What does 4+16+4 mean in double glazing quotes, and why do argon gas and a Low-E coating make a difference? We explain the structure of insulated glass units, spacer choice and the insulation values of different glazing units side by side.
The first technical expression anyone replacing windows comes across is usually "4+16+4 double glazing". Alongside it, terms such as "argon-filled", "Low-E coated" and "warm edge" also appear frequently in quotes. These details directly determine how warm your windows keep you in winter and how cool in summer. In this article we explain in plain language the structure of double glazing, what the numbers mean and the real difference made by argon gas and a Low-E coating.
What Is Double Glazing?
Double glazing (an insulated glass unit) is a glass unit formed by joining two or more glass panes at their edges with a set gap between them. The gap between the panes is created by a profile called a spacer bar, and the edges are sealed. A desiccant inside the spacer keeps the air in the cavity dry and prevents the unit from misting up on the inside.
The insulating power of double glazing comes from the still layer of air or gas between the two panes. It roughly halves heat transfer compared with single glazing; with the right coating and gas fill, this ratio improves much further.
What Does 4+16+4 Mean?
This expression describes the cross-section of the glass unit in millimetres:
- 4: The thickness of the outer pane (4 mm),
- 16: The cavity between the two panes (16 mm),
- 4: The thickness of the inner pane (4 mm).
The total unit thickness is 24 mm. A cavity width in the 14–16 mm range gives the most efficient insulation value for air- or argon-filled units. If the cavity is too narrow, insulation decreases; if it is too wide, circulation (convection) starts in the gas and efficiency drops again. Other combinations are also possible: asymmetric units such as 6+16+4 can be used for sound insulation, and laminated units (for example 44.2+16+4) for safety.
What Does Argon Gas Do?
Argon is a colourless, odourless and harmless noble gas that occurs naturally in the air. Because it is denser than air and has lower thermal conductivity, filling the cavity between the panes with argon reduces heat transfer. In production, most of the cavity is filled with argon, and the edge seal keeps the gas inside for many years.
The effect of argon on its own is limited; it shows its real difference when used together with a Low-E coating. In an uncoated unit argon improves the U-value only slightly, while in a Low-E coated unit the improvement is much more pronounced.
What Is a Low-E Coating?
A Low-E (low emissivity) coating consists of very thin metal oxide layers applied to the glass surface that are almost invisible to the eye. These layers let most visible light through while reflecting long-wave heat radiation. In winter they reflect the room’s heat back inside; in summer, solar control versions block part of the heat coming from outside.
The coating is applied to one of the inner surfaces of the unit. Low-E coatings for thermal insulation are generally placed on the inner pane’s surface facing the cavity (surface 3), while solar control coatings are placed on the outer pane’s surface facing the cavity (surface 2). Choosing the right surface directly affects the coating’s performance.
Comparing the Values
| Glazing unit | Approximate Ug (W/m²K) | Assessment |
|---|---|---|
| Single glazing (4 mm) | 5.8 | Practically no insulation |
| 4+16+4, air-filled | 2.7 | Basic double glazing |
| 4+16+4, argon-filled, uncoated | 2.6 | Small improvement |
| 4+16+4, Low-E, air-filled | 1.4–1.5 | Significant improvement |
| 4+16+4, Low-E + argon | 1.0–1.1 | Recommended standard |
| Triple glazing, two Low-E + argon | 0.5–0.7 | Very high performance |
As the table shows, the biggest leap comes from the Low-E coating, while argon takes that gain a step further. So if your budget is limited, it makes sense to prioritise the Low-E coating first and then add the argon fill.
Spacer Bars: The Warm-Edge Difference
The spacer that separates the panes is traditionally made of aluminium. Because aluminium conducts heat well, a cold strip forms at the edge of the glass and condensation may appear there in winter. Warm-edge spacers made of stainless steel or plastic composite materials reduce heat loss at the edge, lower the risk of condensation and improve the window’s overall Uw value.
Which Unit for Our Region?
In regions with mild winters and hot summers such as Ortaca, Dalaman and Köyceğiz, a Low-E argon-filled 4+16+4 unit with a warm-edge spacer is a balanced solution for most homes. On west and south facades, combined coatings that bring together thermal insulation and solar control significantly reduce the air-conditioning load in summer. In homes affected by road or airport noise, asymmetric or laminated glazing units should be considered.
Conclusion
4+16+4 is a measurement describing the cross-section of a double-glazed unit, but its real performance is determined by the Low-E coating, the argon fill and the choice of spacer. Always ask about these three details when comparing quotes. During a free site survey our team assesses the orientation of your facade and your needs and recommends the most suitable glazing unit for you.
