If a double pane window is better than a single pane window, should I use triple pane windows?
By Don Ames, www.detectenergy.com
This is getting to be an appropriate question, “Should a window retrofit use triple pane windows?” The vinyl window manufactures are getting so good at making energy efficient windows, it doesn’t take much for them to throw in another pane of glass or two. Wonder if we will be considering quad pane windows one of these days?
If you were really interested in saving energy and wanted the best wall construction possible, you wouldn’t put a window in the wall to begin with. A wall without a window has a greater R-value and fewer air leaks. We know how to build a wall that will separate the outdoors from the indoors - what we don’t know how to do is build a wall that will keep the outdoors out and the indoors in and still let light through. Come to think about it, our need to let light in is at the root of the problem.
Actually, letting light in is just one of the needed benefits of having a wall with a window. Being able to see through the wall is another benefit and being able to get out of a room during a fire is another. Energy wise, a door is not much better than a window, so I guess windows are here to stay.
A window with more than one piece of glass is a real blessing for saving energy and helping with indoor comfort. There is more to a vinyl window than just several panes of glass. Two panes of glass also come with an insulated frame, mounting flange, and better air seals. But the most important part is not the glass, but the space between the glass. Considering window performance, it’s the space between the glass that makes the difference.
Since there is more than one pane of glass, there is also a space in the middle, between the two glass pieces. The window manufactures now have the opportunity to put something between the glass panes besides plain old air. Low-E coatings can now cover one of the surfaces of the glass and be in the middle where it is protected. Low-E is a metallic coating that can reflect unwanted rays of the sun. The room stays cooler in the summer sun and the carpet won’t fade as fast.
Another use for the space between the glass panes is to hold a special gas that slows the transmission of heat. The name of the gas that is used makes the windows sound really efficient and high tech. How would you like to have triple pane windows with low-e coating and krypton gas. Just the mention of gas, let alone Krypton, makes the windows sound strong and indestructible. I feel warmer already.
It is the gas, krypton or argon, that is between the glass panes that allow the window to reach higher insulation values. With a triple pane window, there are two spaces to use for Low-E coatings and superman gas.
Let’s look at some comparisons concerning energy efficiency between different window types. Remember, the higher the R-value and the lower the U-value, the better for saving energy. Values listed are representative of an average range for that window type.
Replacing windows usually has a long pay-back period. The ROI ( return on investment ) is not encouraging. However, if the home is well insulated, air sealed, and the heating and cooling system is efficient, window replacement is a logical next step to increasing energy efficiency and saving energy.
In the part of the country where heating degree days out number cooling degree days, choose a replacement window based on the lowest U-value. Whether the window is double pane or triple pane, the U-value should be 0.30 or lower. We are looking for good insulation value here.
In the South, where cooling degree days out number heating degree days, choose a replacement window based on SHGC ( solar heat gain coefficient ). Here, the SHGC should be 0.40 or lower. With this window, we are looking to reduce solar heat gain so the A.C. unit does not have to work so hard or so long.
In the past ten years, we have seen building codes spec windows with lower U-values. The energy Star rebate program and the federal energy tax credits, both have continued to spec windows with lower U-values and greater efficiency. Ten years ago the new windows were fantastic at U-0.40. Five years ago, new windows were wonderful with a U-factor of 0.35. Today, most new windows on the shelf at your favorite building store will have a outstanding U-factor of 0.29.
In the near future, the efficiency ratings of windows will continue to drop. The only way a window manufaturer will be able to reach the energy efficiency ratings of the future is with three or four panes of glass. If you are seriously considering a retrofit window project, you should select the best window for your climate and that means the window will have three panes of glass.
Yes, three pane windows are more expensive than two pane windows. Shop around for the best U-factor and the best SHGC for the best price. If finances are the deciding factor, it will be better to install two triple pane windows than three double pane windows. Stick with the triple pane window and you will have energy efficiency set for the future.
P.S. Buy your windows and window installation from a local glass company or a licensed general contractor. Please don’t buy our windows from a super salesman at a home show that has all the financing you need and a worthless lifetime warranty to go with it.
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