Will Hot Water Crack A Frozen Windshield?

Will hot water crack a frozen windshield or windscreen? We go through why you shouldn't pour boiling water over your car's windshield.

Picture this. It's 6am and you've just woken up to get ready for work. You look outside and see your and your neighbour's cars have frozen over due to the winter weather. This is definitely not a rare occurrence for those living in the UK. You now have multiple options to thaw it. 

1. Wait for the ice to thaw naturally. Not recommended if you're in a rush!

2. Pour warm water over the windshield

3. Scrap off the ice using a specialised squeegee or brush

4. Use your car's heated windscreen technology to thaw the ice

5. Use a de-icer spray to remove all of the ice off your windshield

Did you know it's illegal to drive if your windshield and other windows are obstructed with ice? Rule 229 of the Highway Code states that your "windscreen and other windows must be clear of ice, snow and any obstructions". Failing to follow this rule can result in a £60 fine and up to three points on your licence due to endangering other road users.

What are car windshields made of?

Early 1900s car windshields were made with ordinary glass, but this caused injuries during crashes as the glass shattered. Both modern cars, vans and any other vehicle on the road have windshields made of laminated glass, which is a type of safety glass that holds together when shattered. These windshields have interlaced polyvinyl butyral (PVB), ethylene-vinyl acetate (EVA), or Thermoplastic Polyurethane (TPU) which helps to bond the laminated glass together and gives it high durability and strength. 

Will hot water crack a frozen windshield?

This is a common question that many people ask during the winter months. And the answer is simply no, pouring warm or slightly hot water over a laminated glass windshield can not crack it. This is because of how laminated glass is created and protected by the materials mentioned above. 

So why do experts recommend us not to do it?

This is due to how water can expand when frozen. If you have a small unnoticeable crack on your windshield and you continuously pour water over it, it will become filled. When refreezing happens either during the day or at nighttime, the water stuck in the crack will freeze and expand, causing the small crack to get larger and more noticeable.

If this process happens enough times your windshield will need to be repaired. This is the same process that happens with potholes on the road, as they are constantly subjected to rain and freezing conditions which causes them to expand and grow larger. 

Using boiling water straight from a kettle is not recommended either, as it can cause thermal shock which can cause damage to the windscreen.

So what's the easiest solution to thaw a frozen windshield?

Warm water is the cheapest and easiest way to defrost a frozen windshield. Don't use boiling water as it can cause thermal shock and damage it.

De-icer is also another easy solution to thaw a frozen windshield.

As a Ford owner, I personally find the quickest way is to switch on the heated windscreen function and spray some de-icer at the same time on the side windows. This thaws out the car within 5 minutes and is safe and ready to drive.

However, if you don't have the heated windscreen function (check out our which cars have heated windscreen technology post), the quickest and easiest way is to use warm water from your tap, or use a de-icer and ice scraper to spread the spray across all the frozen surfaces.