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Can U values be trusted? VLT values also?

DirkGently | Posted in General Questions on

I am comparing windows from big company brands M and P. I have been astounded to learn that vinyl frames with foam have better U values than wood windows. For upcoming project in zone 6 NH I would like to get tripple pane casements/fixed/awning and unfortunatelly a few single hungs or sliding.
Once again i am astounded by the slight differece between double glass and tripple glass…..i thought it would be much greater. Some types/brands only offer tripple in certain styles.

Since i will likely end up with a mix of double pane and tripple pane next to each other…..will i have a noticeable VLT from the different glass types? Example: a pair of eye glasses with 1 side  shaded and other clear.

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  1. Expert Member
    BILL WICHERS | | #1

    The usual reason why triple pane windows don't show as much improvement in U value as expected is because the window manufacturer is offering 7/8" triple pane IGUs so that they can fit in a minimally modified frame that would normally have a double pane IGU. You don't get much benefit from triple pane windows when they're only 7/8". For maximum benefit, you want a full 1-3/8" triple pane IGU. You WILL see a difference going up to a full width triple pane IGU like that!

    Visual transmission between the two windows will depend more on the LoE coatings chosen than it will on the number of panes. If you have the same coatings on both, the triple pane will look slightly darker, and it will be noticeable but it won't be as severe as having one clear lens and one tinted sunglass-style lense in your glasses. The difference will be closer to looking like the triple pane window is a little dirty is all.

    I would recommend avoiding having two different window types next to each other, or on opposite sides of a corner -- anywhere where both windows would be visible at the same time. If you keep the different windows seperated into different rooms or other areas that are not simultaneously visible, then you won't notice the differences between them from the inside. From the outside, the differences may still be visible, but they won't be as noticeable.


  2. Expert Member
    PETER G ENGLE PE | | #2

    Yes, vinyl framed windows with foam can have better performance than wood windows. The U-value for the window is an overall measurement, and if there is an NFRC compliance sticker on the window, it should have been tested under standardized conditions. So yes, you can trust the U-values, at least for comparing one window to another. YMMV based on a number of factors that can deviate from the test conditions in the real world.

    The answer to your second question is yes. You might have enough difference in VLT and color that you can notice the different types of glass in a single wall, or especially in a single room. A good rule of thumb is to at least have the same type of glass in a single wall of a single room, so no triple paned casements right next to double-paned double hungs. Take a look at the VLT values listed for mixed windows. More than about 5%, you will probably notice. If possible, get samples of both and look side-by-side. Even doing this in a showroom can help.

  3. walta100 | | #3

    Understand that if you have one or more windows with a much different U value that window will have a colder surface temp.

    You can never have a dew point in the house above the temp of the coldest window. Your choice of windows is a choice about comfort you are selecting you indoor humidity level when you select your U value. Some people would be miserable at 15% other find it comfortable.

    Let’s make up numbers and say it is -20°F outdoors. Window #1 has a surface temp of 22° and #2 is 32°. If your thermostat is set for 72° house #1 will have a relative humidity below 15% and house #2 will be below 23%. Total made up numbers so you can see my point.


  4. DirkGently | | #4

    Fantastic replys, thank you. I wish i could iinsert a Homer Simpson "doh". I did not think about the glass unit thickness at all.
    I definately will not be mixing double pane and tripple panes in same wall......problem is it looks over a deck so casements are not my best choice.
    I prefer local (matthews bros) or big company suppliers as i am confident parts will be available in the future. I am willing to give up some performance for that safety net.

  5. oberon | | #5

    Even without foam fill vinyl windows are going to be more efficient than wood, assuming an equivalent glass make-up.

    Also keep in mind that VLT is based on the entire window and not just glass, so that two windows that are exactly the same size with identical glass packages would have different VLT measurements if the frame of one was wider than the frame of the other, or if one had grids and the other doesn't.

    If you are looking at triples IMO you want a U factor below .2 or it isn't worth the extra dollars compared to the most efficient duals. There are plenty of triple pane options that meet that goal.

    In NH I would be looking for a moderate solar gain coating (dual silver) for the higher VLT and SHGC values, even though the U factor might be a .01 or .02 higher than from a low solar gain (triple silver) coating.

    1. DirkGently | | #6

      My only problem with 2 silver glass is the mostly north slightly west facing windows (view of Moosilauke/Kinsmans ....fellow hikers may take note :) ). This wall is about 2/3 glass which is why i am commited to tripple pane even with moderate U increase.
      VLT is a concern and confusing due to your insight on them rated by entire frame.
      The bulk size of window frames eating up VLT of most vinyl windows are why I always disliked them. My opinion is now changing as i find narrow frame vinyls.
      Looking at Fibertec windows now. But company longevity is a concern for me. I want to be able to get window replacement parts 10 to 20 years from now.

      1. oberon | | #7

        When I mentioned dual silver and triple silver I was talking about the coating on the glass and not the number of glass lites in the IG unit. Triple silver coatings have a lower VLT and lower SHGC than do dual silver coatings.

        If your primary emphasis is VLT with energy performance a secondary consideration, then a dual silver coating would be better for your needs, but if you want to maximize energy performance then a triple with at least a <.2 U factor (even lower is better) is more than likely the right choice.

        Also. and as mentioned in previous replies, triple pane IGU will inherently have a lower VLT than will a two pane IGU, so that when considering the VTL of the coating it might become even more of a concern for someone who wants maximum VLT mixed with acceptable U factor performance. The catch 22 is that the most efficient (lowest) U factor IGU package will potentially have the lowest VLT as well.

        Imagine a 24"x48" window with a 3" frame. The total window area is 1152 sq inches but 34% or 396 sq inches of the total is frame.

        This is a triple pane window with two surfaces coated - one with a 66% VT triple silver coating and one with an 80% single triple layer coating (common configuration).

        66% of window (glass area) has a VT of 53% and 34% (frame) has a VT of zero. The calculated overall VLT of that window is 35%.

        Increase the size of the window to 48"x48" but using the same 3" frame and the same 53% glass light transmittance, the overall calculated VLT will increase to 41%.

        Now change to triple pane window with two glass surfaces coated - one with a 72% VT triple silver coating and one with an 80% single triple layer coating (again a common configuration).

        The glass VLT is now 58% and the 2x4 window will have a calculated VLT of 38% while the 4x4 window will have a calculated VLT of 44%.

        What about a triple pane with no coatings? Triple pane with clear glass with have a VLT of about 73% compared with a coated dual pane (2 layer silver) at 72%, but the dual coated will outperform the uncoated triple in both U factor and SHGC performance.

        Not trying to scare you away from triples, in fact I would very much recommend triple pane windows in your environment, but suggesting a realistic expectation of the "why" of the VLT performance values that you will be seeing as you shop around.

      2. Expert Member
        BILL WICHERS | | #8

        Use a less aggressive LoE coating like Cardinal's LoE-180 with the triple pane windows to keep them from looking too dark. The more aggressive coatings will look darker, and if you're already using triple pane windows, the difference between LoE-180 and the more aggressive coatings isn't all that much in terms of U value, but it does make a big difference in VT.

        For a large amount of glass like you're talking about, be sure to get 1-3/8" full width triple pane IGUs for best performance. Its more than a moderate U value increase between double and triple pane when you are using full width triple pane IGUs.

        Check out Inline too -- they've been around a while and have been easy to work with for me in the past.


        1. oberon | | #9

          I really like 180 in most northern climate applications, and would also recommend it as well, except being west facing windows where using 180 can make a room almost uninhabitable in summer and often very uncomfortable during the shoulder months.

  6. onslow | | #10

    I wouldn't focus overly much on the VLT numbers based on my own experience. I would definitely urge you to focus on the U factors and framing materials. I also look out at mountains and for a couple of reasons chose a low SHGC of .27 which also lowers my VLT to somewhere around 50% on average. If anything, I wish I had lower VLT values for my eastern windows. My guess is that even at 40% VLT you would not notice much.

    As others have already pointed out the reported VLT is affected by total framing percentage of opening, which begs the question of why the test facilities do not also provide center of glass VLT to create less confusion. Like your mountain viewing windows, the framing percentage of my picture windows is pretty small so functional VLT might be near to 60%. This is not really very dark and during the winter months the snow reflection is still quite intense. For summer months the reduction of heat gain is well appreciated. There is certainly no perception of looking through a glass darkly.

    My north windows are also low SGHC partly for simplicity when I ordered windows, but also to make use of the lower VLT. The snow field view is still quite bright. However, if you really wanted to get the max transmission on those windows, their mostly north view might allow you to choose glazing options to maximize transmission without creating a summer heat load problem.

    It is true that if you mix windows of different VLT next to each other you can see the difference. It is also true that if you open a window, the view around the sides will also reveal how much the tinting is. I don't really see that as an issue, but to each their own. Going in and out of house at dusk is probably the only time I perceive the shift in viewing brightness.

    The value of triple or even quad windows comes from humidity points being much better managed as well as personal comfort. Sitting by my large windows at night in the dead of winter is not an issue despite some talk of window cold air flow. None of my heating is under any window. The only curtains or shades we have are in the bedroom.

    The frames are fiberglass pultrusions and seem to take the wide temperature shifts we have in stride. More so than the vinyl ones I left behind. I can't quantify the thermal properties of fiberglass frames beyond noting that the interiors do not seem cold. The window locking mechanisms seem quite firm and the air sealing is excellent.

    One last thing to consider is the percentage of heating load windows represent, especially as you pair with PGH wall R values. Run your numbers and confirm for yourself the benefits of getting .15U (or better) windows over a .22U. Your whole house energy management may benefit more from higher grade windows than piling on wall or ceiling insulation.

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