How a double-glazed unit is built, and every variable that changes its performance
An insulated glass unit is a factory-sealed assembly of two panes separated by a spacer and a controlled cavity. That simple description hides a considerable amount of engineering, because the unit has to survive decades of wind load, barometric pressure swings, UV and temperature cycling without failing — and failure of any single component produces the fogging between panes that means the whole unit gets replaced.
Triple and even quadruple glazing exist, but the double IGU remains the workhorse of energy-efficient construction. It balances cost, weight, manufacturing practicality and thermal performance better than anything else, and for the vast majority of projects it is the right answer.
This page covers what is actually inside a unit, the choices available at every layer, and how those choices change the numbers. If you are comparing quotations, the sections on spacers and cavity width are where most of the unexplained price difference usually lives.
Six components, each of which can fail and each of which can be specified.
The glass lites. The two panes. These carry the coatings, tints and strengthening treatments, and they need not be identical — in fact, on acoustic work they deliberately are not.
The spacer bar. Separates the panes and defines the cavity, running the full perimeter. Its material has a larger effect on performance than most people expect.
The desiccant. A moisture-absorbing material inside the spacer, or integrated into it. It absorbs residual moisture trapped at manufacture and the small amounts that permeate the seals over decades. When the desiccant saturates, the unit fogs.
The cavity. The gap itself. This is the primary thermal break, filled with dry air or an inert gas.
The primary seal. Polyisobutylene applied to the sides of the spacer before bonding to the glass. This is the main barrier against moisture getting in and gas getting out.
The secondary seal. Silicone or polysulfide around the outer perimeter. This provides the structural integrity holding the assembly together and protects the primary seal.
The dual seal arrangement is why a well-made unit retains its gas fill for decades — one seal handles gas retention, the other handles structure, rather than asking a single material to do both.
A makeup is always described from outboard to inboard, and the four glass surfaces are numbered the same way.
Surface 1 faces the weather. Surface 2 is the inner face of the outer pane. Surface 3 is the outer face of the inner pane. Surface 4 faces the room.
Coating position is a real specification decision, not a manufacturing detail. Solar control coatings normally go on surface 2, intercepting solar energy before it enters the cavity. Passive Low-E coatings, which are retaining interior heat rather than rejecting exterior heat, are often placed on surface 3 instead. In a mixed climate like New York the right choice depends on which problem dominates on that elevation.
Surface 4 coatings exist too — they reflect radiant heat back into the room and improve the U-factor, at the cost of a small reduction in visible light. These have to be durable products, since surface 4 gets cleaned.
Clear float is the baseline, with the faint green cast that iron content produces. Low-iron glass removes it for maximum clarity and true colour rendition — worth it where edges are visible or the glass is thick.
Annealed glass is untreated and breaks into jagged shards, so it is not used in safety locations. Tempered is four to five times stronger and granulates when broken; code requires it in doors, sidelights, low windows and wet areas. Heat-strengthened sits between them at roughly twice annealed strength, breaking into larger pieces that tend to stay in the frame — which is why it is preferred for spandrels and for resisting thermal stress.
Low-E coatings divide into two families. Hard coat, applied pyrolytically while the glass is still hot, is very durable but generally lower performance, and can survive exposed positions. Soft coat, sputtered in a vacuum chamber with multiple silver layers, delivers far better performance but is physically delicate and must be protected inside the cavity. Nearly all high-performance residential and commercial glazing uses soft coat — including the units inside Pella, Marvin and Andersen windows.
Tinted glass takes its colour from minerals in the melt. It reduces glare and solar gain, but it does so by absorbing heat rather than reflecting it — the glass itself warms up. Reflective coatings reject solar heat and give daytime privacy, at the cost of a mirrored appearance.
Laminated glass can occupy either lite, and brings acoustic damping, security and impact resistance.
Spacer bars were traditionally aluminium: structurally sound, and a serious thermal conductor. That creates a thermal bridge around the entire perimeter, letting heat cross directly from the warm inner pane to the cold outer one. The result is cold edges and condensation forming at the perimeter first — the classic symptom of an older unit.
| Spacer | Thermal Behaviour | Notes |
|---|---|---|
| Aluminium | Poor — strong thermal bridge | The old standard. Cold edges and perimeter condensation. |
| Stainless steel | Good | Conducts far less than aluminium while keeping box-spacer rigidity. |
| Hybrid plastic/metal | Very good | Engineered plastic with thin metal foil for the vapour barrier. |
| Foam (structural silicone) | Excellent | Conducts almost no heat; flexible, so it accommodates movement and curves. |
| TPS (thermoplastic) | Excellent | Extruded directly onto the glass — no corner keys, so no corner failure points. |
A warm-edge spacer is standard on any high-performance unit today, and the difference shows up in two places: a measurably better whole-unit U-factor, and a much better condensation resistance rating.
The choice among the warm-edge options often comes down to the fabricator's equipment rather than to performance, since the top options are close. Foam and TPS have an advantage on curved units and on very large ones, because they flex. TPS additionally eliminates corner keys, which are a known failure point in rigid spacer systems — corners are where sealed units most often fail first.
If a quotation does not state the spacer, ask. It is one of the easiest places for a cheap unit to hide.
Dry air is the baseline and provides useful insulation on its own.
Argon is denser than air, so it moves more slowly and suppresses the convective loop that carries heat across the cavity. It also conducts heat about a third less readily than air. It is inert, non-toxic and cheap, and it offers the best performance-per-dollar of any upgrade available on a sealed unit.
Krypton is denser again and performs better still, but costs several times more. Its distinctive property is that it reaches optimum performance in a narrow cavity, around 6–8mm. That makes it the answer for triple glazing where total thickness is constrained, or for retrofitting a high-performance unit into a shallow existing frame. In a standard wide cavity, krypton is partly wasted money.
A double unit is outboard lite plus cavity plus inboard lite. Total thickness typically runs from about 18mm to 25mm, though specialist units go well beyond.
Glass thickness is driven by structural load and size. Standard residential work uses around 3mm; light commercial 4mm; heavy commercial and structural applications 6, 8, 10 or 12mm and up. As glass area increases, thickness must increase too, or the pane deflects and bows visibly under pressure differences.
Cavity width is where thermal and acoustic goals genuinely conflict, and this is worth understanding before choosing.
For thermal performance with air or argon, the optimum sits around 12–16mm. Narrower and conduction across the gap increases. Wider and convection currents begin to circulate inside the cavity, which increases heat transfer again. There is a genuine peak, and going wider does not help.
For acoustic performance, wider is simply better — low frequencies in particular need distance between the panes. Specialist acoustic units run 20mm or more, and secondary glazing configurations use 50mm and beyond.
So a unit optimised for heat is not optimised for noise. On a quiet site, take the thermal optimum. On a busy street, the honest conversation is about which problem you are actually solving. See acoustic glass for the full picture.
One design choice deserves singling out because it is cheap, effective and routinely overlooked.
If both panes are the same thickness — 4mm and 4mm — they resonate at the same frequency, and the assembly loses performance exactly there. Sound passes through the coincidence dip in both panes simultaneously.
Making them different — 4mm and 6mm — staggers those dips, so one pane is still blocking while the other is weak. The improvement is meaningful and the cost difference is usually small.
If you are specifying glazing for a noisy elevation and the quotation offers matched pane thicknesses, ask why. This single change is the highest-value acoustic decision available in a sealed unit.
How the choices combine in practice, for three different priorities.
| Priority | Outboard Lite | Cavity | Inboard Lite |
|---|---|---|---|
| Balanced residential cost, code compliance, safety | 3mm clear, tempered | 13mm, warm-edge spacer, 90% argon | 3mm clear, tempered, Low-E |
| High-rise commercial wind load, structural glazing | 6mm clear, heat-strengthened, Low-E on surface 2 | 16mm, stainless spacer, 90% argon | 6mm clear, heat-strengthened |
| Urban acoustic street and traffic noise | 6mm clear, tempered | 12mm, foam spacer, air fill | 8.38mm laminated (4mm + PVB + 4mm) |
The commercial example uses heat-strengthened rather than fully tempered glass deliberately. Tempered carries a small risk of spontaneous failure from nickel sulfide inclusions, and on a high-rise facade that means granules falling to the street. Heat-strengthened is far less prone to it.
The acoustic example combines both techniques — asymmetric thickness and a laminated inboard lite — which is why it reaches noticeably higher STC than either alone.
U-value measures heat transfer through the unit. Lower is better. Low-E coatings and gas fill are the main drivers.
Solar heat gain coefficient is the fraction of solar radiation admitted, from 0 to 1. In cooling-dominated situations you want it low; on a south-facing elevation in a cold climate you may want it higher to capture winter sun.
Visible light transmittance is how much daylight gets through. The point of advanced coatings is keeping this high while pushing SHGC low — tinted glass, by contrast, reduces both together.
STC rates sound reduction. A standard unit sits around 28. Asymmetric thickness and laminated glass can lift that into the high thirties or beyond.
Ask for all four on any unit being quoted. A supplier who can only give you one of them is not giving you enough to compare against anything.
Glass technology moves quickly. Coatings that did not exist a few years ago are standard now, and the maximum sizes fabricators can produce keep changing.
For oversized or jumbo units, unusual acoustic targets, extreme thermal requirements or anything structurally demanding, the constraints are current manufacturing capability rather than theory — and those constraints shift. We check what is actually available and buildable at the time of your project rather than working from last year's data sheet.
One thing worth flagging on very large units: they are sensitive to barometric pressure changes during transport, particularly across altitude. Where that becomes a structural risk, the remedy is capillary tubes — and a unit with capillary tubes must breathe air, which means giving up the gas fill. It is a genuine trade-off, and one we will explain rather than decide for you. See argon gas fill for the detail.
Send us the openings, the performance targets, the elevation orientations and any acoustic or security requirements, and we will specify the makeup rather than quoting a generic unit.
Low-E Glass Argon Gas Fill Acoustic Glass Laminated Glass Tempered Glass Architectural Glass
Tell us about the opening and we will specify it for you.
