What Is a Sealed Unit Window? The Parts Inside Your Double Glazing

A sealed unit is the glass part of a double glazed window: two or more panes held apart by a spacer bar and sealed around the edge so dry air or gas is trapped between them. It sits inside the frame as one piece. When people talk about replacing a misted or cracked double glazed pane, they usually mean replacing this unit, not the window frame.

The parts of a sealed unit

Every sealed unit is built from the same handful of parts, even though thicknesses and glass types vary. Knowing them makes it easier to understand what has gone wrong when a window mists up or lets in cold.

• Two panes of glass, sometimes three in triple glazing
• A spacer bar running round the edge that holds the panes apart
• Desiccant inside the spacer that soaks up any moisture in the cavity
• A primary seal between the spacer and the glass
• A secondary seal round the outside edge that bonds the unit together
• Air or an inert gas such as argon filling the gap

What the spacer and desiccant do

The spacer sets the width of the cavity, which affects how well the unit insulates. It is hollow and filled with desiccant, small beads that absorb moisture. When the unit is new, that desiccant keeps the air in the cavity dry so the inner faces of the glass stay clear.

Spacers come in aluminium and in so-called warm edge materials that conduct less heat. You can often see the spacer as the grey, black or coloured strip just inside the frame when you look closely at the edge of the glass.

Why sealed units fail

Over the years the edge seals are exposed to sun, frost, rain and the frame moving slightly with temperature. Eventually a tiny gap can open. Damp air creeps in, the desiccant becomes saturated, and moisture begins to condense between the panes. That is the familiar misted or cloudy look that cannot be wiped away.

Water sitting in the bottom of the frame, blocked drainage slots and poorly fitted glazing packers all speed this up. Our condensation between glass panes guide explains how to tell failed seals from ordinary condensation on the room side.

Can a sealed unit be repaired?

Once the seal has failed and moisture is inside, the practical fix is a new unit. Drilling and drying services exist, but they do not restore the original seal or any gas fill. We replace the unit with a new one made to the size of the old one, fitted back into the existing frame.

That is usually far less disruptive than a new window. If the frame, hinges and locks are sound, the glass can be swapped in a single visit once the unit has been made. Our sealed unit replacement vs new windows guide weighs up when a whole window makes more sense.

Choosing a replacement unit

A new unit gives you a chance to improve on the old one. Depending on the frame depth, you may be able to have low emissivity glass and gas fill for better insulation, safety glass where the position calls for it, or obscure glass for privacy. Our low-E and argon filled units guide covers the energy side in more detail.

The overall thickness has to match what the frame can hold, so the key measurement is the unit thickness as well as the width and height. Our guide to measuring a double glazed unit shows how to take those measurements if you want a rough idea before we visit.

Signs your sealed unit needs attention

Mist or droplets between the panes, a white film on the inside faces, streaks that come and go with the weather, or a crack in either pane are all signs the unit has failed. A draught round the glass is usually a gasket or bead problem rather than the unit itself, and is worth mentioning when you get in touch.

Getting a sealed unit replaced

Mark measures the existing unit, orders a replacement to suit the frame and position, and fits it on a return visit. Our misted double glazed units service page covers the job itself, and double glazing repairs covers the hinges, handles and seals around it. Send a photo from the quote page or call 07535 916982.

Call Mark on 07535 916982 or request a callback.