Glass is great at conducting both heat and cold. Unfortunately, this means that single-paned windows easily conduct heat or cold into the home—and it’s never the one you want! When it’s hot outside, and you’d love your home to be a cool sanctuary, heat is transferred into the house through every single-paned window. Similarly, when the weather outside is cold, you want your home to be toasty, but the cold seems to pass through the windows like the glass isn’t even there.
In order to counteract this lack of insulation through windows, your HVAC system must work harder to heat or cool the house, raising your energy bills. So, if your HVAC bills are high, I would recommend checking all the windows in the house using one or all of the methods detailed below before calling in an HVAC specialist.
Check for a spacer at the bottom, behind the inner glass pane. Spacers mean two panes. Pinch the glass between thumb and finger. A close pinch means one pane. Hold a lighted match to the glass and view side-on. Two flame reflections mean two panes. Compare glass temperature and noise level to a known window.
Main Difference Between Single and Double Pane Window
There are four different and effective methods by which you can determine if your windows are single- or double-paned.
Three of these methods rely on the primary difference between the two windows: a single-paned window is a single sheet of glass, while a double-paned window is two sheets of glass with a spacer between them.
The last one relies on the most important result of this difference: better insulation from double-paned windows.
Method 1: Visual Check for a Spacer
The spacers in double-paned windows create spaces. These are air-tight and are either filled with insulating gases, such as argon and krypton, or vacuum-sealed (heat cannot move through a vacuum).
Typically, you should be able to see the spacer. Pretend like you know there are two panes of glass and look in between them. If the window is double-paned, then there will likely be a dark-colored strip along the bottom of the window, behind the first pane and in front of the second.
If the spacers blend into the window frame or if you can’t quite get the angle correct when peering down, then you can try some of the other methods.
Method 2: Pinch Glass Between Thumb and Finger
The second method to assess whether the window in question is single- or double-paned is to open the window and put your thumb on the inside of the window and your forefinger on the other side of the window (pinching the glass).
Make sure that your finger and thumb overlap so the distance between them can be more easily seen.
If there seems to be no space between your thumb and forefinger (almost as if they are touching), then this window is most likely a single-paned window. If there seems to be a space between your thumb and forefinger, then this window is most likely a double-paned window.
A single-paned window is just a single sheet of glass with no spacing system, so the distance between your fingers is very small so that they appear to be touching each other.
If double-paned windows were simply two glass sheets pressed together, then this method might be less effective. However, the presence of the space between the two sheets makes the difference in thickness between single- and double-paned windows relatively noticeable.
One downfall to this method is the fact that judging the space between your fingers is not an exact measurement. A thicker sheet of glass in a single-paned window might give you the impression that the window is double-paned, especially if you have another single-pane window with thinner glass to which you are comparing it.
A second limitation is that this method can only be applied to opening windows. If the window has no way of opening, then you cannot pinch the glass between your fingers.
A third limitation is if the glass is vacuum-sealed as opposed to double-glazed. A vacuum-sealed window is still double-paned, but the panes are much closer together.
Method 3: Hold Match to Glass and Count Reflections
The first two are the most common methods by which to distinguish between single- and double-paned windows, but I rather like this third one. Mainly because it involves fire so it seems more magical and sensational.
For this method, you will hold a lighted match to the glass and see how many flame reflections you see in your window. You will need to look from a slightly off-center angle, i.e., don’t look straight at the window. Rather stand to the side and look at it from that angle. If you were to stand directly behind the flame, then the reflections may overlap, making them harder to distinguish.
For a double-paned window, when looking at the match in the window, you should see two reflections of the flame. This is because the flame’s light is not only reflected off the inside pane but also the outside pane.
For a single-paned window, the flame will only have one reflection, since there is only one pane of glass to reflect the light of the flame.
This method doesn’t have to be done with a match if you don’t have any available. Most devices with a light source will show their reflections as long as they do not overlap too much to distinguish between each reflection. Alternatives that should work include a lighter’s flame, flashlight (not too big), or a laser pointer.
The best time of day to test this is in the early morning, evening, or night. This ensures that the reflections can be easily seen and are not obscured by sunlight.
Method 4: Compare to Known Window
If you have tried the previous methods but would like to know for certain that your windows are either single- or double-paned, you can compare them to windows in your home that you know for certain are either single- or double-paned.
Let’s assume you know that one window in your house is double-paned because the previous window broke and you replaced it with a double pane.
Comparing Glass Temperature
Feel the known window with your hand. Then, go and feel the unknown one.
In Winter, if the known windowpane is noticeably warmer than the unknown one, this unknown window is most likely single-paned. In Summer, if the known windowpane is cooler than the unknown one, then the unknown window is most likely not double-glazed.
The main confounding variable in this scenario is the location of the two windows, and it must be taken into consideration for this method.
If one is south-facing and the other is north-facing, then this can cause a temperature difference in the glass of the two windows, even if they are both single- or double-paned. Similarly, there are natural temperature differences between upper floor windows and ground floor windows or one window can be sheltered by a wall or a tree whereas the other is always in full and direct sunlight.
You do have to be aware that there are sometimes factors that make double- and triple-paned glass feel colder.
Comparing Noise Level
The noise level can also be tested. This should be done when the same constant sound is happening for both tests (for example, someone is mowing their lawn at about the same distance from both windows).
If the noise level near the unknown window is higher than the noise level near the known double-paned window, then the unknown window is most likely single-paned.
If the known window is single-paned, the opposite will occur.
If the known window and unknown window have very similar temperatures or noise levels, then it is likely that they are the same type of window, whether they are both single-paned or double-paned.