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Insulated Glass spacer blocks


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.

3/25/15       #2: Insulated Glass spacer blocks ...


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?

3/26/15       #3: Insulated Glass spacer blocks ...
Joe Calhoon

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.

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.


3/26/15       #4: Insulated Glass spacer blocks ...

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.
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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3/26/15       #5: Insulated Glass spacer blocks ...
Joe Calhoon

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.

View higher quality, full size image (1632 X 1224)

3/27/15       #7: Insulated Glass spacer blocks ...
Mark Ledvina


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.

3/28/15       #8: Insulated Glass spacer blocks ...
David R Sochar Member


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.”

3/28/15       #9: Insulated Glass spacer blocks ...
Joe Calhoon

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

4/1/15       #10: Insulated Glass spacer blocks ...
Mark Ledvina

Thank you Joe for your comments. I will address them as best as I can in this post:

Shipping: GMG does not intend to make any money on shipping. Our goal is to become psychologically as close to our customers as possible. We therefore have attached ourselves to a good freight line, run all our business through them, receive a substantial discount, and pass that discount on to our customers. We package and crate our product very carefully and do not have many instances of breakage. It is a challenge to come up with economical, "bomb proof", packaging solutions. We prefer to make our income off of our product rather than its transportation. Longer distances take a couple more days to get delivery, but we try to keep our production prompt to insure best possible ship dates. Your distance from us in Colorado, Joe, is actually not so much of a problem as your altitude is. (which is solved by installation of breather tubes) Anyway, we ship all over the states: Texas, Mid-west, Washington, East Coast states, and have sent some product into Canada.

Bedding: As I indicated earlier, Neutral Cure silicone seems to be the best with small IG units. It is always important to consider the "critical mass" point I made in my earlier post. Butyl glazing tapes are available and effective, but still require a "cap bead" of preferably silicone. Once you are used to it, bedding with neutral cure silicone in a manner that allows a consistent "squeeze out" that gets cut off after it begins to cure, leaves a clean, consistent look. That silicone may not be able to be painted, but it can be bridged with paint or chosen to be a color close to that slated for sash paint. Weep holes seem to require extra wood to put them into and contribute to a more clunky-funky sort of look. If they do their job long enough, for the most part, they will eventually be filled with paint. However, in large IG glazing, weep holes are just about essential.

Glazing Putty: It can be done on dual seal silicone/pib IG units without causing unit failure. I have done several larger jobs using them without hearing of any problems. It is important to seal these units with silicone into the glazing cavity on both sides, taking particular care to make a continuous seal of the putty side of the unit to the glazing bar. Silicone is impervious to oils, but can/does allow solvents to migrate directly through it. Solvents then can attack the pib part of the seal, causing unit failure. If you use glazing putty on IG units, be very careful. Keep the solvents from contacting the sealant.

Stops, outside or inside: Everything I have seen has shown less deterioration of the exterior of the sash if the exterior stop is integrated into the basic sash; and profiled, applied stops are used to the interior. In historic work, an exterior stop has to be as small as possible, usually mimicking putty. Small applied pieces of wood often seem to come loose, especially if paint maintenance is delayed or neglected.

Glass Pocket: I do not exactly know which direction we are talking when discussing the depth. When we make insulated glass, the overall thickness of the unit depends upon thickness of glass and type of spacer and intention. Single strength IG at its thinnest can be 3/8" thick. If the glass or spacer thickness changes the overall thickness changes. In the plane of the IG unit, we (not everyone) call the sightline the measurement that the sealant band around the perimeter takes up and must be hidden behind the stops. Our regular narrow sightline is 5/16". We can go to 9/32" in small IG. We recommend using as large a sightline as possible. (Of course within reason.) Larger units require a larger sightline.

One more point I think important is that if sloppily made IG is used in otherwise very nice millwork, the customer will never see the excellence of the joinery or the beauty of the wood. If the glass is well made, the customer will never see the glass and judge the product on the merits of the millwork. I think this is important.

I hope this answers some of your questions. Feel free to ask more if you desire. It may take me a little while to post, but you are always welcome to call me. I will tell you what I think I know. Mark

4/2/15       #11: Insulated Glass spacer blocks ...
Joe Calhoon

Hi Mark,
It was great to talk with you yesterday. This post is timely. See attached picture of a job to price that just showed up. These have ¾” wide bars. With Green Mt glass I should be able to do 7/8” or so. And they can supply wavy glass in the IGU.

I think the shipping will not be a problem especially with the smaller sizes involved in historic work. Altitude is a major problem. Shop at 7500 feet and most jobs at 8 to 10 thousand foot elevation. We know breather tubes well. This has kept us from using gas fill in our passive house type work. In Germany they have an interesting way to deal with this. They compress the IG units at the factory before sealing and shipping to high altitude. I have no idea of the process to do this.

I agree about the weep holes, a must for large units but I think your method for small TDL is good.

Good to know about the glazing putty. Will be more historically accurate.

Stops inside or out? This is as big an argument as SDL vs. TDL. The UK, Northern European coastal countries and Australia go glass beads on the outside. Most of the US and continental Europe go bead inside. Myself, I am a bead inside guy.
I think glass bead outside comes from the historical glazing putty outside and profile inside. One good argument for this is in coastal areas in driving rain stop outside will provide a built in dam to help keep water out. The biggest problem with stop outside is water getting under the bottom stop and causing rot. The Danes developed an aluminum piece to replace the bottom bead to prevent this.

Glass pocket depth I am referring the distance in from stile edge. Not the depth from the face of stile. Normal pocket depth for IG has been about ½” for a long time. As we all know this does not work well for TDL. In Europe pocket depths are increasing for energy efficiency and to block the sunlight away from the spacers. See attached picture of passive house glass pocket with heat mirror IG. Pocket depth in this system is 21mm (13/16). Good for a lot of reasons but hard on the eyes in some cases. As I said IG brings many compromises.

Mark thanks again for all the information!

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11/1/16       #12: Insulated Glass spacer blocks ...
Dave Member

I know this is an old thread but I have a related question not covered in topic. I'm in process of doing some fixed 15 lite sash.
True divided lite, insulated glass, traditional m/t joinery in redwood. Applied stops to inside. Paint outside, clear varnish inside.

Typically I have been using Dow corning 795- neutral cure for this type of application. Planned on painting/ finish prior to glazing.

Question: What is recommendation, best practice on wood prep prior to bedding glass. Prime the glass rabbet ? Prime/paint rabbet? Leave raw?

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