There are actually quite a few differences, so maybe this is a good time to do a series of articles on the important basics that anyone who is considering window replacement should know before they start shopping around.
There are two categories of windows available to retailers: (1) “new construction” windows are the ones you see at the hardware store, and (2) “replacement windows” are what you can get from reputable companies like us. And I don’t use the word “reputable” lightly… it is essential that you understand the difference between them because there are some less-than-honorable window dealers who will be looking for an opportunity to rip you off.
So lets start with a description of the two types of windows:
“New Construction” windows can be found at the large hardware stores and are manufactured for those places where the walls and framework are not yet finished, such as the construction of a new building or house, or when a wall is being completely rebuilt or reframed. If your house still has the original windows, then that’s what you have. “New construction” windows are manufactured in bulk in predetermined stock sizes; they are then stored away in warehouses so builders can buy the sizes and quantities they need “off the shelf”. Once they are delivered to the construction site, the builder can make sure the windows are a secure fit by positioning each window into the newly framed wall first, and then adding the last of the load-bearing wood to make sure the frame wood is up tight against all sides of the windows to allow for proper sealing. If you go to a big builders supply or hardware store that has a display of these windows, you can recognize a “new construction” window by the presence of nailing fins sticking out of the outside of at least two sides of the window frame; these fins are what the builder nails to the frame wood/studs to secure the window in place.
“Replacement” windows are designed to work the opposite way: to precisely fit into a wall and window opening that already exists, such as a finished wall that cannot be re-framed to fit the shape of the new window. In those cases, the window must be customized and special ordered in precise dimensions to fit the shape of the frame. For that reason, these windows do NOT have nailing fins; instead they are manufactured to be attached by long screws that go through the window frame into the load-bearing wood. This customization is necessary because the structure of houses and buildings shift over time, so the shapes of the window openings can have slight changes, too.
For example, a window opening that was originally built to be a perfect rectangle has probably shifted by ¼ to ½ inch in every direction. That may not seem like much of a change, but its enough to guarantee that if you try to install a perfectly rectangular “new construction” window in an opening that has shifted there will be gaps around the outside of the window that will result in air and water leaking into the wall and the house. Or, some part of the window opening could have possibly NARROWED, and could cause the uneven frame wood to put excessive pressure on the window and quickly make it difficult (if not impossible) to operate, or even cause the glass to crack.
To prevent these problems, it is necessary to make precision measurements of each and every opening, measuring all four sides and the angles in between to within approximately 1/8th of an inch each way, so that a precisely-measured custom window can be made specifically to the dimensions and shape of each window opening. That means that no matter how many windows are in your order, no two windows will be the same size and shape… each one must be custom-made, one at a time. That means the manufacturer must re-tool for each individual window, so each one requires a little more care and precision to manufacture and install, and that makes them a little more expensive. But the advantage is that it will guarantee that each window will fit snugly inside the opening it was made for and will still allow room for minor movement of the house in the future.
“Replacement” windows also have a little different appearance than the “new construction”. As you can see in the picture below, the frame in the window is heavier and slightly wider so it can effectively hold the weight of the extra-thick glass and other energy-efficient features that we’ll discuss in other articles.