Double glazing is now a household commodity that we almost take for granted. Coming as standard now in most windows and doors, we would expect nothing less than good quality glazing that lasts. 

However, double glazing can on very rare occasions spontaneously crack, or collapse inward causing a shatter effect look on your glass. 

Cracked Double Glazed Window

These situations can occur at any time of the year, and for all nothing might have touched the window or flew into the window to cause such a break, there is a reason behind these mysterious breakages. 

Window Glazing 

Double glazing is ultimately constructed by placing two sheets of glass either side of a vacuum, or in most cases a layer of inert gas.  This helps to create an invisible barrier which helps to contain heat hence keeping drafts out of your home, the temperature stable, and your energy bills down. 

A double-glazed window comes as a sealed unit.  The pressure placed around the glass is therefore constant, unlike the air pressure it is exposed to externally and internally. 

For all this pressure on the glass is minimal, you still might start and notice changes to the glass within the double-glazed unit.  

These changes might occur because of external pressures, and the window may slightly move inwards. 

Now, in most cases, the glass will be strong enough to withstand most pressure.  However, there are some instances where this is not the case, and the window glass cracks, showing what is known as stress cracks. 

Most emergency glazing and window repairs took place during the winter months when the colder outdoor temperatures contrasting with the heat and the warmth of our homes with the central heating cranked up, both make for very different temperatures on either side of the glass! 

Main causes of cracks in double glazing 

Size of window 

The size and shape of your window can determine which ones are most at risk of breaking.  Thinking about it in terms of aspect ratio, if you have a small square or even a circular shaped window, you will find that these are typically at very low risk of suffering a break.  This is because shorter panes of glass have less opportunity to flex than what larger panes do. So, in these cases, a tall, very narrow window, will most likely be at greater risk of breaks or indeed stress cracks showing. 

Problems with the window unit itself 

Even with the best technology in the world, human error is still inevitable.  Double glazing requires precision engineering to get perfectly right every time and, in some cases, unfortunately, there are manufacturing errors.  Things such as practices, machinery, workforce skills, and environmental factors all play a part in the manufacturing of double-glazed window units. 

Scratch on the surface 

Ultimately to make double glazed units, glass has to be cut down to size.  If a pane of glass is unfortunate enough to get scratched across any part of it, that part then becomes the weakest part of the window, becoming exposed to more of the elements faster and turning into something much more sinister in the long run.  The risk of scratches occurring is increased when glass cutting is more complicated due to window design, or when engineering bevelled glass. 

Humidity and temperature 

With winter being the most popular season for window repairs to be carried out because of the differing temperatures indoors and out, we also need to take into consideration the temperature of the gas inside the unit. 

For example, if your double-glazed unit is installed and fitted on a hot summer’s day, the atmospheric pressure will be low, posing a greater risk to the glass during the winter. 

Moisture within the unit also poses the same problems.  Moisture is often absorbed during the manufacturing process using a desiccant substance, which helps to prevent condensation droplets from being created inside the window.  However, this process also changes the pressure of the window, so again, windows that are manufactured on hot and humid days will be at higher risk of cracking with the temperature drops! 

Glass strength 

Window glass cracking can most certainly be reduced if the glass being used is of sufficient strength.   

A thicker pane of glass will be less likely to suffer from stress cracks than a thinner one, and at Alan Carnall, we’d always recommend opting for a 6mm pane over the most common 4mm.  

However, this isn’t to say the 4mm won’t be strong enough, as other factors also need to be taken into consideration.  Such as, where the stress is coming from, the size of the unit required, and how your house is arranged. 

Manufacturers now put in place much more stringent environmental controls within factories, helping to reduce the impact of these environmental factors and reduce the chances of a break happening and a claim of negligence being sought. 

Most manufacturers will now offer a guarantee against these spontaneous breakages and can offer glass replacement services. 

An increase in temperature from outside 

To help windows from cracking in those hot summer months, it’s essential to make sure that heat is dispersed fully across the glass.  Helping to reduce the impact that a full beam of sunlight in one spot may have on the window. 

Those windows that are fully exposed to the sun during the summer do have more chance of cracking in the winter.  This is because as the glass cools during the night, a beam of sunlight will then suddenly appear again during the day.  But this beam will only raise the temperature of where it hits, meaning that more than likely the corners of the windows are left out in the cold and take much longer for the temperature to reach them.  This temperature difference is what starts stress cracks appearing. 

To help disperse the heat more evenly in the winter months, you can place a pale-coloured blind/curtain inside the glass.  This helps reflect the heat onto the glass and evenly distribute the temperature. 

The temperature inside the home 

Within our homes and especially during the winter months, the temperature inside can also fluctuate, as we have our central heating systems on and off throughout.  Most problems to windows occur if your radiator or fire is directly next to the window. 

To help prevent any cracks from occurring, if your radiator is directly underneath the window, you can provide a little protection with curtains that fall behind the radiator, while still making sure airflow to the window from the rest of the room is adequate. 

If you’re looking for new double-glazed windows and doors, or you have an emergency glazing situation and need a window repair, call the team at Alan Carnall today and see how we can help you. 

Read the next article: Window Repairs vs. Window Replacements