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Community and Q&A

Value of i89Coating in Triple-Pane Windows

BILL WICHERS | Posted in Green Products and Materials on

Is there really much value to cardinal’s i89 coating when dealing with triple pane windows? I can see the benefit for double pane windows, but according to cardinal, the U value difference in 1-3/8” argon filled triple pane windows is only 0.01 — not much.

The data is here:

I’m looking at the low-E 180 coating for the “regular” low-E coatings. I don’t have info yet on the price difference to add the i89 coating.


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  1. GBA Editor
    Martin Holladay | | #1

    When it comes to evaluating glazing performance, what matters are U-factor, SHGC, and visible transmittance (VT). You don't really care how these numbers are achieved -- that's the responsibility of the researchers at places like Cardinal Corp.

    If the difference in U-factors between two coating options is only 0.01, that's how you evaluate the performance difference between the two options.

    1. Expert Member
      BILL WICHERS | | #3

      In the triple pane IGUs, i89 makes a very minimal difference in U value, VT and SHGC. The same coating makes a much bigger difference in double pane assemblies, and cardinal markets it as making double pane perform like triple pane (low end triple pane anyway).

      It seems that the i89 coating is into the range of diminishing returns when used in the better 1-3/8” IGU. Looks like it will just come down to a cost difference which I should know sometime this coming week.


  2. charlie_sullivan | | #2

    For readers who don't know, i89 is a low-e coating in the "4th surface", the side of the glass facing the interior of the house.

    Based on the U-factor, you'll get about 8% lower heat loss through the windows with the i89 coating. (It's hard to be sure it's not just rounding error when the numbers are reported as 0.012 and 0.011, but if you also display it in SI units, you see that that it really is an 8% difference). Once you get the cost, you can decide whether that's worth it.

    Normally you don't care at all whether that 8% was achieve with a different fill gas, a different coating, or what, but for a window you are sitting next to, the temperature of the glass can affect your comfort, in addition to affecting the heating costs, and the interior i89 coating will make a difference in terms of the mean radiant temperature you experience in that location than you would get through some other way of improving the U-factor by 8%. I'm not quite sure how to quantify the thermal comfort benefit. With triple-pane low-e, it certainly won't be uncomfortable to sit near those windows, but the i89 coating will more or less eliminate any minor effect there.

    So if the pricing comes back with only a very small adder for i89, you might just go for it everywhere.

    If it comes back expensive, just go with regular low-e

    And if it comes back questionable, you could consider running the numbers on energy loss differences and compare to saving energy other ways, and/or consider getting it just for windows you expect to sit near.

  3. Jon_R | | #4

    Quantification of window comfort (including inside coating):

    1. charlie_sullivan | | #8

      Cool, thanks for posting that link!

  4. onslow | | #5


    One additional consideration to think about. Prior posters on this site have noted that the I89 coating on the final inside surface seems to have the effect of lowering the glass temperature slightly. The infrared reflection properties will make the glass seem warmer to a person because it will reflect the heat you emit just as well as it reflects the ambient room heat. I am unclear on whether the IR reflecting property is bi-directional and would be helpful against heat loads in the summer season.

    I do not have any threads to point to, but I do recall at least a few posters commenting on the coating lowering the glass surface temp enough to create condensation when not expected. I think the effect of the coating is much like hanging cellular shades in front of the window. The heat blocked by the shade or coating will of course not be available to the glass. Perhaps with very active air flow by the windows from heater vents located below them, the conductive element of heat transfer might still keep the glass temp safely above the dew point.

    1. Expert Member
      BILL WICHERS | | #6

      That’s the kind of thing I’m wondering too. My question as to the value of the coating was basically if there were any downsides to the i89 coating besides cost that would offset the very small relative improvement the coating makes when used with a triple pane window. On a double pane window, the i89 coating makes a much bigger relative improvement compared to the same window without it.


      1. charlie_sullivan | | #7

        On the summer vs. winter question, the effect it has on heat loss and comfort for a human near a window is identical in the summer vs. the winter. However, it has essentially zero effect on the solar radiation coming through a window: it will still allow the benefit of that in the winter and the annoyance of it in the summer.

        The condensation concern is a real concern if you use it on double pane but less of a problem on triple pane.

        1. lance_p | | #9

          Are there any hard numbers relating to reduced condensation resistance? I'm interested because a prime target of my build is to use windows that allow a high indoor RH of 40% in the colder end of the 6A climate zone, Ottawa Ontario. I would gladly pay a slight energy penalty to guarantee I don't have condensation issues, and I can't see comfort being an issue with otherwise efficient triple pane IGUs.

          1. Expert Member
            BILL WICHERS | | #10

            I’m doing similar things to be able to maintain a bit higher humidity in the winter. From what I’ve seen data-wise, I suspect the issue with increased condensation is a concern with i89 applied to double pane windows, but much less if any of a concern when applied to triple pane windows. My thinking is that the data shows a very small performance difference for i89 when comparing triple pane windows for all of the important performance characteristics. With double pane windows i89 makes a much bigger performance difference as a percentage and I think that’s where the noticeable increase in condensation is coming from.

            Unfortunately, there doesn’t seem to be much practical expierience with i89 used with triple pane windows. If the price difference isn’t too much I’m going to order my sliding doors (I have three big 12 foot wide sliding doors to replace) with i89 on 1-3/8” triple pane assemblies and next year maybe I’ll have some practical expierience I can share.


          2. Jon_R | | #11

            The Payette tool also indicates when condensation occurs. Inside coating does have an effect.

          3. Deleted | | #12


  5. gusfhb | | #13

    We have 4 large sliders we built with triple pane Cardinal glass, with the , perhaps memory is failing, but i81 interior coating

    when you stand with your nose to the window when it is zero out, it reflects the heat of your body back at you. When you aim a thermal imaging camera at it, it gives you are perfect thermal image of yourself.

    I think, personally it is not just the numerical value, but numerical value plus incremental cost.

    You would not pay 20 percent more to increase an R 30 wall to an R31 but perhaps an R5 to R6 would be worth it

    I thought so.

    1. lance_p | | #14

      FYI, any ordinary glass will give you a nice reflection with an IR camera. The lenses in IR cameras are made with special materials that allow IR wavelengths to pass more easily than with regular glass lenses.

      My (very limited) understanding is that coatings amplify the natural tendency of regular plate glass to reflect IR wavelengths, shifting the reflective qualities of the glass to higher and higher frequency (shorter) wavelengths.

      Since stopping at a specific value is not possible, the higher we shift the reflective qualities the more visible light transmittance we lose in the lower (warmer) part of the spectrum, and the more "blue" the window looks. If that is incorrect, I hope someone can set me straight.

      1. Expert Member
        BILL WICHERS | | #15

        From my read of the cardinal data sheets, the coatings are all multi-layer coatings so they act as what are known as “thin film” filters. I’m familiar with these from work since I work with both the facility side and optical design for fiber optic networks (yeah, weird mix, but people pay me so I’m happy :-).

        Thin film filters work by exploiting varying refractive indices of different materials. This allow the filters to be frequency selective, even very selective (optical networks have channel spacings less than 1nm apart, for example).

        In window glass, the filters are design with an optical passband (the range of wavelengths that can pass through the coating) that includes the visible portion of the light spectrum. Most heat energy is in the infrared spectrum, which is the longer wavelengths and this is filtered out. Wavelengths that are “filtered out” are reflected, so the filter basically lets you choose which wavelengths are transmitted (passed through) or are reflected.

        Once you filter out the long wavelengths, if you want to block (reflect) more energy you have to start filtering out wavelengths closer to the visible range, and eventually you start to get into the visible range. The blue tint you see when you look through loE windows is because the longer wavelengths are filtered out and that reduces the amount of red light that passes through. Blue wavelengths are the shorter wavelengths. The filters aren’t perfect, so as you get closer to the visible spectrum, you actually start to reduce the amount of visible light passing through too (the curves on the edge of the filter passbands are known as the “skirts”).

        Some energy is passed in the visible spectrum too, so to really get aggressive with limiting energy loss, or gain, through glass, you have to start filtering out the visible spectrum too, usually on the red end of the spectrum. Eventually you run into a limit where the filtering is so severe that people don’t like the way the window looks.


        1. lance_p | | #16

          Thanks for the more technical explanation, Bill!

      2. gusfhb | | #17

        AS you might imagine I have also aimed the camera at regular glass, there is a difference

        Same with IR thermometer, you can pick up your reflected temperature in a regular window, you will pick up your reflected temp with the internal coating.

        The only downside was at the time Cardinal would only go up to IIRC 1 3/32 or so total thickness and I would have preferred larger spacers

        1. Sofiane | | #18

          Hello Zephyr and Lance,

          It seems like I deleted my post instead of editing it last time. I was wondering if you made a decision on your triple pane windows? I'm in zone 6A, actually just across the river from Lance, and currently looking to change my windows as well.

          I'm thinking the small increase in insulation probably isn't worth the extra cost and increased risk of condensation, but would gladly take the input of anyone who's tried them in a similar climate to mine.

          1. Expert Member
            BILL WICHERS | | #19

            I’m planning on going with i89 on full thickness (1-3/8”) triple pane windows. The other coatings are loE 180 on two other surfaces. The added cost for i89 was low enough they didn’t bother quoting both with and without, and it provides a bit better insulating performance. I’m not too worried about increased condensation risk since the nature of the triple pane window already helps to minimize that.


  6. PAsun | | #20

    This post contains information very similar to what I have been trying to find out. I am going to be adding passive solar sunroom to the back of my home and I am looking for window companies that offer glass with SHGC in the .60-.70 range. I have had no luck finding such window manufacturers. I'll create a new topic for this since I do have other questions and I was not planning on using triple pane glass since I think it was in the Cardinal literature that the addition of i89 coatings on double pane windows often eliminates the need for triple pane. Doesn't more layers of glass also reduce the SHGC?

    1. Expert Member
      BILL WICHERS | | #21

      Cardinal doesn’t say i89 eliminates the need for triple pane IGUs. What they say is that i89 can make a double pane IGU approach the performance of a triple pane IGU, but if you look closely you’ll see that i89 makes a double pane IGU perform similarly to a very low-end triple pane assembly.

      Cardinals high solar gain coating is loE-180. Any window manufacture using cardinal IGUs should be able to order it, but some may not want to if it’s not a normal option for them.

      More layers of glass does reduce SHGC, because less energy makes it through when there are more layers. The extra layer also helps trap more heat inside, so the overall effect of the additional layer of glass may still be beneficial to you. Triple pane windows will hold heat better than double pane windows, assuming you’re using the better (thicker) triple pane IGUs.


      1. PAsun | | #22

        Thanks Bill. Your right I should not have written "eliminates" & should have said "comes close" instead. Do you think it's more important to retain more heat with triple pane glass knowing it will reduce the SHGC which will decrease the effectiveness of the sun directly heating the thermal mass?


        1. Sofiane | | #23

          Although I am not qualified to give you a precise answer, my understanding is It really depends, principally on your climate zone, your room's orientation and the surface of window that you are planning to add.

          Obviously with a north facing window you would be better off with the lowest possible u value window. Even then, there is a cost benefit analysis that you would need to do. I decided to go with triple pane because I thought the increased comfort of tripe pane had some value beyond the decrease in heating cost that would be rather minimal. Your conclusion might be different.

        2. Expert Member
          BILL WICHERS | | #24

          I would look at it as a ratio. Cardinal has an excellent technical document that I linked in one of my other posts here. That document gives very specific information for each of their IGUs and coating options.

          If you are looking for high SHGC, some of the differences are only a few percent, but the increase in U value is more than that. Basically the loss in solar energy gain is more than offset by a decrease in the loss of heat to the outdoors. This is probably most significant for north-facing windows where more energy is likely to be lost over a day than will be gained, so you gain more overall with a lower U value even at the expense of a slightly reduced SHGC.


  7. PAsun | | #25

    Thanks Bill and Sofiane. North wall is not a concern regarding heat loss since it will have a well insulated door, no windows and insulation of R-30 or more. Gable shingled roof will have R-49 insulation. Glazing no more than 25% of the 300' of floor area of sunroom. Thermal mass of 400-450' of concrete. Need to find window companies that make good quality high SHGC windows with a U-factor of 30 or less.

  8. HFF | | #26

    I am considering Cardinal double glazed loE 366 units and am curious if anyone can give me a rough idea of the additional cost for the i89 coating? Trying to pin down my builder's supplier is like chasing my own tail.

  9. jonny_h | | #27

    @Zephyr7 / Bill: Did you end up getting the LoE-180 on S2 / S5 + i89 combination, and are you happy with it after a year of actual experience? I'm trying to finish up an order with Inline, they use Cardinal glass (but I believe make their own IGUs) and are offering 2x 272 or 2x 366, both of which seem a bit aggressive from a VT standpoint without making much (if any) difference to U-value. Looking through their NFRC listings it seems like a 2x 180 +i89 achieves slightly better U-value and much better VT. (I'm not concerned with SHGC except in one specific location, where I will go with a more aggressive coating)

    1. Expert Member
      BILL WICHERS | | #28

      I ended up postponing that project since the quote came back at some $40k+ for the three sliding doors. Turns out they are big enough to be extra custom -- Pella won't even make them this way anymore. I'll revisit that project at some point, but for now I have other more pressing things to spend money on :-)

      The more aggressive coatings, especially 366, really are too much on a triple pane window in my opinion. You get a very noticeable darkening, and a bit of a greenish/gray tint too. It is especially noticeable if you have ANY other windows with less aggressive coatings in the same general area where they are visible at the same time as the more aggressive window. You want to make sure you match coatings of all windows that are simultaneously visible.

      If you're going with a triple pane window, I'd stick with 180 unless you are in an area where solar gain is a really big concern (down south somewhere, for windows facing in directions that get blasted with sunlight for long periods of time). If solar gain is a big concern, and you mention it is for you in one spot, consider a more aggressive coating there. I would recommend against putting such a more aggressive window near a less aggressive window since you'll see the difference between them and it will look "weird". If you have no choice, I'd probably just take the hit on reduced SHGC to preserve aesthetics, but that's a call you'll need to make for your particular project.

      If you're in the Northern lattitudes, I'd rely on the triple pane part of the window to do the heavy lifting in terms of energy efficiency, and use a less aggressive LoE coating like 180 to better preserve your VT. Remember that VT is already more of an issue with a triple pane window since the glass of the third pane blocks some light too, leaving even less light to "start with" compared to a double pane window for the purposes of LoE coating selection.


      1. jonny_h | | #29

        Thanks! Yeah, I was all worried about the price of lumber until I learned how much windows cost.....

        After reviewing more data on the Cardinal coatings (thanks for the link to the technical guide in another old post), I definitely agree with your assessment. I'm in Zone 5 / Cleveland area, which is.... not known for its sunny weather.... so capturing what little natural light there is in the winter matters a fair bit more than reducing solar heat gain in the summer. From reading about the coatings, it seems like the U-value improvement from the heavier coatings is quite minimal, and they're mostly reducing SHGC (and also VT, especially on longer-wavelength side of the spectrum). I'm asking my Inline sales rep if they can offer any of the LoE-180 only coating options -- it seems odd that a Canadian company would only offer such aggressive coatings.

        In the one location where I care about SHGC, it's a large west-facing patio door that gets zero sun in winter and several hours of sun in summer. The existing door is clear double-pane, and I added a tinted window film last summer (which helped substantially). I imagine that any low-E coating will be better than clear glass, but since I'm already used to tinted film on that side of the room, it probably wouldn't kill me aesthetically to have a different / more aggressive coating than on the remainder of the house... Will need to think about that a bit more.

        Heck, if my timeline keeps getting delayed, the border will be open and I can just drive up to Toronto and see the windows myself...... as it is I was supposed to have this order in 3 months ago!

        1. Expert Member
          BILL WICHERS | | #30

          Inline had no problems getting me LoE 180 when they quoted the project. They have access to all of the Cardinal line. They were pretty responsive -- nothing was a problem for them, they just needed some extra time to quote my spec is all.

          I really think people worry too much about things other than VT. VT is important too! You are correct that even the least aggressive LoE coatings are a lot better than clear glass. If you only have minimal issues with solar gain with clear glass, you're probably fine with the LoE-180 coating. I'd only go with '366 if you had to deal with brutal sunlight roasting your room.


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