You know what question I get asked most often? «Artem, tell us about glass units». People understand it’s important, but completely don’t understand how to choose correctly. As a result, they either overpay for unnecessary options, or save money on the wrong thing — and then get condensation on the glass, a green tint or a sauna effect in summer.
Over years of work I’ve tried hundreds of glass unit combinations. Seen how laminated glass cracks from its own weight. Replaced glass units that turned green like a bottle. Explained to clients why water runs down the glass in winter. All this taught me one thing: a glass unit needs to be selected systematically, understanding the logic of each parameter.
In this article I’ll tell you how to properly choose a glass unit for aluminum glazing — from glass thickness to frame type. No fluff, concrete, with examples from practice.
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What is an insulated glass unit and what it consists of
Let’s start with the basics. A glass unit is a sealed structure of several glass panes separated by a spacer frame. The most common option is a double-chamber glass unit of three panes.
The design is simple: glass, spacer frame, glass, another frame, third glass. The frame is an aluminum or plastic profile that sets the distance between the panes. Inside the frame is filled molecular sieve — a material that absorbs moisture from the air inside the chambers. This is needed so the glass unit doesn’t fog up from inside.
Around the perimeter everything is sealed with two layers: primary sealing with butyl (between frame and glass), secondary — with polysulfide sealant or silicone (along the outer contour). The result is a completely sealed structure.
The glass unit formula is written like this: 6-16-4-16-6. Decoding: first glass 6 mm, chamber 16 mm, second glass 4 mm, chamber 16 mm, third glass 6 mm. Total thickness — 48 mm.
Now let’s figure out how to choose each parameter.
Glass thickness — depends on the size of the insulated glass unit
The first rule to remember: the larger the glass unit area, the thicker the glass should be. This is due to strength — thin glass over a large area can crack from wind loads, temperature fluctuations, or even its own weight.
Here is an approximate classification by area:
Small glass units (up to 1 m²) — 4 mm glass can be used. These are standard apartment windows, small sashes.
Medium glass units (1–2.5 m²) — I recommend 6 mm. These include medium-sized glazing, balcony doors, sliding systems.
Large glass units (2.5–4 m²) — 8 mm glass. Panoramic windows, large façade systems.
Very large glass units (4–6 m²) — 10 mm glass. These are substantial glazing systems in private homes and commercial buildings.
Extra-large glass units (6+ m²) — 12 mm glass and thicker. Full-wall panoramic glazing, double-height spaces.
These are not strict limits but general guidelines. The exact thickness is calculated based on dimensions, regional wind loads, and installation method. However, the overall principle remains: the larger the area — the thicker the glass.
An important nuance about glass tint
Standard glass has a greenish tint. This is especially noticeable at the edge — glass that is 10–12 mm thick looks distinctly green. With thinner glass, like 4 mm, it is almost unnoticeable, but when you assemble a double-glazed unit with three panes of 10–12 mm each, it becomes clearly green.
I had a project where the client wanted panoramic glazing with maximum transparency. We made a unit with three panes of 10 mm — standard float glass. Once installed, the client was shocked: «Why is it green?!» I had to explain that this is normal for that thickness.
How not to make a mistake with thickness on adjacent panes
Another important point: if a single glazing system includes glass units of different sizes, try to keep the glass thickness (nominal) fairly consistent. For example, 12 mm next to 10 mm is fine. Or 10 and 8, or 8 and 6.
But if one unit in the same glazing system is large with 12 mm glass, and right next to it there is a small unit with 4 mm glass — the difference in tint will be noticeable. Thicker glass appears greener, while thinner glass is more transparent. Visually, this stands out.
That’s why, during design, I try to keep glass thickness consistent within a single façade. If different thicknesses are required, I limit the step to one or two increments, no more.
Low-iron glass — when it is really needed
Standard float glass appears green due to iron impurities in its composition. The thicker the glass, the stronger the green tint. For small windows this is not critical, but for panoramic glazing it can become an issue.
Low-iron glass (ultra-clear, extra-clear) is glass with reduced iron content. It is almost colorless and highly transparent. At the edge, standard glass looks green, while low-iron glass appears light, almost white.
I have samples for comparison. When I show clients two pieces of glass side by side — standard and low-iron, both 10 mm thick — the difference is very noticeable. One is distinctly green, the other is clear. And that’s just a single pane, while in a glass unit there are three — so the difference becomes even more pronounced.
When low-iron glass is justified
Low-iron glass is about 40–50% more expensive than standard glass. That’s why it doesn’t make sense to use it everywhere. Here are the situations where it truly makes a difference:
- Large panoramic glazing with thick glass. When a glass unit is made of three panes of 10–12 mm each, the green tint becomes very noticeable. Low-iron glass solves this issue.
- Premium projects where maximum transparency matters. High-end homes, showrooms, restaurants with panoramic views — color rendering is critical there.
- Multifunctional glass on a low-iron base. More on this later, but in short: multifunctional coatings on low-iron glass provide better color rendering from the inside.
For standard windows, smaller glazing, and budget-limited projects — regular glass is perfectly sufficient. The greenish tint is there, but it’s not critical.
Russian low-iron glass
Coatings on glass — energy efficiency and solar protection
Modern technologies make it possible to apply ultra-thin metal layers (mainly silver) onto glass. These coatings significantly improve the performance of a glass unit. There are three main types: energy-efficient (low-E glass), solar control, and multifunctional.
Low-E glass — we install it always, without exception
Low-E glass (energy-efficient glass) is a coating that reflects heat back into the room. In winter, heat from the interior does not escape outside, and in summer, the cool air from air conditioning stays inside.
The coating is applied to the inner pane, facing the cavity. It is completely invisible, does not affect transparency, and does not add reflectivity. It’s simply a thin layer of silver that acts as a heat reflector.
Low-E glass should be included in a glass unit by default. Always. It’s not an option — it’s a standard. Even if you are not planning any other coatings, Low-E is essential.
Why? Because it significantly — really significantly — improves thermal performance. A glass unit with Low-E coating is one and a half to two times more energy-efficient than one without it. At the same time, the cost increase is minimal — negligible at the scale of the entire project.
I use Low-E glass on all my projects by default. I don’t even discuss it with clients — I simply include it in the specification. Saving on this makes no sense.
Multifunctional glass — solar protection plus energy efficiency
Multifunctional glass (multi-functional, MF) is a coating that works in two directions: it prevents heat from escaping inside (like Low-E glass) and reflects solar radiation from the outside.
It is applied to the outer pane, facing the cavity. This creates a dual effect: in winter it retains heat, and in summer it prevents the space from overheating due to sunlight.
Multifunctional glass is especially relevant for:
- South-facing façades exposed to direct sunlight
- Panoramic glazing with large surface areas
- Spaces that tend to overheat in summer (greenhouse effect)Air-conditioned rooms — reduced load on the cooling system
Combination of Low-E glass + multifunctional
You can apply both coatings at the same time: a multifunctional coating on the outside and low-emissivity (Low-E) glass on the inside. This creates two layers of protection — an insulated glass unit with excellent solar control.
This doesn’t significantly increase the cost, but it provides maximum comfort. I use this approach on projects where the client wants ideal performance and is willing to pay for it.
Reflectivity — an inevitable consequence of solar protection
Any multifunctional coating creates a mirrored effect on the outside. This is physics — to reflect solar radiation, the glass must also reflect visible light.
The higher the level of solar protection, the more reflective the glass becomes. This is a trade-off you have to accept. If you want solar protection — you get a mirrored facade.
The degree of reflectivity depends on the specific type of coating. There are glass options with strong reflectivity (reflecting 50-60% of light), with moderate reflectivity (30-40%), and nearly neutral ones (20-25%).
Tints of multifunctional glass
Multifunctional glass comes in different shades:
- Neutral (clear, colorless)
- Silver (gray tint)
- Bronze (warm brown)
- Blue (bluish)
- Green
- Titanium (dark gray, almost graphite)
Choosing a shade is a matter of taste and architectural concept. Some prefer neutral clear glass, others bronze, and some graphite.
How to choose a specific glass — sample method
Each manufacturer offers dozens of types of multifunctional glass. Guardian, AGC, Saint-Gobain — all of them have their own product lines with different shades and levels of reflectivity.
How do you understand which glass you need? The simplest option is sample books with small pieces of glass. You look through them at light and compare the shades. But with a single small sample, it’s difficult to predict how an entire facade will look.
The best way is to order A4-sized samples. These are single-chamber insulated glass units the size of a sheet of paper, where the outer pane is multifunctional glass of the required type.
We do it like this: we select several options from the line of interest. For example, Guardian Neutral — we take three variants with different levels of reflectivity. We order A4 samples, go outside, place them in the sunlight, and review them together with the client and the architect.
We compare which shade looks better and which level of reflectivity is acceptable. Once the choice is made — that’s it, the decision is finalized.
We always keep such samples for different types of glass. It’s a small expense, but it saves a huge amount of time and stress. Because when insulated glass units are delivered to the site and it turns out the shade is wrong — rework can cost hundreds of thousands of dollars.
Multifunctional glass on a low-iron base
Coatings can be applied both on standard green glass and on low-iron glass. The second option is more expensive, but it provides better color rendering from the inside.
Why is this important? From the outside, we see reflectivity or tint — that’s a given. But from the inside, it’s important that everything remains as clear as possible, without color distortion. So that trees look green, the sky looks blue, and the sun looks yellow.
High-quality multifunctional glass on a low-iron base (for example, Guardian on UltraClear or AGC on Crystal Vision) provides excellent color rendering from the inside. You look out the window and see the world as it is, without a green or gray filter.
This is the most expensive option — low-iron base plus multifunctional coating. But for premium projects where aesthetics matter, it is worth it.
My choice — Guardian
I often work with Guardian. Their range is based not just on low-iron glass, but on extra-clear glass — something in between standard and low-iron. It is lighter than regular float glass, but more affordable than ultra-clear.
Light transmission is very good, and the price is reasonable. For most projects, this is the optimal balance.
Spacer frame — aluminum vs warm edge frame
Why aluminum spacer is bad?
Aluminum conducts cold very well. The weakest area of an insulated glass unit is the edge zone, where the glass connects to the spacer. That’s where most of the heat loss occurs.
If you install an aluminum spacer, there is a risk of condensation forming on the glass unit. Especially in winter during severe frost. The temperature at the edge of the glass drops lower than in the center, the dew point shifts — and small water droplets appear along the perimeter.
I have seen many photos and videos where such droplets form at the bottom and along the edges of insulated glass units. This happens precisely because of the aluminum spacer — it creates a thermal bridge.
Warm edge frame — the solution
Glass safety — tempering and laminated glass
Tempered glass — the standard for most projects
Tempering is a thermal treatment after which glass becomes 5–7 times stronger. When it breaks, it shatters into small fragments with blunt edges that do not cut.
For large insulated glass units, tempering on both sides is mandatory. This is a requirement for strength and safety. For small windows, in principle, you can leave annealed glass, but I always prefer to temper. It’s not a significant cost increase, while safety is far more important.
Another advantage of tempered glass: the edges are chamfered and processed before tempering. This reduces the risk of microcracks that can propagate from the edge during installation or due to temperature fluctuations.
I have never had a case where tempered glass cracked on its own. But with annealed glass, it has happened — especially with large formats.
Laminated glass — when it is really needed
Laminated glass is two panes of glass bonded together with a special interlayer film. When it breaks, the shards remain attached to the film and do not fall apart. This provides maximum safety and additional protection against intrusion.
But laminated glass has its drawbacks:
First — it is more expensive than tempering. Significantly more expensive.
Second — it is thicker and heavier. An insulated glass unit with laminated glass becomes more массивный, and it is more difficult to install.
Third — laminated glass is usually made from annealed glass. It is possible to make laminated glass from two tempered panes, but that is even more expensive.
My experience with laminated glass in large insulated glass units
I once had an unsuccessful experience with laminated glass. The client wanted large insulated glass units with maximum safety. We made them with tempered glass on the outside and laminated glass made of annealed panes on the inside.
We installed them on setting blocks — and the problems began. The weight was significant, the laminated unit pressed on the setting blocks, creating point stresses — and then a crack formed in the laminated glass. We replaced the unit. Installed a new one — and again a crack appeared a month later.
We struggled replacing those glass units. In the end, we remade them with laminated glass made from tempered panes — and the problem disappeared. But it cost a lot of money and nerves.
Since then, for large formats I use laminated glass only made from tempered panes. Or I don’t use laminated glass at all, and instead install two tempered panes — cheaper, simpler, and more reliable.
When laminated glass is justified
Laminated glass makes sense when:
- Protection against intrusion is required (ground floors, commercial properties)
- Enhanced sound insulation is needed (laminated glass dampens sound better than standard glass)
- There are specific safety requirements (schools, hospitals)
In other cases, double tempered glass is more than sufficient. Safety is ensured, while the insulated glass unit is lighter, more affordable, and easier to install.
How not to overpay — cost optimization
An insulated glass unit can be expensive or very expensive. The difference can be several times. Here’s what affects the price:
Size of the insulated glass unit. The larger the area, the thicker the glass, the higher the manufacturing cost, and the more complex the installation. A panoramic glass unit of 3 x 4 meters can cost 5-10 times more than a standard window of 1 x 1.5 meters.
Low-iron glass. Adds 40-50% to the cost compared to standard glass.
Multifunctional coating. Adds 20-30% to the cost.
Multifunctional coating on low-iron glass. The most expensive option, it can increase the cost by 1.5-2 times.
Laminated glass. More expensive than standard glass, and even more expensive if made from tempered panes.
How to optimize? If the budget is limited:
- Divide large glazing panels into smaller sections. Several smaller insulated glass units are cheaper than one giant one.
- Use low-iron glass only where it’s critical. Not for the entire house, but for the main facade or the living room.
- Use multifunctional glass only on southern facades. Northern sides can be left with standard Low-E glass.
- Instead of laminated glass — use two tempered panes. Cheaper and simpler.
With this approach, the price can differ by two to three times while maintaining comparable characteristics. This is real savings without sacrificing quality.
Quality control — what to check upon delivery
When insulated glass units are delivered to the site, I always check:
Glass unit specification — compliance with the order. There have been cases where a double-glazed unit was replaced with a single-chamber one, or the thickness was incorrect.
Spacer frame color — especially important. I once had an issue where the contract specified a black frame, but the designer submitted an aluminum one. Everything had to be replaced upon delivery.
Coatings — I check for the presence of Low-E and multifunctional coatings. They can be visually identified by a slight reflection.
Seal quality — I look for any sealant leaks and whether everything is properly sealed.
Tempered glass — tempered glass should have a marking in the corner. If it’s missing, the glass is not tempered.
All these checks take 10–15 minutes per batch, but they save months of rework.
Choosing an insulated glass unit is not just about picking something from a catalog. It’s a systematic process where every parameter affects the final result. Glass thickness, low-iron clarity, coatings, spacer frame, safety — all of this must be carefully considered and correctly combined.
If done right, you’ll get glazing that performs well for years: clear, warm, condensation-free, without a green tint, and safe. If you cut costs in the wrong places or overlook details, you’ll end up paying for replacements yourself.
I hope this article helps you make the right choice.
