What is actually inside a double glazed unit?
Two panes, a spacer bar full of desiccant, two seals, a gas fill and an invisible metallic coating. What each part does, why units mist up, and why a misted unit cannot be cleaned.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
Short answer
A sealed unit is two or three panes held apart by a spacer bar that contains a drying agent, sealed at the edge by a primary seal that keeps the gas in and a secondary seal that holds the assembly together. The cavity holds dry air or argon. One inner surface usually carries a low-emissivity coating. When the seals fail, moist air gets in, the desiccant saturates and the unit mists — permanently.
Almost every complaint about double glazing comes down to a component in this list, and none of those components is visible from either side of the glass. Knowing what is in there makes the difference between accepting a quote to replace a whole window and knowing that only the sealed unit needs changing.
Step by step
- Start with the two panes.Each is a separate sheet, and their specification can differ — one toughened and one not, one laminated, one patterned. What went in is what has to come out, which is why the corner marks matter when ordering a replacement.
- Look at the spacer bar.The frame around the edge that holds the panes apart. Traditionally aluminium, and now often a warm-edge polymer or composite, which conducts less heat and reduces condensation at the edge of the glass in cold weather.
- Understand the desiccant.The spacer bar is hollow and filled with a drying agent, which breathes into the cavity through small perforations along its inner face. Its whole job is to absorb the small amount of moisture sealed in at manufacture, and any that gets in later.
- Understand the two seals.A primary seal between the spacer and each pane is the gas and moisture barrier. A secondary seal filling the channel around the outside is the structural one that holds the unit together. The unit fails when either gives up.
- Understand the cavity fill.Dry air, or more often argon, which conducts heat less well than air. Argon leaks out slowly through the seals over the life of the unit, and there is no way to top it up or to check it without instruments.
- Find the low-emissivity coating.A metallic layer only nanometres thick on one of the surfaces inside the cavity, which reflects long-wave heat back into the room. It is always on an inner surface so it cannot be worn or cleaned off, and it is why a modern unit outperforms an old one of the same thickness.
- See why the unit sits on setting blocks.Small blocks in the frame carry the weight of the unit and hold it clear of the bottom of the rebate. A unit sitting in water, in a frame whose drainage slots are blocked, fails early. Keeping frame drainage clear genuinely extends unit life.
- Recognise the failure.Misting or streaking between the panes that comes and goes with the weather, and eventually permanent deposits on the inner surfaces. That is seal failure. The unit is replaced; the frame is not.
Tips
- Only the sealed unit needs replacing when a unit mists. Any quote to replace the whole window because of misting is worth a second opinion.
- Drainage slots along the bottom of a uPVC frame are there to let out the water that inevitably gets in. Cleaning them out once a year is the cheapest maintenance in the house.
- Condensation on the room side of the inner pane is humidity in the house, not a fault. Condensation on the outside face on a cold clear morning is a sign the unit is performing well.
Common mistakes
- Drilling a misted unit to let the moisture out — Products exist that do this and they are contentious for good reason. It breaches the seal deliberately, the cavity becomes vented, the coating and the panes can deteriorate, and the thermal performance is lost.
- Blocking or sealing the frame drainage slots — Water then stands in the rebate against the secondary seal, which is exactly what causes early unit failure.
- Ordering a replacement unit by measuring the visible glass — The unit is larger than the visible area because part of it sits in the rebate. GuideHQ has a guide on measuring one properly, and this is the most common ordering error.
If it doesn't work
Misting between the panes that clears in dry weather
Cause: Early seal failure — moisture is entering and the desiccant is still coping intermittently — Fix: The unit is failing and will get worse. Plan the replacement rather than waiting for permanent deposits, and check the frame drainage while it is out.
A permanent white or rainbow deposit on an inner surface
Cause: Long-term seal failure, with the coating and glass surface now damaged — Fix: Replace the unit. There is nothing that can be cleaned or restored once the inner surfaces have been wet for a long period.
Condensation on the outside of the glass in the morning
Cause: The outer pane is colder than the dew point because so little heat is escaping through the unit — Fix: Nothing to fix. It is evidence of a well-performing unit and it clears as the sun warms the glass.
Questions people ask
How long does a sealed unit last?
Manufacturers give guarantees rather than lifespans, and how long a unit lasts depends heavily on the frame drainage, the exposure and how it was installed. What is clear is that units are consumable and frames often outlive them.
Can a misted unit be repaired?
Not restored. Drilling and venting products claim to clear the misting, and they do so by permanently breaching the sealed cavity, which loses the gas fill and leaves the coating exposed. Replacing the unit is the honest answer.
Is triple glazing three of the same thing?
Three panes, two cavities, two spacer bars, and considerably more weight — enough that hinges and frames have to be designed for it. The thermal gain over a good double unit is real but smaller than the pane count suggests.
What is a warm edge spacer?
A spacer made from a low-conductivity material rather than aluminium, which reduces heat loss at the perimeter of the glass and reduces condensation forming along the bottom edge of the pane in cold weather.