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Surface 4 low-e… condensation issues?

davidmeiland | Posted in General Questions on

I’m just about to order windows for a project, and leaning towards spec’ing some of the units as double-pane with low-e on surfaces 2 and 4. My sales rep is trying to get current information on exactly which coatings are available, but it appears to be Cardinal 180 on surface 2 and i81 or similar on surface 4. He mentioned that there was uncertainty about the surface 4 coating because there had been some condensation issues when this product line came out. He was NOT clear on the details so I may have this all wrong, but is seems counter-intuitive to me–wouldn’t a better u-value mean warmer glass and less likelihood of condensation? I’d appreciate any comments on this type of glazing spec. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    You're right. It's counterintuitive, but it's true.

    In a comment posted below my article, Rating Windows for Condensation Resistance, Greg Smith / Oberon wrote, "I am curious if you have had any thoughts about a follow up blog on window condensation when using surface 4 coatings? That could make for worthwhile discussion point as well. At the GANA (Glass Association of North America) conference in June, in Minneapolis, Jim Larsen and Tracy Rogers had an interesting debate on the potential for increased condensation when using surface 4 LowE coatings. The gist was that while surface 4 coatings do lower the interior glasss temperature, what is the potential effect on condensation resistance when using them?"

    In response to this comment, I wrote, "A note to GBA readers about surface 4 coatings: Greg is referring to a relatively new type of double glazing: one that includes a hard-coat (pyrolitic) low-e coating on surface #4 (that is, the innermost surface of glass facing the interior of the house). The use of a surface 4 coating allows double-glazing to have two low-e coatings, thereby lowering the U-factor of the IGU. Somewhat paradoxically (counterintuitively?), a surface 4 coating reflects radiant heat energy back into the room, and IGUs with surface 4 coatings have a colder interior surface than conventional low-e double-glazing without the surface 4 coating. The glazing performs better, but the surface is colder. However, if you are sitting near the window stark naked, the glass will feel warmer (even though it is colder). ... IGUs with surface 4 coatings will have a slightly increased risk of condensation compared to other types of double glazing."

    We need more research on this issue to determine whether the potential for more condensation is significant or trivial. According to a recent news report on a Natural Resources Canada (NRCan) website, "NRCan is also becoming concerned about potential condensation issues when a low-e coating is placed on Surface 4 of a double glazed product. Both the ER review and the review of the Surface 4 condensation issue should take at least several months to complete and will delay the estimated implementation date of the next technical specification until late 2014 or early 2015. NRCan will update everyone as more information becomes available."

  2. gusfhb | | #2

    I have several triple pane with the i81 coating and have no significant condensation issues. My older double pane casements condense all of the time[enough that there is always a little mildew by spring]

    I have one double pane with the i81 and it does not seem to condense worse than any other.

    Taking the window screens off the casements in the winter improves the condensation issue.....

  3. GBA Editor
    Martin Holladay | | #3

    More info on Cardinal i81: Click here.

    Also, check out Cardinal Bulletin CG06 - 04/13. The bulletin notes, "When used on the # 4 glass surface in a dual pane IG unit or # 6 glass surface in a triple glazed unit it will reflect roomside radiation back into the room. The roomside pane of glass will be colder than the same glass construction that does not include the LoĒ-i89™ coating. This will result in a slightly colder roomside pane in winter conditions with the potential of having a higher probability of roomside glass condensation in the winter. This is very dependent on the outdoor ambient temperature, indoor room temperature, and the % RH in the room."

  4. davidmeiland | | #4

    This topic made for some good scientific pondering. I think I understand *why* a surface 4 coating leads to colder glass in spite of a higher U-factor.

    I talked to the salesman about it a bit more, and he offered me triple-glazed for the same price, so that's what we're doing.

  5. oberon | | #5

    Weird, ain't it?

    As Martin pointed out, while the glass surface temperature is colder with a surface 4 coating, if you take all your clothes off and sit next to the glass it feels warmer to you. The simplest explanation is that the coating is reflecting heat back into the room rather than that heat being absorbed by the glass. When the glass absorbs room heat, glass temperature rises. When the surface 4 coating reflects heat back into the room, air temperature rises.

    As a general rule, in a home with dual pane windows, condensation is typically much more common along the edge where the glass meets the sash, especially the bottom edge of the glass. This is the typical "cold spot" on most windows.

    Other than in high humidity situations, condensation is not particularly common over the entire glass pane unless it is really cold outside, and in those locations a triple pane window is a very real option to get maximum performance.

    But in less severe locations, adding a surface 4 coating is going to lower the center-of-glass temperature about 9 degrees F over an identical surface 2 coated IGU without a surface 4 coating.

    Center-of-glass (CoG) is all glass area that isn't within 2 1/2" of the edge of the glass.

    In all cases, edge-of-glass (EoG) is going to be lower than is the CoG temperature. And while surface 4 coatings lower CoG about 9 degrees F, they only lower EoG about 3 degrees F, so the effect of the surface 4 coating is less in the area where window condensation is more likely to occur.

    Obviously gven that information, when using a surface 4 coating a low thermally conductive spacer system becomes even more critical than in a more conventional IGU, at least where condensation is concerned. Basically any spacer other than aluminum is considered to be low thermal conductance, and relatively few IGU's are being made with aluminum these days, probably less than 5% give or take a few.

    One last comment. In David's original question, he mentioned Cardinal's I81 coating. In Martin's second reply: Cardinal Technical Bulletin CG06 - 04/13 refers to I89 rather than I81.

    I89 is Cardinal's newest coating; eventually to be replacing I81. Like I81 before it, I89 is being produced and marketed to be used as a surface 4, U-value enhancement coating, but what is also being discussed in some circles is that I89 may be arguably the best High Solar Heat Gain LowE coating available when used in a stand-alone surface 3 application.

    Case in point for comparison:

    clear glass/clear glass dual pane IGU has a Visible Transmittance of 82.4, U-value .452, and SHGC .778.

    LoE-180 on IG surface 3 results in VT 79.3, U - .255, SHGC .686.

    I89 on surface 3 has VT 80.4, U-.285, SHGC .746.

    It will be interesting to see if I89 will be offered (or even available if requested) as a stand-alone surface 3 coating for high solar heat gain applications.



  6. user-4243359 | | #6

    I'm hoping to revive this older post on i89 Low E coatings now that these glazings have been on the market for a few years. Can folks share please their experiences? For an upcharge of about $40 per unit, the dramatically better U-value seems a good investment, provided the downsides (condensation, durability) aren't significant. Thanks.

  7. Expert Member
    Dana Dorsett | | #7

    I think Greg Smith was arrested for sitting in front of his warmer-feeling windows without any clothes on, but don't have any details on that. :-)

  8. user-6504396 | | #8

    I am thinking of using Cardinal 272 on surface #2 and i89 on surface #4 for our new house window package. I found this link with test data from Cardinal. 272 plus i89 reduces the surface temperature of the inward facing glass from 56F to 47F (not sure of outside temp used) and raised the Mean Radiant Temperature from 65F to 68F. Center of glass U factor improved from 0.30 to 0.23 with air fill. i89 MRT performance beats even the triple pain windows without i89.

  9. Expert Member
    Dana Dorsett | | #9

    Kevin: Take a look at p22. There is a table of the indoor RH above which there is center of glass condensation at three different outdoor temperatures over a wide range of glazing options, pane separation etc. At the bottom of the page in note #3 it specifies:

    "3) %RH: Percent relative humidity at indoor temperature of 70° F (21° C). Maximum indoor relative humidity before condensation starts to appear"

    For LoE-272(#2) + L0E-i89® (#4) with 9.8mm separation condensation isn't an issue at 0F outdoors unless the interior RH is north of 42%. That's actually a fairly high RH to experiences at 0F outdoors in houses with mechanical ventilation.

    The same glass at -20F becomes condensing when it's north of 34%. That's still a very human-healthy an comfortable interior RH, and easy to control to by ventilation rate.

    Working backwards using psychrometric calculators, 34% RH @ 70F corresponds to a surface temperature of 40F, while 42% RH @ 70F corresponds to a surface temp of 46F.

    So with a house maintained at 35% RH, south of the cool half of zone 6 condensation events of any duration would be pretty rare, only during the worst cold snaps, and not necessarily every year. In zone 7 it would happen several times per year, with possibly copious window condensation during sustained cold snaps.

    If the house is kept at 40% indoors even during the coldest weather (not recommended for wood-sheathed framed houses) condensation events, even copious condensation is likely to occur multiple times every year even in zone 5, or even the colder edges of zone 4.

    For most people it's not really an issue south of US climate zone 6, and even in zone 6 it's easy enough to control the indoor RH with the ventilation rates to keep it from becoming a problem. In zone 7 or colder it's worth considering triple-panes.

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