Cold weather and a cracked windshield

[Jason S]

Due to job relocation, we moved to Kansas early last year. I've never lived this far north before and am still having to get used to weeks on end of highs below freezing. 

The day before yesterday, the high was 11 and overnight temp was near 0. 

Came out the next morning to start my '05 Chevrolet pickup to warm it up and noticed the windshield was cracked.  I presumed it was cracked all the way across the windshield. 

Today, I got to looking closer and it was three separate cracks, in line with each other.  The cracks were separated by about 1 1/2 to 2-inches directly over the partitions for the defroster vents.  Very odd...

[Dr_Snooz]

Do you suppose the defroster caused undue thermal stress? Very strange indeed.

[Jason S]

Do you suppose the defroster caused undue thermal stress? Very strange indeed.

It appears that residual heat coming up through the defroster vents caused localized 'hot' spots.

I shut off the truck around 6 pm the previous evening. I guess it's possible that as temperatures fell overnight there was enough heat in those three areas that the glass cracked.

At work, there is a 2005 Chevrolet pickup that experienced a similar occurrence last year. The windshield cracked during a spell of very cold temps.  However, the work truck had not been run for several days prior to cracking.

[enaberif]

Its very commonnd when it gets extremely cold for windshields to crack when the heat is turned on. I watched a 2" crack expand on a windshield once because of this very thing.

There was probably a rock chip or ding in the glass to compound this.

[fitz]

  A glass company in Mass runs radio ads telling people to run the heat on the floor vent (not defrost) until the interior warms up.  They say once the interior is warm, then turn on the defrost.
  They say that by letting the windshield warm up slowly you reduce the chance of turning stone chips into cracks.
  I lost a few windshields before I learned this trick.

[Jason S]

Its very commonnd when it gets extremely cold for windshields to crack when the heat is turned on

Possibly it can happen in reverse?  Vehicle off, no heat- but drop in outside temperature?

They say once the interior is warm, then turn on the defrost.

I can understand that; cracks from applying heat to cold glass...

All I can think is enough heat remained in the cab and vents with the truck off, plus cold and getting colder outside...  I still think it odd...

[VileZambonie]

rust or moisture will cause this

[enaberif]

Its very commonnd when it gets extremely cold for windshields to crack when the heat is turned on

Possibly it can happen in reverse?  Vehicle off, no heat- but drop in outside temperature?

They say once the interior is warm, then turn on the defrost.

I can understand that; cracks from applying heat to cold glass...

All I can think is enough heat remained in the cab and vents with the truck off, plus cold and getting colder outside...  I still think it odd...

Look along all edges of the cracks(s) and see if there are any pits or chips in the glass.

[fitz]

When it happened to mine there were stone chips that turned into cracks.
The vehicles had snow on the windshields and I let them run for extended periods with the defrost on high.
I lost 3 windshields doing this (I should have figured it out after the first time).
Now I just let the vehicles warm up with the floor vent on. It hasn't happened since.