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Community and Q&A

Visible Light Transmittance Through Residential Windows – How Low is Too Low???

gordy_b | Posted in General Questions on

The 2x-pane insulated glass units (IGUs) from the ’60s in the true divided lite living-room window (attached) are being replaced with 2x-pane IGUs which will have one or two coatings (zone 5a).  These IGUs will necessarily be at least a bit darker than the originals.

But, from the inside we don’t want the glass and the view of the outside to be dark, like you see in some offices.  You know, where it looks like the glass was specified only for thermal purposes, with no thought to representing outside brightness and colors well.

We are getting small samples with different coatings on them.  But, I feel the darkness might look different when scaled up to the full 25-pane window…

So, subjective advice needed…  How low can Visible Light Transmittance be before the average person walks into a room with a large window and says “Huh, that window is dark”???


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  1. Expert Member
    BILL WICHERS | | #1

    The best way to determine what is acceptable TO YOU is to go to a showroom and look through various LoE coatings and see what is "too low" for you :-)

    I personally don't like LoE 366 (and equivalents from other manufacturers), because it's too dark and it noticeably tints things a bit greenish to my eye. I like LoE 180, and I like triple pane IGUs where the U factor isn't impacted as much by the specifics of the coatings since you have two insulating air spaces to help out too. LoE 272 is an in-between, part way between 180 and 366 in terms of performance, but it might be a happy medium in terms of visual light transmission.

    There are a few other LoE variants that are less common, and there is also the i89 interior side coating that can help boost performance a bit. I personally prefer LoE 180 with i89 on a double (or better yet, triple) pane IGU, but I'm on the northern edge of CZ5 on a heavily wooded lot, and I have a lot of North-facing windows, so I'm almost entirely concerned with U factor for winter heating performance. Solar gain is much, much less of an issue for me, and the higher numbered coatings are mostly geared towards SHGC. Your application might be different if you have more need for better SHGC numbers to help keep your cooling costs down.

    One thing you do want to avoid is mixing different LoE coated windows in the same visual area, which would mean on the same room or on the same side of the house. There is a very noticeable difference between, for example, LoE 180 and LoE 366 coated windows if they are installed right next to each other and it will look weird. If you use them on opposite sides of the house, you'll not notice the difference, unless you're on a corner room. In places where you can see both at the same time, the 366 window will look much darker than the other, and will have a noticeable colored tint to it. If all your windows are the same, you don't really notice the darkness and the tint as much.


  2. gordy_b | | #2

    Thanks Bill!

    I've been all over Cardinal's site and haven't seen anything about showrooms. I did find a Chicago window-maker near me that uses certain Cardinal products and has a showroom. I'll check this out.

    Good info about 366. I was tempted by its lower U and great UV reduction. But, one of the people in the mix is an artist who wants a roughly true view of the brightness and color outside - "dark and noticeably tints things a bit greenish" likely isn't going to cut it. Probably looking at 270 or 272. These are shades that will be acceptable all the way around the house too which, as you point out, is important so the diffs don't stand out.

    I'm looking into room-side i89, which apparently does not darken the window or alter the colors from the outside. --------------IF SOMEONE KNOWS DIFFERENTLY, please let me know as the window-maker whose showroom I will be going to does not carry i89.---------------------


  3. [email protected] | | #3

    Since you visited the Cardinal site, you know that the 66 in 366 is percent VLT, and so on with 180 (80%), 270, 272.... When you say you are looking at one coating or two are you talking 180 (single silver layer) versus 270 or 272 (dual silver layer), or do you mean something else?

    180 is indistinguishable from clear glass, you won't know it's there from just looking.
    270 and 272 are color neutral, and in reasonably bright light you won't be able to see them.
    366, like all triple layer coatings with similar VLT, does have a slight greenish tint that some people find objectionable. Actually 366 is less green than the other comparable coatings from other manufacturers, but you can see it in the right conditions.

    i89 on surface 4 will lower condensation resistance of the IGU, but it will still be higher than for clear glass. You can't see i89, not color, tint, or darkening.

    The VLT listed for the coatings is when used in an IGU, so it does take into account both lites as well as the coating. By comparison a clear glass dual pane has a VLT of 82% and a clear triple 75%.

    Cardinal has no show rooms because they sell glass and/or IGU's to window manufacturers, or else coated or other value added glass occasionally to small IGU or other specialty shops., not to homeowners.

  4. gordy_b | | #4


    Thanks - "... 366 is less green... but... can see it in the right conditions" keeps it in the running. 452+ and 340 are definitely out based on the darkening seen in the Cardinal catalogue.

    270 and 272 were still in the running based on the shading we saw in the catalogue but it's good to know they are color neutral.

    i89 and 180 being clear is great - no need to nix them for aesthetic reasons.

    So we'll check out the 366 and 270 at the showroom (no one I've called carries 272).

    I think that takes care of color. So I'm going to start up the original "how to make this the close to the best divided lite it can be" rehab post which was more technical in nature.

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