Thermal Shock – Why I don’t clean windows on Frosty Days

Double glazed cracked window

In brief, thermal shock is where a sheet of glass either expands or contracts very quickly due to hot expansion and damage happens as a result.

Most often the problem is rapid expansion, from very cold glass getting hot water put on it. Why would this happen? Well, have you never heard that if you put water on a car windscreen to defrost it on a frosty morning, that you should never use ‘hot’ water, just warm/tepid water. Otherwise you can break the windscreen? That is the most common form of thermal shock in day to day life – Frozen glass + Hot water = Thermal Shock

The problem is made worse if there are any little cracks or imperfections in the glass already. If so, these will crack open faster than a perfect sheet of glass.

Vehicle windscreens are a special type of glass, made to withstand a lot of flexing and movement. So, if glass like this can crack with thermal shock, just imagine what glass that doesn’t have this ‘flexibility’ treatment can do. Glass such as ‘architectural glass’ for instance, the glass that your household windows are made from. That type of glass is even more prone to breaking with thermal shock.

Though the imperfection can occur anywhere, the most confusing for homeowners, is when the imperfection is on the glass edge, usually behind the frame and thus, it cannot be seen. So, the owner believes that there was nothing wrong with his/her window and that someone has broken it. Actually, impact damage and stress or shock damage looks different, but I am not going into that right now. Except to say that you can normally see the point of impact with impact damage and the crack pattern it often different with more cracks outward from the point of impact. Look at the photograph of the double glazed window, it is an obvious stress crack from the edge behind the frame. The origin is hard to see in this photograph, but it is clearly a stress crack rather than an impact crack.

We are interested in stress or shock damage, which is often, though not always, leading from an imperfection in the glass.

There are various reasons for imperfections in window glass, such as:

    • Edge Damage from poor handling by the factory, transport, fitter or anyone else with any contact with the glass from the moment it comes to the end of the production line.


    • Poor cutting technique or tool. Can leave a slight imperfection that isn’t noticed at the edge of the glass.


    • Wrong glazing sizes. Not enough clearance has been left for the glass moment within the fame. (Glass needs to move, contract and expand in the frame under normal conditions)


    • Poor frame design can result in the same problem as wrong glazing sizes.


    • Frame debris when glazing. Bits of dirt, stone, glass, nails etc left in the frame to rest against the edge of the glass. (Glass needs to contract and expand in the frame so increased pressure can damage the edge if it expands against a hard/sharp object)


    • Warped frame. A frame that is not totally straight can warp the glass within causing stress damage.


    • Poor fitting blocks. Glass is set on special blocks, the wrong size of block or even the wrong type/material of block can lead to glass edge damage.


    • Damage from site works. Glass is easily damage from the slightest thing. When glass is being put in, sawing, cutting drilling, moving, hammering, metal cutting, stone cutting is also often done around the glass. Any of this can cause slight or not so slight, damage. I have see this a lot.


    • Previous thermal stresses. Glass can flex too much under strong thermal conditions e.g. a very hot day, something amplified the suns rays (the same was as a magnifying glass can, many forest fires are stared like this). Previous stresses like this cause damage that can’t bee seen yet. Actually a very sunny day alone can crack and break a window.


    • Solar Glass (Low E etc. coated glass to retain heat). The very nature of this glass can cause the glass to heat more and result in thermal stress damage.


    • Normal impact damage. Day to day life takes it toll on everything. It’s easy for a window to get impact damage and you not notice it. Every impact does not result in a very obvious crack, some are microscopic in size.


    • Glass can contain nickel sulfide impurities at the manufacturing stage. This is a risk in glass production that can result in glass damage a number of years after the glass has been fitted. It’s a little ‘ticking time bomb’. The crystals can change and re-shape in the glass resulting in stresses and damage. This can even e the the cause of spontaneous breakage for no apparent reason that can be seen in glass.
      Other manufacturing error. Glass production is complex, and even though the largest, most reputable companies have good quality control, imperfections can get through.


  • Cheap glass. If the glass is a cheap glass, a common problem in today’s society of competitive pricing, money saving etc there are likely to be more imperfections in the glass at production level. A cheaper glass company cannot afford to be as selective about the glass it sends out to market. They also cannot afford to spend as much money on high end production equipment, or quality inspection equipment.


Single Glazed Cracked Window
On this single glazed cracked window. You can clearly see how the crack started at the edge, behind the frame to the right. Then spread from that point of damage. This is a stress/shock crack, not an impact crack from something hitting the window.

As you can see, there are many reasons why glass can get imperfections and damage. This damage can lead to breakage all on it’s own. Or cause the sort of minor damage that is the start of a crack from thermal shock damage.

Thermal shock is a problem for window cleaners still, especially water fed pole cleaners. We use hot water (well some of us use hot water most use cold water, which is useless in the cold weather). The hot/warm water hits the frozen glass and any imperfections will simply crack open. Not what you want, nor what the window cleaner wants.

So, quite often, a water fed pole window cleaner just doesn’t clean windows on frosty mornings.


If you are interested, you can get some more information on glass breakage from a few more sources:

  1. Wikipedia – Spontaneous Glass Breakage
  2. UK Centre For Window And Cladding Technology
  3. Windows Online UK


Sometimes, there can be a sort of hazy look to windows if you catch the light at the right angle. It almost looks like there is something on the glass, or maybe it has some sort of residue from the window cleaner? Well. glass is clear right, so of it looks like there is a distortion right over the pane, it means something has been done badly by the window cleaner – right?

Sorry – wrong.

Architectural glass today is more complicated than you may think. Various things can affect the look of the glass and the article below explains why this can sometimes happen – and it’s not the window cleaner’s fault 🙂 The article is by Pilkington, a wold leader in glass manufacture and the main glass manufacturer for household window panes in this part of the world.



Why have I got a haze/milky appearance on my windows?

Haze is an optical phenomenon which makes the glass look like it is covered in a very fine, uniform layer of dust when viewed from an oblique angle or viewed under strong light incident on the glass at an oblique angle. Our original Pilkington K Glass™ can, under certain lighting conditions, display this phenomenon to a limited extent.

The reason for this is that the Pilkington K Glass™ coating is not as smooth as the glass surface. While this is not obvious to the eye when examining the glass, some people who regularly handle Pilkington K Glass™ can tell which side the coating is on by the feel of it.

The optical effect of the slightly rougher surface is to scatter a small proportion of the light incident on it (in exactly the same way as a thin layer of dust would, which is why it looks similar).  With Pilkington K Glass™ the amount of scattered light is generally less than half of one percent of the light coming through the window, so under most viewing conditions it is not obvious. However, when incident sunlight is at an oblique angle and the view through the glass is of a shaded area, then the scattered light can become more visible, giving rise to the appearance of haze.

Nowadays low-emissivity glass is coated using alternative technologies so that our latest low-emissivity glass products; Pilkington K Glass™ S and Pilkington Optitherm™ do not suffer from this phenomenon to such a noticeable degree.

Article Copyright © Pilkington Glass