Glass insulation explained: what is inside a double-glazed window?

What is an insulated glass unit?

An insulated glass unit (IGU) is a factory-made assembly of two or more panes of glass, held apart at the edges and sealed so the space between them stays dry and closed. That trapped layer of still air or gas resists heat far better than a single pane.

The idea is old: an “insulation pane” of two sealed lites was patented in 1865, and commercial units appeared in the 1930s and 1940s. The big step came in the 1970s, when Low-E coatings were developed. Guardian notes that these nearly invisible coatings almost doubled the insulating performance of an IGU.

The six parts of an insulated glass unit

How the unit slows heat

Heat moves in three ways, and a well-designed unit addresses each one.

• Radiation: a Low-E or solar-control coating reflects much of the sun's near-infrared heat away from the building while still passing visible light.
• Convection: air circulates inside the cavity and carries heat across. The cavity width and a heavier gas such as argon slow that circulation.
• Conduction: heat travels from glass to spacer to glass along the edge. A warm-edge spacer reduces this path.

Surface numbers and where the coating goes

Glass surfaces are counted from the outside in. In a double-glazed unit, surface #1 faces the street, #2 and #3 face the cavity, and #4 faces the room. Soft-coat Low-E and solar-control coatings are usually placed inside the cavity, where they are protected.

In a hot climate like Riyadh's, the solar-control coating is usually on surface #2, so the sun's heat is reflected before it crosses the cavity. Vitro Architectural Glass, for example, advises placing solar-control Low-E coatings on surface #2 of a double-glazed unit. Always confirm the position on the glass data for your chosen product.

Air or argon?

Argon is heavier than air and slows convection in the cavity, which lowers the U-value. It diffuses out slowly over the years; Guardian cites about 1% a year as an accepted rate, and the remaining gas keeps insulating.

Argon does not improve sound insulation. Pilkington states that air, argon and krypton make no difference acoustically. If noise is the concern, the panes themselves have to change (see our guide to sound insulation).

Single, double or triple glazing in Riyadh?

• Single 6 mm glass has a U-value around 5.7 W/m²K and offers little protection from heat. It is not a good choice for air-conditioned rooms.
• Double glazing with a solar-control or Low-E coating is the usual specification for Saudi villas and façades. The coating choice has a large effect on solar heat gain.
• Triple glazing adds a pane and a second cavity, which lowers the U-value further but adds weight and frame depth. In a cooling climate, solar heat gain often matters more, so compare the complete options with your designer (see our high-performance glass guide).

Why fog appears between the panes

From the day it is made, a sealed unit lets a tiny amount of moisture through its edge seal. The desiccant absorbs it. When the desiccant is saturated, moisture collects in the cavity and appears as fog or droplets that cannot be wiped away. This is called seal failure.

A failed unit cannot be dried out in place. The usual remedy is to replace the sealed glass unit; a sound aluminium frame can normally stay. Good edge seals, correct glazing and drainage in the frame all help the unit last longer.

Insulating glass is tested against standards such as ASTM E2190 in the United States and the EN 1279 series in Europe, whose parts include long-term tests for moisture penetration (EN 1279-2) and gas leakage (EN 1279-3).

What to ask your supplier

1. The full build-up in writing, for example: 6 mm solar-control glass (coating on #2) + 16 mm argon cavity + 6 mm clear glass.
2. The calculated values for that build-up: U-value, SHGC (or g-value) and light transmission.
3. The spacer type, and whether it is a warm-edge spacer.
4. The secondary sealant, especially for structural glazing or curtain walls, where a UV-stable structural silicone is normally required.
5. Which standard the units are made and tested to, and the warranty terms for the sealed units.

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