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


Master Guild Of Window Cleaners Member ImageWarrenpoint Window Cleaning is a member of the Master Guild of Window Cleaners. The Guild will only accept companies that are proven to be professional, insured, trained and comply with the Guild’s Code of Conduct. This means that you are not getting some sort of ‘cowboy’ window cleaner, but a professional company.

[Click on their logo to the right to take you to our page on the Master Guild Web Site]


Why Is There A Master Guild of Window Cleaners?

Window cleaning is no longer the simple job of the untrained as it used to be. Glass is more complex both the make up and the shapes and designs. Many of the properties we go to have treated or coated glass that needs special attention. Even the framework today is no longer the basic ‘bit that holds the glass in place’ as it used to be. Keeping up with the new developments in the window industry as well as the suitable cleaning techniques is only one area that makes us stand out from the old school window cleaner. I (Paul) don’ hide the fact that I started learing to clean windows years ago with a chamois (shammy)- old school, but the industry has moved on in keeping with people’s desire for more efficient homes (including the glass in the windows). Did you know that even ‘self cleaning glass’ needs cleaning? Well it does from time to time, and you can’t tackle those windows like you would the standard plain float glass windows.


Why Does It Matter If WWC Is A Member?

Being part of the Master Guild of Window Cleaners means that we must keep up to date with cleaning practices, health and safety and other aspects of this industry that are beneficial to you – the customer of our services. Do you know when traditional cleaning should be used instead of water fed pole or the opposite way around? If WFP, what determines what type of brush should be used (i.e. the bristle material), or even if a brush is the most suitable? When is hot water best used on glass, and when should it not be used?

As a member of the  Master Guild of Window Cleaners, we acknowledge our responsibility to know these things, to treat our customers (and staff) with respect. Putting into place best practices to care for your property and our staff’s wellbeing. And of course, if something does go wrong, to be properly covered by a suitable insurance policy that actually covers the work that we do.

The Master Guild also sit on the HSE and the Cleaning Industry Liaison Forum. This forum provides advice and guidance on health and safety within the cleaning industry. Sitting on the Board are interested parties from all aspects of the cleaning industry including the Master Guid of Window Cleaners.

So, if you are watching us work, look at how we work and how thorough we are. If you are an adult now, maybe you remember back to your childhood window cleaner if you used one. He quite possibly had the nickname of ‘Splash Harry’ or ‘Zorro’ for his practice of doing as little actual cleaning as possible and still getting paid 🙂


Master Guild of Window Cleaners Membership Card


Warrenpoint Window Cleaning Is A Member of the Master Guild of Window Cleaners.


Spiders Web on Window Surround & Frames

Spiders Web on Window FrameYes, you can tell the seasons on the year in this part of the country by what needs cleaned off window frames, sills and window surround. As I clean all this as well as the glass, I am always aware what the most prolific type of dirt / debris is at any point in the year as it changes season by season.

I clean my customers windows regularly, so the things that need cleaned off change as we go through the year. There is less and less mould and algae accumulating on the sills now, so less work with the scrubbing brush round the sills for me. But, the spiders webs are appearing more and more. The fine webs are no problem, but the blobs of web in the corners of the window surround and jammed between the top of the frame and the surround are starting to accumulate.


The spider’s web blobs occur for 2 main reasons:

  1. Primarily, because spiders feed on smaller insects that get trapped in their web. The spider then wraps up the insect in web in preparation, stores it close by and keeps it for consumption later.
  2. Spiders webs don’t really like water. They are fine with morning dew and light showers etc, but not good in bad weather. Rain and wind destroy the web, often resulting in a shapeless blob of destroyed web sticking to something near the site of the original web.


Getting the fine webs off (such as the spider’s web photo at the top of this page) is no problem, though the blobs do cause take a bit more work (see photos below). They are very sticky, and are usually well jammed into a corner. Since I clean these off all my customers windows, I have become rather practised at it, and have even designed my own tool for removing them safely and cleanly. I have had to do this as none of the usual window cleaners tools are really up to the job – we have a bigger spider blob issue in this part of the country than elsewhere it seems.

The only good thing for me is that spiders generally don’t go above the ground floor windows. Yes, you do get a few, but as a rule, I only have to remove the mess on the ground floor windows, though that mess does gets very bad as we get into better weather and the insect population increases.

Anyway, the spiders are back out of hibernation or wherever the go in the winter, and my spider’s web removal tools are out on nearly every job these days, and we are only just at the start of spring!

My customers can be assured that I take pride in my work, and my company tag line “The ‘Complete window’ cleaner” is not just some marketing label claim, but I do clean the complete window – spiders webs and all.


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Here is an example of spiders web I cleaned from a window the other day:

Spiders Web on Window Surround & Frames
Spiders Web Cleaned Off Window Surround & Frames


Spiders Webs Removed As Standard by Warrenpoint Window Cleaning


Raining on Window

Raining on WindowYou Shouldn’t Clean Windows In The Rain – Is That Really True?

Rain water is actually very good for window cleaning. Yes, all those people who say that you shouldn’t clean windows in the rain are actually wrong. OK, it used to be true, but that was back in the Industrial age, when the air was filled with industrial smog. The rain that came through that smog was contaminated with dirt, smoke, industrial pollution etc. In fact in the UK and Ireland, acid rain was much more common than it is today due to the pollutants that were picked up by the rain from industrial air pollution. And all that pollution only made the rain water dirty, so what was the difference between cleaning windows in the rain and cleaning them ‘before’ the rain? And if you live in the UK or Ireland, then for more than half the year, you are never far from a rain shower anyway. Well, err…


Rain Water Is Harvested By Window Cleaners

Nowadays, the story is different and rain water is actually harvested (collected) by many window cleaners, and used to clean customers windows. Yes, instead of avoiding rain water, professional window cleaners are actually using it to clean windows in preference to tap water. Don’t believe me? Do an online search for “rainwater harvesting window cleaning”.


Rain Water Has Fewer Contaminates Than Tap Water

Rain water normally contains far fewer contaminates in solution than tap water in many parts of the country, and it more suited to cleaning and removing dirt than ‘clean’ tap water. If you measure the number of contaminates in solution in rainwater using a TDS meter, the reading ranges from very low to zero in many parts of the country. Whereas, tap water normally has between 30 – 200 times more solids dissolved into it. These figures are important to window cleaners, and you can read some more on how we do it in this article – What Is TDS and Why Is It Important To Window Cleaners.


Wait! I have Seen Dirty Windows After The Rain

Yes, sometimes a window does look more dirty after the rain. This generally happens if the window hadn’t been clean in a while, and the rain shower was only light on the glass – not a wash out.

Sometimes after a light shower, you can see dirty streaks on your windows. Well, these aren’t actually from new dirt, but existing dirt on the window. A raindrop lands on the glass and starts to run down it. As it goes, it absorbs some of the dirt on the glass (remember the lack of dissolved solids we just discussed in rain water). We know that a drop of water tries to stay as a drop / a lump / a bump of water, this is because of the physics of surface tension, anyway… The drop creates a line. This line is outlined with both the dirt already on the glass at the edge of the clean streak, and some of the dirt which was absorbed by the drop and deposited to the outside of the drop – NOT new dirt. The dirt deposited to the outside of the drop is due to the science of surface tension and absorbency & saturation.

As well as this, if the rain stops and the rain drop evaporates, the absorbed dirt is then deposited on the glass in one point where the rain drop was. This creates a dense area of dirt – the dirt that was absorbed in the rain drop.

So, taking all of this into consideration, when you look at a window after the rain, you can see the outlined streak of the path of the raindrop. It makes the window look dirtier to you, when in actual fact, it isn’t. A line has been created in the glass, outlined with dirt already there and ending in a smear of dirt which is prominent to the eye. So, although all that has happened is that the dirt already on the glass has been repositioned by the clean rain water, it looks like the rain has made the window dirty!


Dirty Windows In The Autumn & Winter

You may have noticed that your windows actually get dirtier in the Autumn and Winter when there is more rain, so how come if the rain doesn’t make them dirty? Well, not only does it rain, but it is also more windy and that are more storms. Dirt is actually carried in the wind and storms, and that is what causes most problems. And for those of us close to the sea, you know that sea water can be carrier in the air too, and that makes creates a white haze over your windows with the sea salt. It’s not the rain that makes them dirty, it’s the dirt and salt etc. carried on the wind that causes most of the problems.


Can You Clean Windows In The Rain?

So, yes you can clean windows in the rain. The next time you see your window cleaner out in the rain, don’t think “he shouldn’t be cleaning windows in the rain”. Instead feel sorry for him, because frankly it’s miserable working outside in that weather. Cleaning windows in the rain is not a problem – rain water is normally clean – cleaner than tap water.

OK, there is one other thing that makes a difference when cleaning windows ‘in’ the rain. If you use the traditional style of window cleaning, it is more difficult to finish them when it is raining – not impossible, but it is more difficult. So, that is just a problem for the window cleaner, not the owner of the windows. Anyway, this doesn’t apply to me, as I generally do not use the traditional method, but a pure water cleaning technique. This technique is not affected by the rain, so anyone using this method can happily clean widows in the rain with no ill effects to the windows.


Do ‘I’ Clean Windows In The Rain?

Yes, I do. I offer a regular service to my customers, and if I am going to do that, then I need to be out working in the rainy days too. I offer a 100% satisfaction guarantee anyway, if I have cleaned your windows (raining or not) and they are not cleaned right, I will happily do them again for free. I would rather clean them again, and you be happy, rather than you be unhappy with my service. So, I ‘put my money where my mouth is’ regarding cleaning windows in the rain.

Some days it is raining too heavily for practical or sometime safe working conditions. Then, on those days I do not clean windows. Otherwise, yes you will see me out working in the rain, as my customers want a regular, reliable service, and it makes no difference at all to the finished cleanliness of your windows.

So, forget about the myth about not cleaning windows in the rain – it’s just an ‘old wives tale’ and doesn’t apply in today’s climate or my window cleaning method.

TDS is short for “Total Dissolved Solids”. It is a reading that we as window cleaners, take from the water we use to clean windows, when using a pure water cleaning system. It is not a factor in ‘traditional window cleaning‘, just pure water window cleaning, also called water fed pole cleaning / reach and wash etc.

The TDS reading tells us of the number of particles of solid matter such as very small particles of metals, calcium and other minerals and salts etc that are held in suspension, or dissolved in the water. These ‘dissolved’ solids are very small – less than two micrometers in size and undetectable by eye. They stay in suspension in normal conditions and don’t sink, but stay dissolved as part of the fluid.

Wikipedia describes TDS as “Total Dissolved Solids (often abbreviated TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in: molecular, ionized or micro-granular (colloidal sol) suspended form.”

This shows you that a TDS reading can also be taken from other liquids, but as a window cleaner, I am interested in the TDS of water that I am going to use for cleaning windows.


How Is TDS in Water Measured?

TDS is measured using an electronic device which analyses the water using electrical conductivity, then tells us how many Parts Per Million (PPM) are dissolved solids. This way, we can get exact details of what level of dissolved solids we have in our water.

TDS Monitor
In Line TDS Monitor

TDS Meter
Handheld TDS Meter


Why Is TDS Important To Window Cleaners?

Have you ever washed an outside window, then rinsed it off with fresh tap water, maybe from a garden hose or just a bucket and cloth? When it dries on the glass, you end up with spots or dirty marks all over the glass. Your nice clean window that you spent so much time and work on are no longer clean.

How did this happen when you used fresh clean water? These marks are the solids that were dissolved in the water and didn’t run off the glass, but stayed in suspension until the water dried, and they were left on the window. Windows are great at showing up any dirty marks – especially on sunny days.

Maybe if you rinsed off your windows with bottled water afterwards? No, you may get an even worse result! Bottler water, sometimes called ‘mineral water’ contains a lot on TDS in the form of minerals. You could get lots and lots of white spots all over your nice windows by rinsing off with bottled water.

You can see why TDS is important to a professional window cleaner. You wouldn’t want to pay someone to leave your windows with spots or streaks every time them came to ‘clean’ your windows would you?


Water Marks on Window Water Marks on Glass Window


What TDS Level is Good for Window Cleaning?

The lower the TDS, the better. In general we want zero ppm, or very close to it in the water we use for cleaning windows. In my opinion, anything between 0 – 10 ppm is good for cleaning windows (for my local tap water). It is not reasonable to expect to be able to get that level of purity from normal tap water, so we must have a way of removing these dissolved solids before cleaning windows with the water.

TDS of Water for Window Cleaning - 000 parts per million
Ideal TDS of Water for Window Cleaning – 000 (Zero) parts per million


How Do You Get Rid Of These Dissolved Solids From The Water?

There are a number of ways, and again, I am just interested in how a professional window cleaner deals with the problem. We have two main methods of removing these dissolved solids – Reverse Osmosis and De-ionisation.

Reverse Osmosis

Basically, water is put through an extremely fine sieve with holes so small that the majority of these dissolved solids can’t go through. The much purer water is collected at the other side and now has a far lower TDS reading.


Ion Exchange Resin
Ion Exchange Resin Used for Deionisation of Water

De-ionisation, also sometimes called Demineralisation, is a chemical process rather than a physical one like reverse osmosis. Water is passed through a special substance called an “ion exchange resin”. A chemical process takes place which ‘exchanges’ the dissolved minerals with hydrogen ions and hydroxide ions. These new ions then combine to form water. Since most dissolved solids in tap water are salts (the things that cause spotting on windows), this process is very efficient and creates a high level of water purity similar to that of distilled water.

There are other methods used to lower the TDS such as distillation and carbon filtration, but these are not generally used by window cleaners. Well, not quite true… carbon filters are often used as first stage filtration when the primary method is reverse osmosis.


It is generally thought that deionised water is the best for our purposes as window cleaners. Though each of these methods above has it’s positives and negatives. Often the choice of which purification method a window cleaner will use, comes down to how much dissolved solids are in the water source. Generally, if there is a high TDS in the original source water then it is more cost-effective method to use the reverse osmosis process followed by finishing or ‘polishing’ as it is sometimes called, the water by de-ionisation. Treating only with de-ionisation is only cost-effective if the source water TDS is low enough.

Just when you thought that a window cleaner just needed a bucket of soapy water, a cloth and a squeegee! Well, actually many still do just that – I still see them working in the same areas that I do. But, the modern window cleaner keeping with updated methods, and looking to give his or her customers something a little bit more, has these things to consider as well.




Water Fed Pole Cleaning

Water Fed Pole CleaningWater Fed Pole Cleaning – Good or Bad?

I have been asked this, or a similar question quite often. And usually because the person has had a bad experience of a window cleaner using a water fed pole cleaning system [1].

First of all, let me answer the question – Is the water fed pole cleaning system any good? Yes, when in the right hands it does an excellent job of cleaning windows. It has now become the standard method of cleaning windows commercially on low-rise buildings.

So, how come some people have had bad experiences with window cleaners using one? Well, there are two answers to this…


Answer One – A Natural Occasional Problem

The first answer is that occasionally, on the first or second window clean with purified water, you can get a run down the window (dirty brown or white). This happens after the window cleaner has cleaned the windows and has now gone. The purified water used to clean the windows has probably gotten under a top window seal. Purified water is like a magnet to dirt and has come in contact with old dirt or old soap under the seal, attracted it, then run down the glass after the window cleaner has gone. Then the water dries and leaves only the impurities it picked up, thus you get a dirty stain or white soap stain on the glass.

I tell my new residential customers that this can ‘occasionally’ happen, and to simply call me if it does and I will come round and re-clean the window ASAP – for free of course. After one or two cleans with a water fed pole cleaning method, this can’t happen (if the job has been done right) as all the old ingrained dirt and soap is cleaned away. It also generally doesn’t happen on commercial buildings, as pure water cleaning is the normal method, and dirt or soap doesn’t get to build up in the seals. Though, it only happens to the occasional house anyway, it is not the normal result of water fed pole cleaning.


Answer Two – ‘Cowboy’ Window Cleaners

The other problem is sadly more likely to be the reason for the poor quality clean. And that is people starting up as window cleaners with no experience or understanding of what they are doing. They look at the pure water cleaning system, think that this is great because no skill is needed, and you just brush over the glass a bit and the windows are clean – Wrong! The equipment needed to clean windows with pure water is easy to buy, and it “looks” like pure water cleaning is simply a matter of giving a window a quick brush to get it clean. This is so wrong, it does takes skill, experience and an understanding of what the process is actually doing, to get it right.

Like every tool, it has it’s place and it cannot replace every other piece of window cleaning equipment. Though a lot of new window cleaners think that it will replace all the other tools and even replace experience. This is the biggest reason these days why some customers have had bad experiences.

There are just too many amateurs that have bought water fed cleaning kit off eBay or Gumtree, and set themselves up as professional window cleaners.


Common Mistakes Made By Non-Professional Window Cleaners


Spotting On The Windows

Due to a couple of reasons – the water has not been purifed properly and some impurities – usually a mineral has been left in the water. Though the water can be pure and still have spotting if the cleaning process isn’t done right.

The Wrong Brush

There are different types of brushes for water fed pole cleaning, and they are not all equal. First of all, a suitable WFP brush should be used that will not damage the glass, I have seen people using a cheap brush they bought in a supermarket and converting it for window cleaning use. This can lead to glass damage if the bristle material or tip finish is not suitable – some of these brushes are suitable – many are not. Even within the specialist brushes, there are differnt types of bristle material, bristle weights, brush sizes and configurations. These are made for different jobs, and an understanding of what you are using for which job is needed to give the best clean.

Beliving That WFP Is All That Is Needed

A water fed pole is an excellent cleaning too in the right hands, but it is not the answer to every window cleaning problem. Though I use water fed poles myself for the majority of my cleaning, I also carry a large supply of hand tools and other cleaning products. There will often be marks on windows that will not easily be removed with a water fed pole. Having the right tool for the job, and knowing how to use it is imperitive.

Soap & Dirt From Window Seals

I mentioned a natural occasional problem above that can sometimes happen to anyone. Though, a non-professional will have this problem much more often as they do not have the understaning of what is happening, or how to try to prevent it.

A Poor Clean

A poor clean in general can happen for a number of reasons. Though the most common being that they simply don’t know what technique gives a good clean, or they are trying to get the job done too quickly. The wrong brush, dirty equipment, poorly maintained equipment, being tight with resin or RO membranes are all other problems that can give a poor clean generally.


So, yes you can get an excellent result with a water fed pole / pure water cleaning system, if it is done by a skilled window cleaner. In my opinion, a water fed pole window cleaning system, used properly is the best way of cleaning ‘most’ windows on low-rise buildings. It gives you a better and more thorough clean that traditional window cleaning. It is also a much safer way of cleaning rather than traditional clean up ladders.

If you are still doubtful about one being used, but want to ask me to clean your windows, then remember this. I offer a 100% satisfaction guarantee. If the windows are cleaned right, I will come round and re-clean the for free ASAP and with no quibbles. I have a number of regular customers who had previous bad experiences with another window cleaner doing a poor job with a water fed pole. I convinced them it works well, maybe I can convince you also. By the way, when I started cleaning windows – I used a shammy (chamois leather) and newspaper. I learned how to clean windows a long time ago!


Paul Signature




[1] Water Pole Cleaning System. This is also known as a “Pole Cleaning”, “Pure Water Cleaning”, “Reach and Wash” and some other names. The cleaning method is described here: Pure Water Window Cleaning