The term glaze is accurate but somewhat misleading when it comes to windows. A glaze is something you put on top of something else. In this case, you’re adding a windowpane on top of a windowpane, so you’re glazing the glass of the window with another layer of the same material. That’s why double-glazed windows are also called double-pane windows.
In This Article
- What Is a Double-Glazed Window?
- Why Do Double-Pane Windows Have a Gap?
- How Much of a Difference Does Double-Glazing Make?
- Is Double-Glazing Worth It? The Cost of Double-Glazed Windows
- Benefits of Installing Double-Glazed Windows
- Disadvantages of Installing Double-Pane Windows
- What to Look for in Double-Glaze Windows
- Is There a Cheaper Way to Get Double-Glazing?
What Is a Double-Glazed Window?
A double-glazed window features two panes of glass instead of one. In colder areas, it’s common to even have triple-pane windows.
You might wonder if you could get the same effect as a double-glazed window from a single-paned window with thicker glass. The answer is no. Double-pane windows are more effective because not only is the total glass thicker than in a standard single-pane window, but there are also gaps between the sheets of glass.
Why Do Double-Pane Windows Have a Gap?
If you had a single sheet of glass or two or more layers of glass touching one another, the temperature of the warm air outside would transfer directly to the colder air outside. In other words, glass conducts heat energy. The result is that you would lose heat through the glass. That’s why when we touch a window, it’s colder than the wall. The wall has insulation to keep the heat in, whereas a single-pane doesn’t.
That’s where the gap comes in. The space between the panes of glass isn’t empty. It’s filled with air or other gases that act as insulation. The gas doesn’t conduct the heat energy as well as glass does, so heat doesn’t escape your home as much through the window glass if you have a space between the panes.
How Much of a Difference Does Double-Glazing Make?
So, the two panes of glass with a gap in the middle for air or other gas is supposed to reduce your heat loss, but how big of a difference does it really make?
When you talk about how well a material insulates, you want to look at the R-value. It’s the material’s resistance to heat transfer. The higher the number, the greater the material’s resistance.
The wall of a house filled with fiberglass insulation generally has an R-value of between 12 and 15, depending on the materials used to finish the outside and inside surfaces. A single-pane window has an R-value of less than 1. If you replace that window with a double-pane window filled with air, you more than double the R-value to 2.04. If you used argon instead of air to fill the gap, you would almost quadruple the R-value to 3.8.
So, even the best double-pane window only gives you an R-value of 3.8. That’s a lot less than the 12 or 15 level of insulation your wall provides, but it’s a lot better than a single-pane window.
You can increase the window’s overall R-value by adding thick curtains or window film. If you can, redirect vents away from the windows so that the warm air blows into the center of the room rather than blowing up toward the window.
Is Double-Glazing Worth It? The Cost of Double-Glazed Windows
So, if double-pane windows don’t give you nearly as much insulation as the surrounding wall, are they worth the cost? What’s the payoff of installing double-glazed windows instead of single-pane windows?
The cost for double-pane windows ranges from around $385 to $850 per window. That’s a big range, but it depends on the company you buy from, whether you want air or gas filler, and the material you choose for your window framing. Typical choices include aluminum, fiberglass, vinyl, and wood. Wood is the most expensive, with aluminum being the least. The cost of labor averages $38 per hour, so it depends on how many windows you need to be installed.
The cost to install a single-pane window is more like $275. Because the goal is reducing heat loss, and yes, the same principle applies when you’re trying to cool your home, you need to consider how much you’ll save in energy bills. According to Energy Star, people who switch from single-pane windows to double-glazing save between 21 and 31% on their heating and cooling costs.
For easy math, if you’re heating bill in the winter was normally $300 with single-pane windows, it might be as low as $210. That’s almost $100 in savings in one month. If you had 15 windows in your house and replaced them all with double-pane windows for an average of $620, your total installation cost would be $9300. Saving $100 on utility bills per month would mean the windows would pay themselves off in value after 93 months, or almost 8 years.
It’s certainly a long-term investment, but the energy savings aren’t the only benefit of installing double-pane windows.
Benefits of Installing Double-Glazed Windows
Money matters aside, what’s the benefit of double-pane windows in terms of your standard of living?
The first benefit is the added R-value of the window. There’s nothing as frustrating as sitting by your window and feeling a draft. Yes, your window will still feel colder than the rest of the wall, but replacing single-pane with double-pane windows will certainly make a difference in your comfort level. Remember, the best double-glazed windows decrease heat transfer by almost four times. That means sitting by your window will be four times as comfortable as with your single-pane window.
If you have single-pane windows and don’t have a dehumidifier running, you might notice droplets of water on your window or the wall surrounding your window. This is a result of the colder air around your window meeting the warmer air inside your home. When you replace the single-pane windows with double-pane windows, the heat loss isn’t as drastic, so there’s less condensation. Plus, whatever condensation does occur is usually confined between the two panes of glass, not running down the inside surface.
Just as that extra layer of glass and gas provide better insulation against air temperatures, so it provides more of a sound barrier. If you have noisy kids playing next door, a neighborhood dog that won’t stop barking, or heavy traffic on the road outside your house, switching to double-glaze windows can make a big difference. One study found that double-pane windows can reduce noise infiltration by 90%. Note that this somewhat depends on the framing material as well.
Rise in Property Value
Okay, back to money. Even if you don’t live in your home long enough for the savings in utility bills to cover the cost of installation, you can still recoup your expenses by charging your buyer a higher price for the home. Installing double-pane windows shows interested purchasers that you’ve made an effort to update the home and maintain it. They’re willing to pay more for a house that already has high-quality windows because this means they won’t have to install better insulating windows themselves.
Disadvantages of Installing Double-Pane Windows
The first obvious disadvantage is the added cost over single-pane windows, but why else might you not want to install double-glazed windows?
Need to Replace vs. Repair
If one of the window panes cracks or breaks, you can’t replace just that one layer. Because of the way these windows are built and installed, you have to replace the whole window. And, again, that’s a fairly large expense.
Even if there’s no problem with the window panes themselves, but a gap forms in your window frame, you want to replace the whole window. That’s because the gas between your panes has escaped through the framing, and so the window isn’t providing an effective insulation layer. In that case, you have two panes of glass, but you don’t have the benefit of double-glazed windows because you no longer have the insulating gas pocket between the panes.
So far, we’ve been focusing on reducing heat loss in the winter. You get the same benefit if you’re actively cooling your home in the summer. The heat from outside doesn’t penetrate into the room as easily as it would if you had single-pane windows. That reduces your air conditioning cost because the air conditioner doesn’t have to work as hard to cool the space.
But if you’re not using air conditioning, these windows can actually keep your house warmer because they don’t let the hot air inside your home escape as well. Again, it’s that whole concept of reducing heat loss. I would still recommend these if you live in an area with cold winters, but if you don’t have much cold weather, double-pane windows may do you more harm than good in terms of keeping your space comfortable.
What to Look for in Double-Glaze Windows
Now that you know how double-pane windows work and what they’re supposed to do, you’ll understand what to look for to ensure you’re getting the highest quality. Ask your installer about these features if they don’t include them in their documentation. Remember, double-glazed windows are a significant investment, so you deserve to know what you’re paying for.
Spacing between Glass Layers
That gap between the panes is where most of your insulation will come from. It’s the most important component of the window. Manufacturers design the windows to have a gap of between ¼ inch and ¾ inch, or 6 and 20mm. A half-inch or 12mm should be the minimum gap you accept. The bigger the spacing, the better the R-value of the window. Some manufacturers make slimmer gaps so as not to spend as much on the gas to fill them. Of course, the larger the gap, the more gas needed to fill it, so the more expensive the window will be to you.
A double-glazed window can have air or another gas between the layers. Air is the least expensive but also the least effective in reducing heat loss. It doubles the R-value of the window, but you can quadruple the value by using argon instead. Xenon is also a good, though less common, gas filler.
Gas will also provide better noise reduction and reduce condensation better than air will.
Type of Glass Coating
Your glass panes should be coated with a transparent finish. This is something you will never notice, and it doesn’t make the window any less difficult to clean. The coating simply adds another layer to the glass to provide an additional benefit. The benefit you desire will determine the type of coating you prefer.
A low-E coating is the best for reducing energy transfer through the glass. Silver coating does the same. Laminate is better for soundproofing. It is possible to have more than one coating on your windows, and many window manufacturers coat one side of the window in one material and the other side in another.
If your window doesn’t have any coating, make double sure that the gap between the windows is at least ½ inch and that it’s filled with gas rather than air.
Window Framing Material
Last but not least (coating is least important) is the material used for the frame. Wood is most expensive because of the work involved in working with it and is best used for older homes to fit the building style. Otherwise, go with vinyl or fiberglass for a durable, clean frame. Aluminum is the least expensive and is susceptible to bending.
Is There a Cheaper Way to Get Double-Glazing?
If you’re still not sold on paying for double-pane windows, you can consider the alternative of adding another pane of glass to your existing window. The secondary sheet of glass attaches to the frame using magnetic strips. You do still need to pay for the cut pieces of glass to fit your frame, but installation can be done yourself, so it may be a good DIY option.
Besides cost savings, the benefits of double-glazing your windows with this method is that you’ll be able to replace the secondary pane more easily than in an installed double-glazed window, and you can remove the pane to clean all sides of the windows and remove condensation. The downside is that you can’t fill the gap with gas. Your insulation will be air only, which doesn’t give you as high of an R-value, so your energy savings will be less.