Are insulated glass spacer blocks necessary in true divided lite double hung window units with small glass openings like 7x9?
Most glass manufactures offer a 3/8 sight line after adding your spacer of 1/8 that would have your total bar at 1 1/4 with a 1/4" stem.
I have seen larger manufactures windows made without spacer blacks ( in an effort to keep munition bars smaller I'm assuming) but haven't seen the results after years of use. I'm new to IG units in true divided lite and want to make sure I have all my bases covered. I'm aware of the need for weep holes but not sure what else is necessary. Thank you.

Tyler,

If your IG units are sealed with hot-melt, then they require a spacer block to support the individual panes and stop them sliding out of alignment and breaking the seal. Hot melt will soften as the temperature rises so this could still be a problem even with your smaller units.

Also, if the unit is not installed properly the manufacturer will not stand by its warranty.

I have found it prohibitive to make small units because most manufacturers have a 3 foot minimum. Also it requires such large Muntin bars to hide the spacer and seal.

Have you thought of true simulated divided light where you apply both interior and exterior muntin bars and fill the space between the panes for a more authentic look?

Are you making your own IG units?

Tyler,
Anytime you introduce IG into a historic sash there will be compromises. We generally go SDL on these small sizes. I know that in some historic situations this is not possible. In this case we order the glass with the slimmest edges available allowing 1/16” each side and using 1/16 glazing blocks. Our IG usually grows in size from the fabricator and we do not attempt to get tighter. On these small units we do not attempt to block the corners to transfer weight like you would for large units.

I have heard the East coast guys here talk about Green Mountain Glass for narrow clean edge lines. Not sure if I have the name quite right. Larger shops doing historic work often manufacture their own IG and will not guarantee the seal because it is more exposed to sunlight.
I don’t know if glass like this from the UK is available here.

http://www.slimliteglass.co.uk/slimlite-ultra-clear-self-cleaning-double-
glazed-units/timber-windows.html

I was called to jury duty this morning in our historic courthouse. Waiting through the selection process I was admiring the large second story double hung windows with the old wavy glass and curved transoms above. Thinking about what it would cost to reproduce and how to handle new glass choices. I decided IGU would totally ruin the beauty of these windows.

Joe

Read,
I have thought of using simulated divided light Like you mention, however the market I serve doesn't allow for it. I'm not currently set up to make my own IG units although like Joe said I have heard of window manufactures that are making their own.
Joe,
Thanks for the info on green mountain I will have to check them out. I have been thinking of using a design like the attached image. This would allow for the stops to be placed on the interior. I know what you mean about IGUs ruining the beauty of the windows but for some reason I have some contractors that are pushing the idea.
Thank you both

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Tyler,
I think you are on the right track for your neighborhood. See what is availble for IG and plan the cutters accordingly.

With some of my tooling I can get to 1 1/8" bars. We did one large job with 1" bars and ended up rejecting a lot of the IG for bent spacers.

I prefer the Euro SDL where I can go 5/8" or 3/4" wide but that may not fly in your area. If they see you as a repeat customer some of the IG producers might give a break on minimum sizes.

We are going to tour 4 top UK joineries before Ligna. It will be interesting to see how they deal with balancing energy concerns and historical preservation.
Joe

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To all:
I am Mark Ledvina from Green Mountain Glass. I am very honored to be referenced in such a positive manner. My company builds IG units for the high-end custom wood window market. I also worked for a significant time in the historic window restoration market. I will be happy to give you information about whatever it is I can actually help you. I would like to respond to a couple of points that have been mentioned:

The initial question about setting blocks: they do not appear to be necessary in installation of small ( approx.1 square foot ) IG or monolithic glass. Most of the IG in historic windows and doors requires too narrow of a muntin to be able to achieve without the glass being quite tight in the opening. On rarer occasions, when there is a significantly larger space in the opening, setting blocks are helpful. They must, however, be properly installed. (Usually two on the bottom supporting both pieces of the unit's glass, each located 1/4 of the distance from the corner. It is also helpful to install them on the sides to prevent lateral movement of the glass in the glass opening. Assuming the millwork will never spend much time upside down, they are optional on the top) It is much more important to properly bed the units into their openings. A continuous bead of bedding is necessary on both surfaces. Despite the fact that it is not able to be painted, Neutral Cure Silicone is the best performing bedding that I have found. It is also a good idea to integrate a channel or groove into the glass seats of the millwork in order to maintain a critical mass of the silicone. Thinking about it, the ability of silicone to stretch is extraordinary. However, this stretching is only a percentage of the silicone's mass. When the bedding is squeezed out during the bedding process, it can get very thin. Therefore, it is important to maintain that critical mass. sooo... without getting too deep into the bedding issue, it is important to bed both sides of the glass for both support and maintenance of a continuous seal of the glazing chamber. And also, if the glazing chamber is full up tight with glass and sealant, There is no place for the possible moisture to gather. The tightness of the glass in the opening can be an issue only if the door or window is not rigid enough to support the glass. I personally have removed many pieces of unbroken glass from historic windows where it was fitted very tightly with, of all things, glass shims, and bedded in rock hard glazing putty.

Another issue I would like to touch upon is that of Simulated Divided Lights or SDL IG or monolithic glazing. There is a serious issue that is seldom addressed: that is that a single piece of glass reflects in one plane. In True Divided Lights, or TDL glazing, each unit or piece of glass reflects slightly differently. Not everyone picks up on this fact, but those of us that are used to "the real thing" can very much appreciate the difference. Along with that, by putting "all the eggs in one basket" repairs mostly need to trash the window or door if the glass or part of the relatively fragile, glue-on gridwork gets damaged. A TDL window or door also just feels so much nicer when it is handled, opened or closed. Just the feeling of substance rather than imitation. I urge everyone to always attempt to make TDL for quality and aesthetic reasons.

Feel free to call me at Green Mt. Glass if you would like to discuss anything related to glass with which I may help you. 6038264660.

Mark at Green Mountain helped us "get right with glass" years ago, and we have provided better glass - better products - for our customers ever since.

Re-read that part "...if the glazing chamber is full up tight with glass and sealant, there is no place for the possible moisture to gather". This limits failures and makes for longer life of the unit. A side benefit of this sealant on both sides, is that it prevents subsequent finishing from running down inside a dry set interior glass stop that is then visible from the outside of the product. The sealant also helps with (gawd help us!) any muntins that may not present a perfectly flat surface from piece to piece.

I will also second his caution on SDL with all the do-dads inside and outside that eventually fail, rendering the entire assembly useless, and difficult to replace.

To quote Stephen Mouzon in his book “Traditional Construction Practices." In discussing myriad reasons for divided lights, “First…the resulting play of light and shadow across the retina of the eye is physically more pleasurable than the harsher, more glaring light of an undivided window. Secondly, divided light windows are inextricably linked in the minds of almost everyone with various human-based languages of architecture. Any attempt to build in these styles with single pane windows is likely to be viewed as a cheap substitute for the real thing.”

Mark,
Thank you for posting. I wish your company were closer to Colorado. We get a few jobs where the extra freight expense would not be a deal breaker though. Nice to hear from an IG producer interested in building for high end residential. All the IG shops here are high volume commercial operations that think they are doing you a favor by making sloppy units.

Regarding the bedding. There are a few schools of thought on this. We used to do it the way you describe and had no issues other than tough to remove the glass with the entire cavity filled. When we tooled up to build tilt turn windows the German method for IG install is quite a bit different. Their sash has weep and drain slots built into the glass pocket. Typically the IG is set with glazing tape and the profiles have a notch to receive tooled silicone after the glass is installed. Before the inside bead is applied the gap between IG and sash is filled with silicon at the top to add a seal. Not the entire cavity. With the drain slots the cavity is considered the cold area of the window and gets rid of the condensation and pressure that builds up.

In the UK I understand they use butyl in the glass cavity. I am interested to see how that works. They apply stops to the outside of sash and maybe that has something to do with it. From reading the info on the Slimlight they also putty in IG units. I have been asked about this but never thought it would be a good idea for IG. What are your thoughts? That would sure make a historically accurate sash with the benefit of IG. We were just talking to an experienced painter and he mentioned glazing putty does not hold up anymore since they took lead out.

On the subject of SDL I have seen good and bad. I do prefer TDL when we can make it work. We use the European method for SDL. Not the typical fake stick on bars that are common. These end up being as much or more work than TDL and look the same in the end. And then hoping the IG producer gets the inner bars right.

Our two window systems have 13/16” and 9/16” deep glazing pockets. This translates to almost 2” and 1 3/8” minimum TDL width for these systems. So when 1” or ¾” bar width is requested it is simpler to go SDL.

What is the minimum glass pocket depth for your glass? On my historic tool set the glass pocket is 9/16” but with a little change in tooling I could make the bars 5/16 or 3/8”. Then just pull the IG farther away where it meets the sash. This would translate to about 7/8” wide bar.

Our window system with the 13/16” glass pocket is geared to the energy efficient, Passive House and PGH (Pretty Good House). With these type systems you do not want to think TDL for many reasons.

You are so right about the single glass in a SDL reflecting the same. I just looked at my new house door and sure enough I can see the difference. Not going to change it though...
I noticed my Slimlite link does not work. Try this one.

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Historic IG from the UK