if you read their literature. Here is a link to a page with relevant information:

On that web page, Milgard boasts, "Our triple-glazed windows come standard with dual SunCoat Low E glass and dual EdgeGardMAX spacers to interrupt wasteful heat flow through the window and save on heating and cooling costs."

In Missouri, the climate isn't as severe as it is farther north, in Minnesota. Triple glazing isn't necessary in Missouri, but if you can get triple glazing for the same price as double glazing, it makes sense to try to get it. Triple glazing will lower your energy bills and cut down on noise transmission.

Answered by Martin Holladay
Posted Jun 13, 2018 5:25 AM ET


In Missouri a double low-E double pane with low-E coatings on both surface #2 and #4 is a good choice. With argon fill they run a center-glass U-factor of ~0.20, which is comparable to triple pane.

The low-E on #4 lowers the temperature of the glass that is in contact with the room air (since it's reflecting heat back into the room rather than absorbing it, warming the glass). That becomes a window condensation issue in locations where temps colder than -5F can persist for many hours or days, but even in the coldest parts of MO those temps are the exception rather than the rule, unlike the cold edge of US climate zone 6 or colder.

Cardinal's Cardinal LoE-180/LoE-i89 combination is one such glazing, that has the combination of a high SHGC and low U-factor, which makes it great for reaping wintertime solar gain. That's fine for south or north facing windows, but glass with a lower SHGC would be preferable to east & (particularly) west facing windows, since that can increase the cooling load by quite a bit. The high mid-day sun angle in summer reflects much of the mid day sun and can be easily shaded by overhangs or awnings, which makes the high SHGC windows on the south side less of a problem than on west facing windows, which see low sun angles and more direct gain.

See p22 (p25 in PDF pagination) of this document for specs on the glass temperature, indoor RH and outdoor temp at which different combinations of low-E have condensation issues in a 70F room:

From that table that at -20F most dual low-E double pane combinations will have condensation issues at an interior RH of 40% (which is pretty high, actually), but at 0F only the thinnest 1/4" spaced windows would be problematic at 40% (but still not a problem at 30%RH, which is still in the human-healthy and comfortable range.)

A U0.20-ish window is still pretty good- the upcharge for triple panes would be better spent on other building envelope aspects in most houses.

Milgard does offer "4th Surface" low-E double panes in the U0.20 range, which SHOULD be both cheaper and higher SHGC (where desired) than triple-panes:

"4th Surface - 4th Surface turns double-pane windows into triple-pane performers. It’s a durable TCO (Transparent Conductive Oxide) coating that enhances the U-factor of the window by reflecting heat back into the home. 4th Surface provides energy performance levels in a Dual Glazed insulated unit that previously were only attainable with our Triple Glazing option. It also offers more light transmittance and less reflectance than triple glazing."

Answered by Dana Dorsett
Posted Jun 13, 2018 11:40 AM ET

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