How a Small Flame Can Reveal Whether Your Windows Are Truly Double-Pane

The listing may have promised energy-saving windows, or a previous owner may have said so. But if the bedroom still feels cold near the glass, there’s no frame label, and the documents are missing, it can be hard to know what kind of windows you actually have.

You do not need to remove the window to find out. A small flame and about a minute can give you a clue. Because each pane reflects light, holding a flame near the glass creates a row of tiny reflections that can be counted.

What the reflections show

An energy help center that describes the method explains the pattern in simple terms. Single-pane glass shows one flame. Double-pane glass shows two pairs of reflections, or four flames total, all the same color. Triple-pane glass produces three pairs, which means six flames.

Color matters too. If the second reflection, counted from the inside, has a faint violet tint, the glass likely has a low-E coating. That coating is extremely thin, but it helps keep heat indoors in colder weather and reduces solar gain, which can make indoor temperatures more stable.

The impact on energy use can be significant as well. In one Department of Energy lab test, adding low-E storm panels over double-pane clear windows cut annual heating and cooling energy use by about 10%.

How to try the lighter test

1. Darken the room. Turn off nearby lights so the small reflections stand out more clearly on the glass.
2. Hold the flame near the window, but do not let it touch. Use a lighter or lit tealight and look into the pane, not directly at the flame.
3. Count the reflections. One flame points to single glazing, four to double glazing, and six to triple glazing.
4. Inspect the second reflection. From the inside, see whether it has a slight violet cast compared with the others.
5. Try again in another light. Because colors can shift, repeat the check in daylight or with a different lamp before drawing a conclusion.

If you would rather avoid using a flame near the glass, start with the manufacturer’s information. The help center that outlines this test recommends checking product details first when available, then using the flame count as a quick visual confirmation.

When the test can give a misleading answer

This is only a visual check, not a laboratory test. The color of the reflections can change depending on the room light, so a violet tint seen at night may look different during the day.

Older windows can also confuse the result. Some have been fitted with thermal coatings after installation, which can alter the color of the reflections without the window itself being new.

Use caution with the flame, too. Keep it away from curtains, blinds, and anything else that can catch fire, place a tealight on a stable surface, and never leave it unattended while lit.

Count the reflections, check the color, and you can get a better sense of what kind of windows you are living with. That is the basic trick.