GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Does glass thickness affect VT and SHGC?

user941025 | Posted in General Questions on


I’ve ordered a fixed triple glazed window through the person who will be installing it, and it has finally arrived. However, the size that has arrived is not the size I ordered (in writing).

In addition to the issue of whether or not it’ll fit into the RO, there’s the other issue, that at widths greater than 55″ (I’d ordered 55″) the thickness changes from 3mm to 4mm (I think I have that right).

Will I see a change in performance with this thickness change? Can’t remember what the specs I’d anticipated were, but the sticker for this wrong-sized window read U 0.17 (as expected); VT .453; SHGC 0.51. I think (maybe incorrectly, maybe not) that the VT and SHGC have dropped.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user941025 | | #1


    Seems like it'll fit into the R/O, and apparently Fibertec manufactured it at 3mm so the VT and SHGC will be as expected. Both of those things are great.

    But now, my question is, if they've manufactured it thinner than their typical requirements for that width, will this window be more prone to breakage under adverse conditions?

    ...and of course I am actually still curious if the thickness will affect VT and SHGC.

  2. GBA Editor
    Martin Holladay | | #2

    Yes, the thicker the glass, the lower the VT and SHGC. Have you ever seen old-fashioned bullet-proof glass that is 1 inch thick? It certainly cuts down on the VT.

    But I imagine your question is really, "How much difference is there between 3 mm glass and 4 mm glass?" I don't know the answer to that question.

  3. gusfhb | | #3

    I have windows with two 1/4 panes and 2 coatings with VT of 73 percent and SHG of .67. I think 'not much' is the answer

    I am wondering if the change in numbers represents your window vs a theoretical window. IOW if your purchased window was one foot square the VT for the unit would drop while the glass never changed.

    Maybe the manufacturers don't do that math, I dunno

  4. user-626934 | | #4

    Glass thickness will also effect U-value slightly, since the spacing between the panes (gas fill) is changed.

    Also, side note on the window sticker - most manufacturers do not adjust the performance values on the window sticker for different sizes of the same type of window, let alone for different glass thicknesses.

  5. [email protected] | | #5

    For a dual pane with a high solar gain LowE coating, changing from 3mm to 4mm (each lite) will drop SHGC .01 - .02 and drop VT about 1%


  6. user941025 | | #6

    Thanks. It's triple pane, but I'll still assume it's minor, then.

  7. user941025 | | #7

    Quick install question for this wrong-sized window. (Note: it's fiberglass, so it should be dimensionally stable, it seems)

    Is there enough room for installers to get a tip in to foam these gaps?
    R/O is 56-5/8w x 43-7/8h.
    window is 56 x 43.
    At the sill, the beveled slope still needs to get dropped in, so we lose a little there, too.

    Pretty tight. The install is to the center of two layers of exterior polyiso, which is helpful, but I still see room for access trouble, and an air gap is an air gap.

    Foamable? Unlikely? Not an issue?

  8. GBA Editor
    Martin Holladay | | #8

    If everything is plumb, level, and square -- a big if -- then the gap is caulkable and will work.

    However, if Fibertec screwed up your order, they owe you a window with the correct dimensions.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |