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Triple glazing or high-end double glazing?

Josh7447 | Posted in Energy Efficiency and Durability on

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!

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  1. GBA Editor
    Martin Holladay | | #1

    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:
    Milgard Triple-Glazed Windows Offer Enhanced Efficiency.

    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.

  2. Expert Member
    Dana Dorsett | | #2

    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."

  3. Josh7447 | | #3

    So the total cost difference with Milgard is $1,700 with tax for the whole house to upgrade to triple pane. The triple pane have U-Factors of .30 with SHGC of .25. The double pane with special coatings are actually rated higher with U-Factors of .29 and SHGC of .22. My question is which should I purchase? Will the triple pane hold up better because of reliance on glass instead of coatings? Will they prevent condensation better? Or is this just a waste of money and I should go with the double? Thanks for your help! Josh

  4. GBA Editor
    Martin Holladay | | #4

    User 709 etc.,
    It would still be good to know your name.

    While it is possible for a badly designed triple-glazed window to have a higher (worse) U-factor than a well-designed double-glazed window, it's unlikely -- especially if you are comparing windows from the same manufacturer.

    The Milgard chart below implies that a typical U-factor for one of their triple-glazed windows is 0.27, not 0.30.
    The chart came from here:

    That said, I'll admit that a U-factor of 0.27 is nothing to write home about. It could be better.


  5. Expert Member
    Dana Dorsett | | #5

    I would be surprised if the whole window U-factor of the a 4th Surface® (Milgard terminology) was as high as U0.29 unless that is the ONLY low-E coating.

    Most low-E double panes have some other coating on surface #2 and even with air-fill come in around U0.35-ish, a bit lower with argon fill. Adding the second 4th Surface coating to an air-filled low-E window drops it to U0.25-ish. Adding it do an argon filled low-E would drop it to under U0.22.

    Their ENERGY STAR v6 Northern U0.27 windows have only SunCoat® or SunCoatMAX® Low-e coatings, not 4th Surface®, but they do imply that adding 4th Surface® drops it to U0.20. See the "Milgard Energy Performance Options" near the bottom of this page:

    Their illustration shows SunCoat® on surface #2, and 4th Surface on surface #4.

    In your location there is effectively zero condensation control advantage to going with a triple-pane. A U0.30 triple pane is a waste of extra glass- you can get that kind of performance with a single low-E double pane. Whether clear glass U0.30 triple pane or single low-E U0.30 double pane it would have the same surface temperature at the interior side glass surface (which is what determines the condensation potential). 4th Surface® would lower the temperature of the glass surface a bit while improving the overall performance, but that effect isn't a significant condensation risk in your climate/location.

  6. Josh7447 | | #6

    My name is Josh. I'm confused by this as well. My window rep says the double panes have Milgard's best coatings and they don't make a triple with the same spacers and coatings. It's possible that in my ignorance I am reading these incorrectly. I've removed the sensitive information and I'm attaching the two proposals. I've also quoted Alpen. They're windows are U.17-u.19 but add 30% to the cost. I'm not sure if that is a good payback. Thanks again for your help!

  7. GBA Editor
    Martin Holladay | | #7

    It looks like the double-glazed windows have argon gas, while the triple-glazed windows lack the argon. I'm not sure why Milgard doesn't offer triple-glazing with argon.

  8. Expert Member
    Dana Dorsett | | #8

    The double pane is a single low-E SunCoatMAX® w/argon, nothing on surface #4.

    Their own published diagram shows SunCoat® on surface #2, and 4th Surface on surface #4,so they MUST offer it, and that would be a significant improvement. That's that glass to get quoted (w/ argon fill), and it should still be cheaper than a triple pane.

    The triple pane is air fill with clear glass in the middle and SunCoat® on surfaces #2 & #5. The closer spacing and air fill is probably cutting into the potential performance by quite a bit.

  9. Josh7447 | | #9

    Dana and Martin,

    Thank you for all your advice. I don't know much about this subject and your feedback is greatly appreciated! I'm guessing if I milgard will quote the 4th surface it will bring the cost close to the lower end alpen which is the zr525. This is also a fiberglass window that is triple pane and has a u rating of 0.19. Do you guys have any experience or feedback on which brand of window would be better? Thanks again! Josh

  10. jackofalltrades777 | | #10

    Have you received a quote from Alpen out in Colorado? They have triple pane with heat mirror and fiberglass frames. U-Values are in the 0.15 range on some of the windows. Plus being they use a heat mirror, the glazing doesn't weight as much as 3 panes of glass.

  11. Stockwell | | #11

    I am going with Enerlux windows out of Nebraska. They are very high quality and I think you would find their pricing reasonable(certainly far below Alpen for equivalent performance).

  12. GBA Editor
    Martin Holladay | | #12

    You may want to read this article: "What Windows Should I Buy?"

  13. Josh7447 | | #13

    For those interested, Milgard's 4th surface only gets me to 0.25 which I don't understand. I did reach out to Enerlux along with a couple others in Martin's article. Thanks for all your help.

  14. Expert Member
    Dana Dorsett | | #14

    I'm surprised that SunCoat® + 4th Surface® only gets you to U0.25.

    That would be about right for AIR fill window with only 6mm between the panes or something.

    On this page under the heading "Milgard Energy Performance Options" they claim U0.20:

  15. Josh7447 | | #15

    I asked about that Dana. Crickets. It seems the fourth surface would not be money well spent. Unfortunately, Cascade is taking orders for November so its probably Alpen, Enerlux or just ok double pane. As much as I want a tight house, I'm not sure the savings justifies the extra expense.

  16. Josh7447 | | #17

    I think I'm actually more unsure after reading more on Martin's post about energy expensive windows not being worth the money. I'm trying to build a "pretty good house". Would I be better off purchasing an average window and investing the money elsewhere? Is decreasing the u value by .10 worth 30% more regardless of brand?

  17. GBA Editor
    Martin Holladay | | #18

    Q. "Is decreasing the U value by .10 worth 30% more regardless of brand?"

    A. The best way to answer that question is to model your house design using energy modeling software. You can run two iterations of the software -- one with the double-glazed windows, and one with the triple-glazed windows -- and compare the annual energy costs of the two iterations.

    REMrate or PHPP are two examples of energy modeling software that could help you answer this question.

    If you simply want to focus on the windows, here's what you can do:

    Annual heat loss in BTU through a window is:
    Q = HDD*A*U*24,
    where HDD is the number of heating degree days in your climate,
    A is the area one window (or all the windows if you want total window heat loss),
    U is the rated U-factor,
    and the 24 is a conversion because U has hours in the definition but HDD is day-based.

    You can perform this calculation using two different U-factors. Then you will know how many BTU per year you save when you use the more expensive windows. If you know your fuel costs and the number of BTU in each unit of fuel, as well as your heating appliance efficiency, you can calculate the annual cost difference between these choices.

  18. Josh7447 | | #19

    Me again. Now that I can't go exterior rockwool, I looking to use that money for other energy upgrades and I keep coming back to windows. There is a hefty charge to go from alpens u.19 to u.10 but it seems having an r10 window may provide good payback. Any thoughts? Thanks

  19. brp_nh | | #20

    You're in Missouri, it's not that cold. Any of the Alpen windows will be more than adequate for your climate. Put the money/effort into installing the windows correctly, or solar PV, or save it for unplanned expenses.

    We have Alpen 525 windows in cold and snowy NH, they are great.

  20. STEPHEN SHEEHY | | #21

    The Alpen label posted by Peter shows a VT of .30. Wouldn't that be noticeably dark?

  21. Jon_Lawrence | | #22

    Stephen - I have seen those Alpen R-10 windows in-person and they are very dark. I could see using them on the southern exposure of a mountain top ski house, but that is about it.

    Josh - I just installed triple-pane European windows and went with high solar heat gain, high VT glazing. I don't like dark glass and I understand that I need to provide shading to avoid overheating, but I am fine with that. Given your location, I think you should be fine with a high-quality double-pane windows. Just make sure the closing mechanism and gaskets provide a good seal (tough to beat Tilt-N-Turns in this respect) and they are installed properly. I just finished taping the outside of my windows with a vapor-open tape, to be followed by insulation in the gap between the RO and frame and a vapor-closed tape on the inside. It is going to be tough for any air to get through and should minimize the thermal bridging.

  22. jackofalltrades777 | | #23

    If you want MORE visible light to come in, just downgrade to the 9 or lower Series. The VT is in the mid 0.40 range. The 10 series is more about high R-Value which causes the VT to drop.

    Most homes I have seen, people install shades because the light coming in is too bright and overwhelming. The larger the window, the less VT you need.

    Watch the SHGC,depending on window orientation and climate. East and especially west facing windows are notorious for causing glare and heat coming in. You can't shade them due to the suns angle. My South facing windows are shaded in the summer but in the winter the sun is directly coming in. You have to think about all seasons and how they will effect your windows.

    Most spec homes the house orientation is off as is the logic behind why they put 10 west facing windows in the hot desert sun and no windows on the north side of the home.

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