The Simple Reason Why Your Window Glass Is Wavy

Amy Pawlukiewicz
Written by Amy Pawlukiewicz
Updated August 19, 2021
A sunny living room with vintage windows
Thomas Barwick/DigitalVision via Getty Images

And no, it’s not because something’s wrong

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You’ve bought a late Victorian home and you’re obsessed with the stained glass windows and elaborate, ornate design. But you’ve noticed something peculiar; the window glass has a wavy appearance. What gives?

Is There Something Wrong?

In short, no. While some people believe that the waviness is the result of the glass warping over time, the real reason for the wavy appearance has to do with the way glass was made at the time the home was built. 

There are two types of glass that were used in the 19th century: crown glass and cylinder glass.

Crown Glass

If you have a circular window in your home, it’s most likely crown glass. French glass blowers perfected this technique during the 1320s, when they used to make these windows in front of a furnace to keep the glass hot and fluid, forming its circular shape by blowing and spinning it. But the popularity of crown glass waned, as it could not be used in most types of windows because of its shape.

Cylinder Glass

Homebuilders began using cylinder glass in windows because it was more versatile than crown glass and could be produced in sheets. While cylinder glass was made with the same blowing and heating processes as crown glass, it was then flattened and smoothed out. So while cylinder glass is usually less wavy than its crown counterpart, some waves are still visible.

Should I Replace My Wavy Glass?

Because wavy glass does not mean the windows are damaged, you can rest easy and leave them as-is. Many homeowners love the charm of original windows in a turn-of-the-century home, and it can be a huge selling point. Also, you may be eligible for Historic Preservation Tax Credits for homeowners who restore and retrofit instead of replacing their windows.

Updating the Efficiency of Original Windows

House with vintage windows with a garden view
photonewman/iStock / Getty Images Plus via Getty Images

While original windows can look lovely, sometimes they don’t insulate a home as well as new windows do. The good news: There are several ways you can maintain the efficiency of your original windows without replacing them.

  • Maintain the seal between the window frame and the glass. If the seal isn’t good, your heat and air conditioning will escape through the gap, leading to higher utility bills.

  • Install curtains over the original windows. Heavier curtains can serve as a barrier between the air inside and the air outside, regulating the temperature of your home more effectively. Look for blackout curtains or energy-efficient curtains to maximize your savings.

  • Hire a pro to evaluate your windows and make sure there’s no damage that’s letting air through the cracks.

Wavy window glass is not a flaw—rather, it’s one of the ways your historic home tells a story.  These windows are unique in a way that is not easily replicated today.

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