I'd be surprised if there's anyone in the country who has given this topic more thought in the last 20 years than I have. That's primarily because I learned long ago how important is the distinction betweenTrue Divided and Simulated Divided to my clients. If you have muntins in your existing windows and need a window replacement or are trying to match the originals for an addition, you're vitally concerned with how they look.
In preparation for this page, I read everything other companies and sources had to say about the topic on the first page of results of the Google search for True Divided Lites. And, honestly I'm disappointed in the lack of detail. There is not a great deal of information specifically germane to historic properties. One of the greatest shortcomings I found was that all the sources leads one to believe that one has the option of utilizingInsulated Glass AND True Divided Lites. While technically, this can be done,the reality is that True Divided Lite in standard historical muntin widths andInsulated Glass are mutually exclusive. Let's take a look and see why this is….
This image will be a pretty close, if not identical, illustration of your original muntins . The two most important dimensions in the illustration are, a) the overall thickness of 3/4”, and b) the glazing rebate shoulder which is usually either 3/16” or ¼”. If you do the math and look at the other side,after leaving 1/16” for glass expansion, movement and fit, the glass only rests on a shoulder/shelf of 1/8” – 3/16”.
As you can see, the space on typical Insulated glass units,the glass spacer and sealant (back fill) take up between 7/16” and ½”. When you try to put that onto the 3/16” shoulder, at least ¼” of the spacer is exposed(not buried in the glazing rebate).
So, since under the most optimistic scenario, a ¾” muntin with a ¼” divider and a 1/16” of expansion on both sides leave 3/8” of shoulder to be divided by the two IG units for a spacer allowance of only 3/16”. Get out your ruler and look at 3/16”. Would you be comfortable with a house full of insulated glass windows with only 3/16” of sealant between you and catastrophic glass failure?
The only option at that point is making the muntin wide enough to accommodate the amount of sealant you're comfortable with. Thus,other companies have a minimum of 1 1/8” muntin width! So, we must ask ourselves, is having True Divided Lite, which is only noticeable from close range, worth necessitating a muntin so much wider than the original that you can see if from the street? Of course not! But don't despair, we're having this discussion for a reason. But before we continue, let's revisit the image above for another critical topic.
Please note in image one, the thickness of the sash is 13/8”; probably what you now have or had. But look also at image three. It's essentially an historic styled muntin (which is the same as your sash molding profile) with a big hunk of Insulated Glass stuck in it. Even if the muntin was wide enough to cover the spacers, it's still 1/2” thicker than your original sash and two of those will not fit into most residential jambs that have only 3¼” space for sash and stops. How do other companies overcome this dilemma? They either have no exterior return to the glass or they cheat on the interior profile. In either case the deep rich shadow lines that characterize historic windows are eliminated. Rather than using a full 7/8” of return to the glass on the interior. They shrink the molding profile and return down to 3/8” – ½”,which seriously compromises the historic appearance and cheapens the look. Heirloom windows provides the only solution for both issues we've discussed.
We have several links to VIG on this site and I may have spoken with you about it already so let's just say it's an Insulated Glass technology that allows a unit to be only ONE QUARTER OF AN INCH THICK yet provides around 50% better thermal efficiency as a ¾” standard IG unit. Now let's see how that changes EVERYTHING!
Now, even VIG must have a sealant (not spacer) to connect the two piece of 1/8” glass with only .2mm between. This seal is a glass WELD. Unfortunately, the VIG seal is still too wide to use with a ¾” muntin. However, the VIG is so thin that when applied with our proprietary Simulated Divided Lite technology, the result is a unit that is absolutely indistinguishable fromTrue Divided Lite.
Not only does this arrangement afford the perfect solution for the TDL/SDL challenge but it also allows the Heirloom Ultima to maintain the original molding profile on the interior as well as some good depth on the exterior for the shadow lines and depth we appreciate.
The Heirloom Simulated Divided Lite technology is just aseffective with monolithic glass as with VIG, maybe more so. Take a look…