What Is a Double Glazed Unit? Sealed Units Explained

Most of us live behind double glazing without ever thinking about how it works. A double glazed unit is the sealed glass part of your window, and understanding what sits inside it helps explain why windows mist up, why some feel warmer than others and when a repair is worth it. Here is the plain-English version.

What a double glazed unit actually is

A double glazed unit, also called an insulated glass unit or sealed unit, is two panes of glass held a set distance apart with a dry, sealed gap in between. That whole sandwich is made as a single component in a factory, then fitted into your window frame. The frame (uPVC, timber or aluminium) is separate from the glass unit itself, which is why you can often replace the glass without changing the whole window.

The magic is in the gap. Still, trapped air or gas is a poor conductor of heat, so that layer slows the escape of warmth from your home far better than a single sheet of glass ever could.

The parts inside a sealed unit

Break a unit down and you find a few key components working together:

• The panes of glass. Usually two (double glazing) or three (triple glazing). One or more often has an invisible low-emissivity coating.
• The spacer bar. A slim frame around the perimeter that holds the panes apart at a fixed distance, commonly around 16mm to 20mm for the gap.
• The desiccant. A drying agent hidden inside the spacer bar that soaks up any trace of moisture, keeping the gap crystal clear.
• The gas fill. Many units are filled with argon, an inert gas that insulates better than plain air.
• The perimeter seal. A double seal around the edge that bonds it all together and, crucially, keeps moisture out and gas in.

Get all of that right and you have a unit that stays clear and insulates well for years.

What makes one unit warmer than another

Not all double glazing performs the same. A few things make the difference:

• Low-e coatings that reflect heat back into the room while still letting light in.
• Argon gas rather than air in the gap.
• Warm-edge spacer bars made of composite rather than aluminium, which cut heat loss around the edges.
• The gap size, with an optimal spacing giving the best insulation.
• Triple glazing, which adds a third pane and a second gap for extra performance.

These features feed into a window's energy rating. Modern energy-efficient uPVC windows combine them to keep rooms warmer and cut heating bills, which matters through a Yorkshire winter.

Why sealed units mist up

Here is the part everyone asks about. When you see fog or a milky haze trapped between the panes that you cannot wipe off, the perimeter seal has failed. Once that seal breaks down, damp air leaks into the gap, the desiccant eventually gets overwhelmed, and moisture condenses on the inside of the glass whenever the temperature changes.

Misting is a glass problem, not usually a frame problem. The frame, hinges and locks can be in fine shape while the sealed unit has reached the end of its life. That is why the fix is often a new unit dropped into the existing frame rather than a whole new window. Our repairs team handles exactly this kind of job.

Can a misted unit be repaired?

In practice, a failed sealed unit is replaced rather than repaired. Attempts to "defog" a unit by drilling and drying it out do not restore the gas fill or the seal, so the clarity and performance are never quite the same. A fresh made-to-measure unit is the reliable answer, and it usually comes with its own guarantee against misting, though terms vary by supplier, so check what yours covers.

Rough costs in 2026

Prices depend on the size, the glass spec and access, so use these as a guide and get a quote for your windows. As of 2026, replacing a standard misted sealed unit commonly falls somewhere between about 60 and 180 pounds fitted, with larger, toughened, obscure or triple glazed units costing more. To know your figure, ask us for a price and we will measure up.

FAQs

What is the difference between a double glazed unit and a window?

The double glazed unit is just the sealed glass part, two panes with a gap. The window is the whole thing, including the frame that holds the unit. You can often replace the unit without replacing the frame.

What is inside a double glazed unit?

Two or more panes of glass, a spacer bar holding them apart, a drying desiccant inside that spacer, usually an argon gas fill, and a perimeter seal bonding it all together.

Why does a sealed unit go cloudy between the panes?

The perimeter seal has failed and let damp air into the gap. When the temperature changes, that moisture condenses inside the glass, which is why you cannot wipe it off.

Can you repair a misted double glazed unit?

In practice it is replaced rather than repaired. Drilling and drying does not restore the seal or gas fill, so a new made-to-measure unit is the reliable fix.

Is argon-filled double glazing worth it?

Generally yes. Argon insulates better than plain air, so combined with low-e coatings and warm-edge spacers it improves comfort and can reduce heating bills.

Want to see the warmer, more efficient glazing options available? Download our latest brochure for the full range, or request a free quote and we will talk it through.