• Glass inspection by a homeowner leaning onto a window

A Homeowner’s Guide to Glass Inspection

We wish glass was always perfect. But it’s not. So, we ask that homeowners start with that reality and go from there.

The makers of glass around the world know that their products have inherent imperfections. Therefore, it’s critical to carefully manage the expectations of homeowners, in order to avoid misconceptions and disappointments. That’s why the global glass industry has developed standards and specifications that address the quality of various types of glass used in windows and doors.

It’s important for homeowners to know that, as new types of glass proliferate, the global standards shift to reflect the new realities. Based on the inherent nature of the glass composition itself, each type has a different standard, whether it’s annealed, tempered, laminated, low emissivity, low iron, or triple pane. It’s no longer a case of one size fits all. Every type of glass requires a different method of inspection and has different criteria for blemishes or issues. Welcome to the roller coaster of glass inspection! (To go deeper down the glass distortion rabbit hole, view this blog post.)

inspecting home through a magnifying glass

In most cases, a homeowner will have a window or door that contains an Insulated Glass Unit (IGU) that is made with two or more panes of flat glass. As well, the glass will often have some type of low-emissivity (LoE) coating to boost thermal performance. When safety is a concern or there is a code requirement for safety glazing, the window may also contain tempered or laminated glass. As you can imagine, with these combinations of types in one window or door, sorting out the appropriate criteria for inspection becomes much more complicated. How is a homeowner to know whether an issue or blemish is an actual defect or not?

The following is a general guideline intended to simplify glass inspection for the homeowner. This is not a definitive guide covering all glass-related issues (that is the domain of authorities like the American Architectural Manufacturers Association). That said, we hope this guideline may serve as a reference for homeowners to identify whether an issue in their glass may be a defect or otherwise.

Homeowner’s Glass Inspection Procedure

  1. Thoroughly clean the glass on both sides with glass cleaner.
  2. View the glass from the inside looking out.
  3. Stand 10 feet away, directly in front of the glass at 90° to the surface.
  4. The glass should be illuminated with daylight, but not direct sun.
  5. Look at the background beyond the window, not at the glass itself.
  6. If an issue or blemish is clearly visible in these conditions, measure the size of the blemish.
  7. Take the measurement and compare it to the guidelines in the following tables. Contact the manufacturer if the blemish is noticeable and is outside the allowable limits or incidences.


Maximum sizeSeparation
Point blemishes (dirt, debris, residue, pinhole, spot, finger print, etc.)1/16″24″ min
Linear blemishes (scratches, rubs, marks, etc.)3″24″ min
Glass chip1/4” x 1/2”1 per pane

Other Issues

BowDeviation in flatness spanning the entire pane of glass. Commonly seen in tempered glass and IGUs. Not measurable on site or in an IGU. Positive and deflection is a natural occurrence with seasonal barometric pressure cycling.
DistortionLocalized deviation in flatness that can look like ripples across the glass, or pockets of indentations.Allowed and very common in tempered glass. Not measurable on site or in an IGU.
Strain patternAn optical effect that results from the tempering process appearing as a pattern of dark spots on the glass. The intensity of the issue increases when viewed at steep angles to the glass and with polarized sun glasses. Allowed. This optical issue is characteristic of tempered glass and cannot be completely controlled. It will vary from pane to pane.
FringesAn optical effect that appears as a faint, random, pattern resembling an oil stain. Allowed. This is the result of having exactly matched thickness of glass panes in an IGU (less than 0.0001″ difference in thickness).
Newton ringsThis is an optical issue that appears as concentric rings of rainbow colors near the center of an IGU that has its panes touching each other. There is no industry standard covering this issue. Contact the manufacturer.
Color uniformityReadily apparent glass color variation from window to window, or within a window. It can be the result of the glass tint or glass coating. Allowed within reason. Can be measured with special instrumentation if needed. Consult with manufacturer.
Suction cup marks and label residueTelltale impressions on the glass where a suction cup or label was applied.Allowed if on the outdoor or indoor surfaces of an IGU, and seen only when the glass is wet with ran or condensation. Not allowed if the marks are inside the IGU and noticeable at 10 feet or more.
FoggingA moisture or chemical deposit between the panes of an IGU.Not allowed. Indicates seals have failed or outgassing of materials has occurred.
Sightline infringementAn extension into the daylight opening of an IGU by the sealant, spacer, or area of coating deletion.1/8″ maximum.

Every glass maker, supplier and window and door fabricator wants to deliver satisfaction to homeowners. However, homeowners also need to be aware of the inherent realities of glass, and that beautiful windows and doors can be made with glass containing slight imperfections. Still, the size and/or number of acceptable imperfections are governed by industry standards, which are agreed upon by manufacturers, architects, and builders alike. While glass inspection isn’t always straightforward, we hope this guide may help homeowners determine whether their glass has reached these standards of quality.


Ready to elevate your architectural experience?