Double Glazing: The Science of Trapped Air
The Building Blocks of a Double Glazed Unit
At its heart, a double glazed unit (DGU) is a simple but brilliant sandwich. It's not just two random pieces of glass placed together. It's a carefully engineered system where each component has a specific job. Let's break down the main parts:
- Glass Panes: These are the transparent walls of the sandwich. Typically, the panes are each 4 mm thick. In advanced units, one of the inner surfaces has a special microscopic coating.
- Spacer: This is the frame that holds the two glass panes apart, creating the crucial gap. Spacers are usually made of aluminum or a warmer, less conductive material like polymer. They contain a desiccant (a drying agent) inside to absorb any moisture trapped during manufacturing.
- Primary Sealant: This is a tough, rigid seal that glues the spacer to the glass panes, forming the main structural bond of the unit.
- Secondary Sealant: This is a flexible seal that runs around the entire outside edge of the unit. It acts as a second line of defense, ensuring the unit is airtight and watertight.
- The Gas Fill: The space created by the spacer isn't just empty; it's filled with a gas that is a poor conductor of heat. While dry air is standard, better-performing units use heavier, inert gases like Argon or Krypton.
How Trapped Air Fights Heat Loss: The Three Methods
Heat always moves from a warmer area to a cooler one. In winter, the heat from your cozy living room tries to escape to the cold outside through the windows. In summer, the outside heat tries to get in. Double glazing fights this heat transfer in three key ways: Conduction, Convection, and Radiation.
1. Reducing Conduction: Conduction is how heat travels through a solid material. If you hold an ice cube, heat from your hand conducts through the ice, melting it. A single pane of glass is a good conductor, so heat passes through it easily. Double glazing introduces a gas-filled gap. Gases are much poorer conductors than solids. The molecules in a gas are far apart, making it harder for heat energy to jump from one molecule to the next. So, the heat has a much harder time conducting across the gap.
2. Limiting Convection: Convection is the transfer of heat by the movement of a fluid (like air or water). In a single, wide air gap, air warmed by the inner pane would rise, and cooler air would sink, creating a circulating current that efficiently carries heat across the gap. In the narrow space of a double glazed unit (typically 12-16 mm), there isn't enough room for these large convection currents to form effectively. The air movement is stifled, drastically reducing convective heat transfer.
3. Blocking Radiation (with Low-E Glass): Radiation is heat transfer by invisible infrared waves. All warm objects emit it, including your room's interior and the sun. Standard glass allows a lot of this radiant heat to pass through. Advanced double glazing uses low-emissivity (Low-E) glass. This glass has an ultra-thin, transparent metallic coating on one of the inner surfaces that acts like a mirror for radiant heat. In winter, it reflects the radiant heat from your room back inside. In summer, it can reflect solar radiant heat back outside, keeping you cooler.
| Heat Transfer Method | How It Works | Effect in Single Glazing | Effect in Double Glazing |
|---|---|---|---|
| Conduction | Heat flow through a material | High (glass is a good conductor) | Reduced (gas is a poor conductor) |
| Convection | Heat flow via fluid circulation | N/A (only one air surface) | Minimized (narrow gap prevents currents) |
| Radiation | Heat transfer by infrared waves | High (glass is transparent to IR) | Greatly reduced (with Low-E coating) |
Measuring Performance: Understanding U-Values and Gas Fills
How do we know if one window is better than another? Scientists and engineers use a standard measurement called the U-value (or thermal transmittance). The U-value tells you how much heat is lost through a material. It's calculated using the formula:
$ U = \\frac{Q}{A \\times \\Delta T} $
Where:
$ U $ is the U-value (in W/m²K),
$ Q $ is the rate of heat loss (in Watts, W),
$ A $ is the area of the window (in square meters, m²),
$ \\Delta T $ is the temperature difference across the window (in Kelvin, K).
The lower the U-value, the better the insulation. A single pane window can have a U-value of about 5.0 W/m²K. A standard double glazed unit with an air fill might be around 2.8 W/m²K. A high-performance unit with argon gas and a Low-E coating can achieve a U-value of 1.4 W/m²K or even lower!
Why are gases like Argon better than air? It comes down to their physical properties. Argon is a heavier, inert gas with a higher density and lower thermal conductivity than air. This makes it even more difficult for heat to conduct across the gap. Krypton is even better but more expensive, so it's often used in thinner gaps where maximum performance is needed.
A Real-World Experiment: Feeling the Difference
You can experience the principle of double glazing with a simple experiment at home. On a cold day, touch the windowpane of a single-glazed window (common in older homes). It will feel very cold. Now, touch the glass of a modern double-glazed window in the same room. It will feel much closer to the room's air temperature. Why? The single pane glass is conducting the cold from outside directly to your hand, drawing heat away from it rapidly. The inner pane of the double glazed unit is not as cold because the trapped gas layer has slowed down the heat flow from your warm room to the cold outside. The glass surface stays warmer, which also helps reduce uncomfortable drafts and condensation.
This principle is applied on a large scale in skyscrapers. Imagine a tall glass building in a city with hot summers and cold winters. Without advanced glazing, it would be a nightmare to heat and cool. The energy bills would be enormous. By using double or even triple glazing with Low-E coatings and argon gas fills, these buildings can maintain a comfortable indoor environment year-round while using significantly less energy for air conditioning and heating. This is not just good for the building owner's wallet; it's also better for the environment because it reduces the burning of fossil fuels for energy.
Common Mistakes and Important Questions
Q: If the gap is so good at stopping heat, why not make it very wide?
A: This is a common misunderstanding. There is an optimal range for the gap, typically between 12 mm and 20 mm. If the gap is too small, it doesn't provide enough insulation. If the gap is too wide, it allows convection currents to start up again. In a wide gap, the air near the warm pane heats up, rises, and the cold air near the cool pane sinks, creating a circulating loop that actually transfers heat more effectively. So, a very wide gap can make the insulation worse, not better.
Q: My double glazed window has condensation between the panes. What does this mean?
A: This is a clear sign that the unit has failed. The primary or secondary seal has broken, allowing moist air from the room to enter the sealed space. The desiccant inside the spacer becomes saturated and can no longer absorb the moisture, which then condenses on the cold inner surface of the glass. This fogging or condensation between the panes cannot be wiped away and means the window has lost its insulating properties. The unit needs to be replaced.
Q: Do double glazed windows also block sound?
A: Yes, they provide excellent acoustic insulation as a beneficial side effect! Sound travels as vibrations through the air. When a sound wave hits a single pane of glass, it easily vibrates the glass, which transmits the sound into the room. In a double glazed unit, the two masses of glass (the panes) are separated by the spring (the gas cushion). The sound wave has a much harder time vibrating both panes in sync. Much of the sound energy is lost trying to move the mass of the first pane and then transfer the vibration across the gas gap, significantly reducing noise from traffic, airports, or noisy neighbors.
Footnote
1 DGU: Double Glazed Unit. The sealed assembly of two or more glass panes separated by a spacer.
2 U-value: Thermal Transmittance. A measure of how much heat flows through a material per unit area and temperature difference. Lower values indicate better insulation.
3 Low-E: Low-Emissivity. A microscopic coating on glass that reflects long-wave infrared heat radiation, improving the window's insulating properties.
4 Argon: An inert, non-toxic gas that is denser and less thermally conductive than air, making it a superior fill gas for insulating glass units.