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Community and Q&A

Failed double glass units…moving forward?

Coyotefred | Posted in Green Building Techniques on


I have a large sunspace/sunroom that I’m renovating that was originally built in the late 70s. Basically the design is 3 walls of vertically-mounted glazing–specifically 24 34″ x 76″ double-glass factory-sealed units. We needed to re-do the framing, so we removed all 24 back in April. 16 of the 24 were still good; the rest had lost their seal over the years.

Yesterday I started to do a final cleaning/inspection of the 16 “good” ones and found that all but one had lost their seal too. I’m guessing that the reason is that a few weeks ago my contractor moved the 16 “good” ones from a shaded area to the east side of my house while he was doing some other construction work. From what I’ve read, these insulated units should never be stored in the sun. I’m guessing they heated up and this ruined their seal.

So now I’m looking at another $2500 to replace these additional 16 failed glass units. But I’m also stepping back at this point and wondering about the durability/longevity of these factory-sealed units in general. I mean I get the idea of not leaving a stack a windows in the sun, but at the same time…it’s a *window* for a *sunroom*, so I would have assumed it could handle some of this (it wasn’t a particularly hot time of year or a location that would get more than the morning sun)…

I’ve read about “site built” double-windows where two panes are used; the inner pane is sealed tightly from room air, the outer pane is “vented” to the outside. This sounds like an intriguing way to re-use all of these failed units I have, but the installation sounds a little tricky. And I guess I’m unclear how I could expect a different result than the foggy-inside failed units I have right now.

I suppose the easiest thing is to just ‘bite the bullet’ and spend the money to replace all these units…but any thoughts/advice appreciated!

PS my location is 69341 (western Nebraska), USDA zone 4-5

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  1. Expert Member
    Dana Dorsett | | #1

    People have been able to successfully drill tiny holes in the exterior pane of double-pane windows, which eliminates most fogging issues, and they seem to work just fine for years/decades that way. As I understand it, this is using very tiny diamond drills and multiple holes, not a typical DIY operation. But if you want to give it a shot, here is one online instructional piece using a somewhat larger hole (I haven't personally verified that this method works):

    If these are clear-glass (not low-E) units the performance difference with the tiny vent holes is effectively nil.

  2. GBA Editor
    Martin Holladay | | #2

    Dana is honest about the hole-drilling trick -- noting that he hasn't verified that the method works. Count me a skeptic. I don't think it's worth the time and trouble of experimenting -- especially since you will have to dismantle the installed glazing in your sunroom if the trick fails.

    The edge seals used by glazing manufacturers these days are improvements over the seals used in the 1970s. Your old IGUs lasted almost 40 years; the ones you install this summer should last 50 or 60, easily.

  3. Coyotefred | | #3

    Thanks Dana and Martin. Martin I think the way you put it is the proper perspective. It's frustrating to have to replace all the glass units rather than just a few, but as you say getting 40 years out of the old ones is maybe the best one could expect.

    This does give me the opportunity to choose the width, particularly since we are re-doing the stops and trim as we install them. The originals were 5/8" thick 1ith 1/8" thick glass. The price from my glass shop is nearly the same for 5/8", 3/4" and 1", as well as going with 3/16 rather than 1/8". However, the 1" thick 3/16 at the glass shop was *very* heavy to move so I worry a little about installation compared to 1/8" thick glass. From what little I've read there is little insulation difference between 5/8 and 3/4 or 1", but perhaps this is mistaken. Any other advantages/disadvantages I should consider since I'm ordering new?

  4. Expert Member
    Dana Dorsett | | #4

    Is the sunroom space going to be actively heated (or air conditioned)?

  5. Coyotefred | | #5

    Hi Dana,

    I'm attaching a PDF with several photos of the space that will give you a better idea of what I'm working with. Right now I don't have any plans for any active heating or air conditioning with the exception of the through-roof ventilation fan you'll see in the photo. I'm hoping the 4 sliding patio doors north and south should provide some decent air-flow to cool things off in the summer.

    I'm mainly interested in its winter potential. Last winter--even with several huge gaps in missing windows, it heated up nicely on cold but sunny days. Simply by opening the two patio doors that connect the main floor of the house with the sunroom, a fair amount of heat would flow in. I'm considering some kind of thermostat-controlled vent/fan, however, that could pump heat in even if there was no one there to open doors (and turn off when needed). The floor is 5" poured concrete, but some additional thermal mass wouldn't hurt either....I may install some tiles or brick or something on top of that...

  6. [email protected] | | #6


    As Martin noted, IG sealing systems today are a long way from systems used in the 70's. No comparison at all. Today's IG seals will last well beyond the 40 years that you saw from the original windows in your sunroom.

    Since you're buying new, I am going to strongly recommend that you look into using a high solar gain LowE coating on surface 3 of the new IGU's, particularly if the room is going to be mechanically heated.

    The improvement in thermal performance should make the room more comfortable during most of the year than it would be with uncoated glass, and it will also probably save you a little bit of money as well if you are planning to heat the room.

    Also, 34x76 is a very common size for patio door IG's. Possibly the original IG's in the room were once either intended for, or salvaged from, patio doors - maybe, if you want to consider staying with the same size IGU, you can find a good deal on them, almost certainly better than with a custom size unit.

    And if it turned out that the original IGU's were actually patio door units then attempting to drill holes in the glass might have made for a really interesting experience, and not so much in a good way.

    Per weight difference, 1/8" glass in a 34x76 IGU is going to be about 58lbs each while the 3/16" is going to be pushing 86lbs each. Not an insubstantial difference when you are lugging and lifting.

  7. Coyotefred | | #7

    Hmmmm....doesn't look like my attachment attached. This link should automatically download the PDF with my pictures:

  8. Expert Member
    Dana Dorsett | | #8

    With winter gain potential in mind, clear glass double panes have a net wintertime heat loss. But if you go with a U0.35-ish high solar gain low-E glazing there is a net gain. (In some ways Canadian rating standards for windows are a bit more useful in this regard.) Even though the low-E surfaces cut into the solar heat gained, the reduction in loss more than makes up for it.

    If you were actively heating the space to a particular temp an even lower U-factor might be more appropriate, but it sounds like you're really looking at just maximizing the wintertime net heat gain, and don't mind if there are substantial temperature swings in the sun room overnight.

  9. Coyotefred | | #9

    "Since you're buying new, I am going to strongly recommend that you look into using a high solar gain LowE coating on surface 3 of the new IGU's, particularly if the room is going to be mechanically heated..."

    Greg, I'm afraid you've lost me. What is "surface 3" refer to?

  10. Coyotefred | | #10

    'Any thoughts on whether I'd gain much with 3/4" or 1" thick units rather than 5/8" like were originally installed?

  11. GBA Editor
    Martin Holladay | | #11

    Glazing manufacturers number the various surfaces of an IGU from the outside in. If you are talking about double glazing:

    Surface #1 is the outer surface of the exterior pane of glass.
    Surface #2 is the inner surface of the exterior pane of glass.
    Surface #3 is the outer surface of the interior pane of glass.
    Surface #4 is the inner surface of the interior pane of glass.

    For more information on low-e glazing, see All About Glazing Options.

  12. [email protected] | | #12


    There's not a significant difference in energy performance between 5/8" and 3/4" or 1" IG width, you would improve about U .02 with a with a 3/4" and about U .01 with a 1" overall IGU when compared with the 5/8" unit.

    Adding Cardinals LoE-180 high solar gain coating plus argon fill would improve performance from R2 for the clear IG's to about R3.2 with coating and gas.

    Solar heat gain for the clear dual pane (regardless of overall width) would be about 78% while the IGU with 180 would be about 66%.

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