Simulated Divided-Light Door Details

How to apply false windowpane dividers to large panes of insulated glass. June 26, 2009

I notice that a number of high end door manufacturers in my area are using false mullions on the exterior side of the door. I can see the advantage as the price of small individual thermopanes is enormous. How are they attaching the mullions? Are the mullions attached to the frame and floating on top of the glass or are they adhered to the glass as well?

Forum Responses
(Architectural Woodworking Forum)
From contributor C:
I wouldn't exactly call that a high end door.

From contributor T:
To answer your question, they are not adhered to the glass. They are snapped in place. Different manufacturers handle it differently. One of their sales pitches is the ease of cleaning - simply remove the insert, clean the glass, and re-install it. They can rattle with the wind and movement sometimes, and that can be annoying. In my opinion, the high-end boys still use true divided light construction.

From contributor B:
I disagree with the assumption that simulated divided light construction cannot be considered high end. The traditional width of muntins on older work is usually not more then 7/8" and typically as narrow as 3/4 " or less/ This becomes problematic with individual true divided lights in thermopane as the spacer even in a narrow site line is about 3/8". This times two and wiggle room plus the center portion of the muntins add up pretty quick/ usually at least 1 1/8 or better.

From contributor R:
A lot of manufacturers will use on sheet of insulated glass with the aluminum bead laid out in the muntin pattern and overlay with the narrow 7/8" profile on both sides. As contributor B said they would either snap in for easy cleaning but that leaves a somewhat noticeable reveal or added piece of moulding around the glass. In higher end versions the muntins are permanently mounted with either adhesive or two sided tape and coped into the stiles and rails creating the illusion of a true divided light. The better ones are pretty difficult to tell the difference. Those are usually found in the higher end doors and are considerably less expensive than having individual insulated panes.

From contributor D:
SDL vs. TDL. Mostly it's an architectural question that has been brought mainstream for a variety of issues. Steel and fiberglass doors have no real choice outside of SDL and window manufacturers have been using it for years now. Architectural correctness is only part of it for wood door guys. Historically architecturally accurate muntin bars on single glazed doors were about 7/8" thick, but the wood door guys on the west coast made doors with 1-1/4" bars. There is more history about the why on that, but suffice it say, it was done this way for a reason I won't go into here. Then in the mid 70's TDL doors began to use 1/2" IG to fit the1-1/4" muntin bars being used. Energy efficiency requires thicker IG units and when you go to the thicker glass then the spacers become a problem and they project into the daylight openings of the individual lights. So the trend has been to move to 1-3/8 or heavier bars to cover these spacer situations.

Another part of the problem is that the smaller IG units have a higher failure rate per 1,000 units. If you've ever enjoyed the pleasure of replacing one of these failed units, then you can appreciate those who prefer to use a larger IG unit and apply the muntin bars. Now, in the bad old days, it was done with a removable grid. That grille usually lasted through a couple removals, and then if it wasn't broken, the pins holding the grid in place usually lost the ability to secure the grille to the panel. So it in turn went away. That's why the steel and fiberglass guys use a surround with the light pattern built into it.

The new SDL bars generally use a tape and must be applied in a specific manner. Also, these bars are not generally recommended for jobsite applications. They are best installed in controlled circumstances. The bars are not removable once applied. You can remove the bars to reset them, maybe, if you get to it immediately after application, but in general, once applied, they are permanent and you might as well buy another door, or at least a new glass unit. The adhesives will begin to cure within a few minutes and within 48-72 hours will be fully cured and will be permanent. The Grille Between Glass (GBG) can be an excellent means of preventing the daylight from being visible at an acute angle to the daylight openings. The SDL/GBG can be as effective as a TDL for architectural correctness and can provide a greater flexibility in pattern design.