Why do sealed units fail?
There are a number of reason why sealed units fail, but before we look into these let’s understand what we mean by fail.
A failed unit is a hermetically sealed unit that is no longer air tight, this means the perimeter seal that bonds the two panes of glass together has a hole, separation etc,.
The failure in the seal allows the trapped dry air/gas in the unit to escape and be replaced by humid moisture laden air, after time the moisture in the air turns to steam and this can be seen inside the cavity in the form of fog, over time the moisture content builds up and water droplets start to form on the glass (condensation).
Heat
The main reason why sealed units fail is their exposure to heat, in the northern hemisphere south facing units are exposed to the suns rays more than any other elevation, it is these sun facing units that usually go first. The elevated temperatures causes the units to swell in the summer months putting enormous strains on the seals that eventually give way, in the UK the average life of a sealed unit before it shows signs of fogging is just 8 years.
Poor Fitting
The next most common cause of failure is poor fitting, units that have been fitted into uPVC and aluminium frames are often incorrectly installed and block the windows drainage system, resulting in the units sitting in water that accelerates the seal failure.
Low Quality
Finally units that have been made to poor standards are often fitted with faulty seals, gaps, holes etc in the units seal, In the UK when a sample unit is tested for accreditation they do not test the air tightness of the unit only its ability to prevent condensation inside the unit at the point of testing.
In conclusion even if a sealed unit has been made to the highest standards and is free from flaws, has been installed in high quality frames by installers who know what they are doing, the likely hood of that unit lasting more than eight years is very low. In fact if they are south facing you might be closer to five years.
A hermetically sealed unit will react to the external temperatures, on a cold winters day the gas inside the unit contracts causing the glass panes to buckle and invert this not only puts strains on the seals but also reduces the gap between the two panes of glass significantly reducing the units thermal performance, in fact the problem can be so severe that the two panes of glass actually touch creating a circular effect, this problem is called Newton’s Rings.
So in the winter months when we need the unit to be performing at its best keeping all the heat generated from our heating systems in our property the centre pane value is severely compromised, and not surprisingly when sealed units are tested for accreditation the test samples are so small they are unable to go into negative deflection.
The opposite effect applies in the summer months, until the seal fails and the unit then equalizes.
Why replace your units because a small amount of moisture has entered, we can remove the moisture build up and extend the life of the unit by decades.
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