Except in a couple of rare instances, these two types of windows are mutually exclusive. The choice is clear-cut as to which window you should choose: new-construction windows for remodeling and new construction when the window frame is being built or replacement windows for applications when an existing window needs to be removed.
New-Construction Windows vs. Replacement Windows
this is a new housethis is an additionthis is a severely damaged wall that needs much repair workthe wall is being substantially rebuilt for any reason
Building a New House
Building a new house is the perfect example of new construction; thus, the need for new-construction windows. There is never a need to use replacement windows since no windows are being replaced. All new homes use new-construction windows.
Building an Addition
An addition is a smaller version of a new house. Unless you are dealing with a very unusual circumstance, you will almost always use a new-construction window with additions. The only time you may not want to use a new-construction window is when you are incorporating a portion of an existing wall into your addition.
Replacing an Extremely Damaged Window
When the window is in bad shape, it almost always follows that the peripheral area is in bad shape. The framing around the window, exterior siding, interior drywall, wall studs, and even floor covering and subflooring may have been compromised by termites, rot, or carpenter ants. Even if it is just a matter of removing the exterior window trim, new-construction windows are still the better option. Because you will need to perform extensive carpentry work, it will be just as simple, easy, and cheap to install a new-construction window as it is to put in a replacement window. A new-construction window will be more substantial and will provide better insulation from the elements than a replacement window. Because of the attached nailing fins, it will be more weather-tight than replacement windows.
When to Use Replacement Windows
Generally, use replacement windows when the wall has already been built and cannot be disturbed in any major way (other than removing the existing window and replacing interior trim and exterior casing). Replacement windows are best if:
this is a one-for-one replacement of an existing window this is not a new building under construction it is a new-construction window with nailing fins designed to be detached the wall must stay in place as much as possible
Replacing an Existing Window
Installing a new-construction window in a good wall will create more damage and work for you than installing a replacement window. Window, wall, trim, and siding elements are all connected; when you alter one element, you alter other elements. The great value in using replacement windows is that they are designed to slide in horizontally while replacing a minimum of other elements. Usually, replacement windows do require some minimal, temporary alterations, such as the removal of interior trim. However, as long as the trim is carefully removed, it can usually be replaced on a one-for-one basis. Some painting is usually required after the trim has been replaced.
Using New-Construction Windows With Detachable Fins
If new-construction windows have molded fins and replacement windows do not, does it stand to reason that you can remove the attached fins on the former and turn them into the latter? While this is not common, in a few cases it is possible. Some brands of new-construction windows have fins that are designed to be snapped or cut off. This provides the greatest possible utility to these windows. Generally, it is not advisable to cut off molded fins from new-construction windows that do not have purposely detachable fins. You may compromise the window’s structure or break the glass in the sash while cutting. Also, it can be difficult to smooth down the outer edges of the window enough for the window to tightly slide into place and remain weather-tight.
