Cracked glass

Geyser · May 5

Hello. I recently installed Johnson Daylight Natural DN 50 on a couple of windows. Solar energy rejection is 39%. The windows are double pane and the glass is supposed to be tempered 4mm + tempered 6mm. The roof overhang shades the top 1/3 - 1/2 of the windows.

However, a few days after installation I noticed that one of the glass has cracked, presumably due to thermal stress.

Should I remove the film from the rest of the windows? I installed the film on 24 of these windows (6 tiny houses) so now 1 out of 24 is broken. Or is it more likely that I just got unlucky and the glass had a defect?

Is shading from the roof overhang such a big factor?

The glass cracked. Is it really tempered? I thought tempered glass would shatter, but not crack.

Thanks!

Tintguy1980 · May 5

The crack indicates thermal stress, however, the edge of the glass may have an imperfection (bur or clam shell chip). The crack does run 90 degrees out from the edge and curves as it follows any weaknesses in the pane itself. I have attached a pictured of what tempered glass looks like when it breaks and alongside it what a plate (or float) and heat-strengthened glass looks like when it breaks.

The outer panel my be tempered should indicate so by a stamp etched on the glass surface in one corner. It should indicate tempered. Plate glass has no indicators, however, heat-strengthened glass will crack in the same manner as plate glass. I'm an unsure whether heat-strengthened glass requires a stamp.

Tempered glass is 4 times stronger than plate and heat-strengthened is 2 times stronger than plate.

1 in 24 is indicative of edge imperfection.

I could not find the solar absorption rate of DN 50, but if it is similar to competitive films, there should be no issue with that installation setting (including a straight line shadow). This, too, points back to edge imperfection as the cause.

And finally, I spy drapes in a close position, correct? If so, it would be helpful to crack open during sun exposure; this to assist in allowing trapped heat between the glass and drapes to dissipate.

Edited May 5 by Tintguy1980

Geyser · May 6

Hey, thank you for taking the time to reply. It's greatly appreciated!

Ryker · May 6

Whatever that glass is that cracked it is NOT tempered.

DN 50 is pretty high solar absorption and your saying shadows too so you probably need a more reflective film.

Tintguy1980 · May 7

Follow up:

Attached is the performance data for Johnson's Resi/Comm window films and if you look to the bottom you'll see DN50 has a solar absorption of 39%, which is well below the threshold of concern which is generally accepted as 50% absorption (with no shadow). With a straight line shading from the roof this does increase the risk, which is usually low for straight line shading compared to other types. I've also attached a pdf of Johnson's film-to-glass chart which is used to determine how safe a film is. The thicker the glass the higher the risk; you say 6mm tempered and 4mm is the cracked one (interior pane).

The question now is what compass direction do these panels face?

In the end, I'll just suggest you hook back up with the person that installed it and get his input. He had to have been comfortable suggesting that film. I still believe it's a one off cracked panel and the rest will be fine. It takes 2 seasonal cycles to know for sure. Truly your call as to whether or not to replace with something lighter such as DN60.

Here's more helpful info that helps you question the glass type on the interior; is it plate or is it heat-strengthened (HS). I believe it is plate and not tempered or HS. Why?

Plate glass with suffer thermal shock if the edge temperature is different than the center of the glass by 50 degrees F.

Heat strengthened requires 100 degree F difference, and

Tempered requires 200 degree F difference.

Ryker chimed in and that stimulated further review. Good Luck with this. :thumb