Over a period of time we have accumulated heaps of "stars", "half moons" and "chips" in the windscreen of our Subaru without it breaking. Every trip we make we usually get another 'stone-hit' (usually from a small pebble thrown up by a vehicle travelling in the opposite direction but also, sometimes, by vehicles overtaking us) we've been expecting the windscreen to "go"!
It never has!
We reached a point where we decided, 'cos we have 'glass insurance' to get the windscreen replaced. Knowing that Subaru windscreens are very, very expensive, we visited a 'friendly' windscreen replacement company and had it inspected. The conclusion was that the glass was in "... a bad condition ..." but would not be able to be replaced under insurance as it was still, technically, intact. We were not happy and showed it. Our advisor told us to go for a very short drive (like back home) with the windscreen demister on 'extremely high' setting with full fan assistance. The moment we get home we were told to hit the windscreen with a full blast of cold water from the hose.
Sure enough we did this and half an hour later a crack appeared from the driver's side edge, about a third of the way up , where there was a significant 'star' hit, ran towards the centre and then turned and ran down to the bottom of the windscreen. We took the Subaru back to the windscreen replacement company.
Yep! Cracked windscreen with the crack joining two sides, therefore, as the windscreen forms part of the 'structural roof' of the front of the car it was classified as dangerous and able to be replaced under insurance.
Where's all this going?
Well our friendly windscreen man said to us, "I bet you rarely drive with your demister on?" Correct! We only use it to de-ice and then put the car heater on a low setting which stops most misting up or icing up of the windscreen and if we get a build-up we switch over to demist only until it clears.
He explained that laminated glass windscreens are subject to 'thermal fracture'. If you drive with your demister constantly on in cold weather the centre of the window gets 'hot', called a "glass compression zone" while the edges remain cold and the glass there is 'stretched'. If a stone hits the edges and causes an significant impact any crack the crack will run away from the 'compression zone' where the glass is thicker and stronger and turn toward the edges where the glass is 'stretched' and more brittle. Our way of using the demister is more likely to prevent a thermal fracture but would not protect us from an 'impact fracture' - i.e. being hit by a large enough stone , or, a small stone at extremely high velocity. As Subaru's have extra strong laminated windscreens (always insist on a genuine Subaru windscreen replacement) 'impact fractures' are also lessened.
His advice was to use the demister as little as possible and when the windscreen is 'clear' switch to the internal 'face/hand' heating setting in combination with windscreen demisting as this will enable a more consistently spread windscreen glass temperature and lessen the chance of cold edges where the glass is stretched and brittle.
He also said to keep dashboards uncluttered and do not block airvents with things, especially dash mats or mounted applications such as Sat-Nav devices, reversing video screens or those large ornaments some people hang from their rear vision mirrors as all of these things interfere with the flow of warmed air. He pointed out how modern cars are now being built with Sat-Navs and reversing screens mounted into the dashboard instrumentation area as the makers had woken up to the dangers of dash mounted devices towards contributing towards windscreen damage.
Most importantly, avoid placing any 'stick-ons' on your windscreens (rego stickers are usually OK as they are especially designed for windscreens, minimal thickness and have a 'clear space' in them but if you can legally do so, put them on a passenger side window.)