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How important is thickness of window glass?

Luis Rosario | Posted in General Questions on

Greetings,

We are embarking on replacement of all of the windows in our home from the original single pane windows (’97 construction) to something more energy efficient, as well as meeting the storm codes for South FL. Although we’ve found a lot of valuable information online, something that I’ve yet to come across is what difference does it make to go with 5/16″ glass, vs. 7/16.”

Some of the estimators who have come to our home seem to really be pushing the 7/16″ as ‘better,’ while others are telling us that 5/16″ will work just fine. We’re looking to install Lo-E insulated windows.

Any guidance is greatly appreciated!

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Replies

  1. David Meiland | | #1

    5/16" vs 7/16" what? Thickness of each pane? Thickness of air space between? Typical insulated glass here would be 1/8" glass + 3/4" air space + 1/8" glass, total thickness 1". There are a number of typical glass thicknesses, some imperial and some metric, but even 5/16" would be very, very thick, and would have quite a bit of color to it. Maybe it's thicker in storm country, I dunno.

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  2. Luis Rosario | | #2

    Well, like I said earlier, I'm referring to the glass itself. With some of the estimators making a fuss, while others are not, it leads me to wonder if the ones that are making a big deal about it are doing this for legitimate reasons (relevance), or just to distract us from what's really important.

  3. Greg Smith | | #3

    Hi Luis,

    Since you are upgrading in South Florida, I am assuming that you are talking about impact rated laminated glass in your windows at those thicknesses rather than "regular" glass.

    5/16" laminated glass is probably going to be two layers of 1/8" glass sandwiching a layer of .090" plastic interlayer.

    7/16" laminated glass is probably going to be two layers of 3/16" glass sandwiching the .090" interlayer.

    I say probably because there are both slightly thicker and slightly thinner than .090" interlayers used in impact products, although those are much less common than the .090" products.

    Since the vast majority of impact products use one or another version of a .090" interlayer, I am confident that the make-ups in the previous comments were probably correct for the comaprisons you are getting.

    From an impact standpoint, there is very little difference between the thinner glass and thicker glass since it's really the interlayer that stops debris from potentially entering your home during a major storm, but depending on windload requirements where you live there could be advantages to using the thicker glass.

    In order to answer you completely, it would help to know the specific laminated glass make-up (glass plus interlayer), whether or not the estimators are suggesting heat treated glass, if they are quoting monolithic lami versus an IG unit (dual pane) with laminated glass as one one of the two panes, and finally whose windows are you considering?

    Once again, and as a general rule, there really isn't that much difference between the two products (thickensses) in most applications - but possibly, in some cases, there could be a reason to choose one over the other.

    Edit: Reading back, I noticed that you abbreviated the "low emissivity" coating as Lo-E versus Low-E which may be telling since Cardinal Glass refers to their coatings as LoE whereas everyone else uses the abbreviation Low-E for their coatings.

    With one exception, Cardinal does not laminate their coatings inside laminated glass. The exception is LoE³-366 with SentryGlass (SG) interlayer in monolithic (non-IGU) applications.

    Have any of the estimators mentioned the interlayer by name? PVB, polyvinyl butyral, SG, SGP, or SentryGlass, other?

    Regards,

    Oberon

  4. David Meiland | | #4

    Greg, are you saying that some new windows in that market might be non-IGU?

  5. Ron Johnson | | #5

    1/8" - used in storm windows
    3/16" - used for small table tops
    1/4" - used in table tops
    3/8" - used in shower

  6. Luis Rosario | | #6

    To answer some of what Greg is asking, estimators have been showing us heat treated glass. One even went as far as to say that their heat treatment method was better than other companies, but I don't recall getting the actual manufacturer's name for that one. I believe they're manufactured in FL, though. No specific info/name as far as interlayer goes. Samples we've seen have a single layer glass with the air (argon gas?) filled space in between, then the laminated glass.

    The laminated glass is the one that some have been saying 5/16" is sufficient, vs 7/16" being 'better' for other quotes. The companies we've talked to so far carry either PGT Winguard or Sentinel by CGI.

    Other than one particular estimator quoting us a price already, we've yet to receive the actual $ amount estimates for the others that came by to do measurements. We're not going for the one that's already quoted, since he seemed shady. A presentation full of 'But wait, there's more!', and trial closes (pitching a high price, then offering discounts, including getting our permission to be used as a referral, for an additional discount) which we did not care for. If you have a good product and you're a reputable company, you shouldn't need to resort to such tactics.

    Anyway, thanks for the info. Hopefully this helps clarify some more for what we've seen so far. This is foreign territory, so any additional guidance sure helps!

  7. Greg Smith | | #7

    David,

    If I recall correctly (memory), new construction in south Florida requires IGU windows to meet the new energy code - with the caveat that it is possible to build a home with energy performance trade-offs that would allow non-IG glass to be used.

    In the case of a remodel, if changing windows, and if the value of the windows is less than 30% of the value of the home, then the windows do not have to meet the requirements of the new energy code, so once again single or monolithic glazing is acceptable.

    The energy code requires SHGC of .25 or less and requires U-value of .65 or better for non-impact glass and .75 or better for impact glass.

    Reality is that the energy code in south Florida isn't very popular with some people and not everyone is strictly in agreement with enforcement....saying that yes, there is a fair amount of non-IG windows being installed in Florida (and elsewhere), both within code and not within code.

    Luis,

    Laminated glass as thin as 1/4" (~6.5mm) and as thick as 9/16" (~14mm) will meet the impact standards required by either Miami-Dade County or ASTM E1996. The commonality of both the thick lami and thin lami is the thickness of the plastic interlayer between the panes of glass is going to be very consistent...thin glass or thick, the choice of interlayer is very limited and all of them meet the same standards.

    For example 1/4" impact-resistant laminated glass is going to be fabricated with single-strength or 2.2mm glass sandwiching the same interlayer(s) as will 1/2" impact-resistant laminated glass manufactured using two lites of 5.7mm (1/4") glass to either side of the interlayer. Automotive windshields by comparison use two lites of 2.7mm annealed glass and a .030 pvb interlayer. They are also considered to be 1/4" laminated glass.

    There are advantages to using the thicker glass related to structural, or windload, or other type of pressure loading. Any potential or foreseeable advantage that you might gain by going with 7/16" versus 5/16" laminated glass would be in the area of an increase in structural loading or windload resistance - which could be required depending where you live. At best, there may be a very slight advantage in terms of impact resistance between the 7/16" and 5/16" laminated glass, but I would consider it far from a significant advantage and I would not base my decision on that difference if it were my call.

    Like the difference in glass thickness for the laminated product, you may be seeing different thicknesses of "sacrifical" glass (the non-impact glass of the dual pane IGU) offered in various windows. Some of the offerings may be annealed, some may be heat strengthened, and some may be tempered. Once again there is an advantage (and possibly a requirement) in using thicker glass (up to 1/4") or in using heat strengthened or tempered glass in terms of structural or wind loading on the windows. Often code dictates what thicknesses / heat processes may be required in your area - usually based on windzone requirements.

    You may also see some windows with the laminated glass exterior and some interior. From either a windload or an impact standard there is no real difference since the glass/windows had to pass the same testing requirement whether the laminated is inside or outside.

    There are a couple of reasons why window companies place the lami inboard or outboard in the IGU, and typically they relate to the structure of the window sash/frame combination as well as location of Low-E coatings. Some also state that they prefer the lami inboard to keep potential broken glass from the sacrifical lite outside the home. Something that does make some sense to me, but would be far from a deal breaker in either case.

    In your environment, the advantage of argon between the lites of an IGU may not justify the expense unless it's a really low cost option. If you want it, then go for it, if you don't feel the need, then don't.

    Since you are talking IGU or dual pane windows, my earlier thoughts about Cardinal's laminated glass and enclosed LoE coating no longer directly applies. Interlayer is pretty much a non-issue for the consumer since no matter the interlayer (or glass thickness), they all passed the same testing standards.
    .

  8. Luis Rosario | | #8

    Outstanding detail! Thank you very much for the guidance as it really helps us see that perhaps some estimators' heavy emphasis on the thickness of the glass may have been no more than a deterrent to the overall picture, as far as our total investment is concerned. One of the contractor's presentation didn't feel like he was trying to shove that 7/16" down our throats and overall he seemed much more 'user friendly,' if you will, than a few of the others (especially that shady dude), but we just wanted to make sure.

    We had already seen info regarding the SHGC and U-value on the Dept. of Energy website and other related sources, but were intrigued when we found no information about the various thickness in the glass being used.

    Thanks again!

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