One of the questions that I am frequently asked by homeowners and builders alike is " what is the difference between a direct set and a sash set window?"
The short answer is simple; a sash set window is a piece of glass that is placed in a frame, and then that frame is placed in another frame. This outer frame is also the "jamb" of the window. All operating windows are automatically sash set, the sash being the moveable part of the window. Some fixed windows are also sash set. A direct set window is a piece of glass that is stopped into a frame, also known as the jamb. All direct set windows are fixed. No operating window is ever direct set.
Since we have determined that all operating windows must have a sash, the remainder of this discussion will be about non- operating or "fixed" windows.
When considering whether to use sash set or direct set fixed windows, here are a few considerations to help make the best choice.
Cost - under most circumstances, custom sized direct set units will be more economical then sash set for the simple reason that there is less material involved because there is one less frame. However, when using a standard size fixed casement or a standard size picture window, the fact that they are standard and not custom sizes will usually equate to a lower price. This is because the material for standard sizes are usually inventoried in bulk by the manufacturers. Modifying material for a custom size results in more labor, and many times, waste of materials and this will be a cost driver, particularly when multiple custom size windows are used throughout a project. Custom size direct sets when compared to custom size sash set windows however, can be significantly less money.
Siteline alignment - direct set units are not always appropriate particularly if maintaining a vertical or horizontal siteline is desired. If you want all the daylight openings to align on either plane, then you will only achieve that with a sash set window. Below is a photograph of a project currently being built where the decision to spend the extra money (and in this case, it was a pretty substantial upcharge) was obviously a wise investment. Notice how the stiles (the pieces of wood that make up the vertical part of the sash) perfectly align with the stiles of the door below. Because the grill pattern is three lites wide, if they had not had a sash in that frame, the rectangles would have all been different sizes or the bars would not have aligned with the bars in the door below.
This next shot is of a direct sets mounted above other direct sets. Notice that the framing is more narrowline than the previous picture. However, all the rectangles are similar in size and the SDL bars are in alignment with the units below.
There are plenty of times that a direct set works fine, particularly if there are no grills in the windows, or in many cases, a 2 wide grill pattern takes "the sting" out of the different size rectangles because at least the one vertical bar will align with the vertical bar in the window below. Another good use of a direct set window is when the direct set has a grill, but the window below it does not. This is a good illustration of that scenario.
Size - there is a limit to the size that sash set windows can be made and that is because the frame that the piece of glass is glazed into is not substantial enough to handle the weight of larger windows. This is compounded by the fact that larger pieces of glass are many times thicker pieces of glass and are required to be tempered, substantially adding to the weight, so it needs the heavier frame of the "jamb" to carry the load.
A good window professional should help the client explore these options and their ramifications.