Generally we describe Insulated Glass Units (igu's) as double glazing, sealed units etc, a new unit sould be hermitacally sealed (gas tight), over time the gas tight seal looses its effectiveness and the air or other gas such as argon will escape and be replaced with humid air, the moisture in the air will turn to vapor and fogging will be seen inside the unit, shortly after the humidity in the unit will increase and condense on the glass surfaces, if left for a long time streaks and damage will be caused to the glass surfaces.
The average life of a sealed unit in the UK is just 8 Years (GGF Information), north facing units will generally last longer, south facing units exposed to high levels of solar gain will fail very quickly.
Exposure to freezing temperatures causes the unit to go into negative deflection, putting excessive strain on the units seals, high temperatures in the summer has the oposite effect and causes the unit to swell like a balloon, this constant swelling and contraction (solar pumping) eventually breaks the seal.
There is no simple singular answer, the life of a sealed unit depends on a number of factors such as quality of materials used, manufacturing process, transportation, poor fitting, faulty window frames, and the most important is exposure to temperature. These are just the main reasons units fail and by far the most prolific is exposure to the sun.
South Facing units have the shortest life span.
One of the most frequently asked questions we recieve is "Why have my units failed ?"
Why Units Fail
Why Repair? They Will Only Fail Again.