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Frequently Asked Questions
What is a sealed unit?
Panes held apart by a spacer bar.
The edge is sealed all round.
The cavity holds dry air or argon.
The cavity does the insulating.
The seal makes the cavity possible.
How does a unit fail?
By misting, not by breaking.
The edge seal degrades over time.
Moisture enters the cavity.
It condenses between the panes.
It cannot be wiped off.
Why does it happen suddenly?
A desiccant absorbs moisture for years.
It sits inside the spacer bar.
Eventually it becomes saturated.
After that, misting appears quickly.
The seal had been failing long before.
Can a misted unit be repaired?
In practice it is replaced.
The cavity cannot be resealed reliably.
The glass itself is usually sound.
That is what makes it frustrating.
Replacement is a straightforward job.
What shortens a unit's life most?
Standing water in the glazing rebate.
The edge seal sits in it permanently.
Such units fail years earlier.
Rebates must drain and ventilate.
It is a detailing failure, not a product one.
Is a wider cavity always better?
No, there is an optimum.
Beyond it convection starts in the cavity.
Performance then stops improving.
Argon fill helps modestly and cheaply.
Follow the published figures.
What is a warm-edge spacer?
A spacer of lower conductivity than aluminium.
It reduces the cold bridge at the perimeter.
The perimeter is where condensation appears first.
It improves the whole-window figure.
Most modern units use one.