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October  2004


Don't Want to be Bugged?
Then Don't Forget to Consider the 
Variety of Options Regarding Insect Screens
by Steve Ulsh

As a window manufacturer you know that insect screens are an important component of a quality window. A good insect screen is one in which reasonable insect control is provided, and that, in essence, is its only function. An insect screen is not intended to be a security or a retaining device (note that security screens are another product offering entirely, and will not be addressed here). The screen industry has spent much time and money over the past two decades educating the public about what an insect screen will, and won’t, do. By eliminating the term insect from insect screen in the remainder of this article, my intention is not to undermine those efforts. It is done for simplicity only. 

Consider the Hidden Costs 
A screen that is designed to fit well, have excellent retention and install easily into a window can be an asset in marketing and, consequently, in selling a window product. Although the screen itself usually comprises just 10 percent or less of the actual window cost, the effects of poor screen applications can be very costly in the long run. Thus, the screen, often considered to be a minor component of the window, can become a major liability if sufficient thought is not put into its design. 

Some of the adverse screen situations that may occur in the field, and ultimately cost money, include screens that are too difficult to install and/or remove. Many homeowners still prefer to remove their screens during periods of colder weather when they can’t be used for ventilation purposes. Removal of the screens during the fall allows for a clear and unobstructed view. As winter turns to spring and the weather again lends itself to ventilating, consumers place the screens back into their windows. This requires a screen application that allows for easy insertion and removal, to avoid homeowner confusion, not to mention homeowner frustration. 

Screens may also fit too loosely within the window. Both the ease of installation and the fit of screens can be affected by the choice of hardware used to retain the screen within the window. The hardware can also allow for greater flexibility in the size relationship of the screen to the window. In this article, we’ll briefly examine the common types of hardware used for screens and when each one typically is specified. Further, we’ll explain the importance of tolerances when determining correct screen sizes for any window application.

Hardware Options
First, let’s take a look at the most common types of screen hardware, and the window applications with which they tend to work well. There is a multitude of types of hardware available to help retain screens into the windows they’ve been designed to fit. Specifying proper hardware can also help to reduce potential screen rattling. The most common types of hardware include plungers (also known as retractable pins), tension springs and various clip configurations. Of these, plungers are the most frequently used. 

Plungers are spring-loaded pins that protrude from the screen edge and often are used to engage slots or holes that have been fabricated into the window frame. A standard plunger assembly consists of three parts: the plunger itself, a spring for creating pressure and a cap for grasping in order to retract the plunger. Screen retention occurs when the plunger is extended (or in its “relaxed” position) and the plunger tip penetrates into the window slot, securing the screen with the window frame. With the plunger retracted, accomplished by pulling on the cap at the opposite end of the plunger tip thereby compressing the spring, the screen can easily be installed or removed from the window. Plungers are available in varying lengths to affect window engagement, different diameters to accommodate several slot sizes and they may have special tip configurations. These special tips often have different degrees of taper that provide a lead-in for easier insertion of the plunger into the window slot. Plungers may be made of materials that include aluminum, zinc and plastic. 

Tension springs, on the other hand, are often utilized on a screen designed to fit a window that has two pockets into which the screen can be retained. These pockets generally are parallel to each other, and are commonly located on both of the window jambs. With this screen design, tension springs are applied to one of the screen sides on the screen edge. The screen is then installed by first inserting the spring side of the screen into one of the window pockets with pressure, flattening the springs against the base of the pocket. This allows the other side of the screen to be inserted into the opposite jamb pocket as the pressure on the spring side of the screen is released. Thus, the screen becomes retained with tension inside each pocket. External corner keys with stops, which only allow the screen to penetrate into the pocket a pre-determined depth, are often used on the side of the screen opposite the springs. This helps to center the screen between the two retaining pockets. Other-wise, the spring tension would force the screen to be imbedded entirely in the non-spring side screen pocket, causing the screen to be installed deeper into that pocket than in the opposite pocket. As far as material is concerned, most tension springs are made of steel, and they usually have an arced-type configuration.

Next, we have clips. There are many variations of clips and a myriad of applications of clips to screens, and, subsequently, to windows. Because of this, clips and their applications are too numerous and often too specialized to allow for an adequate explanation here. Suffice it to say that, in many cases, these special pieces of hardware are designed for a particular screen to fit a particular window. Therefore, this unique hardware may not necessarily work on any other manufacturer’s window product except the one for which it was created. Contrast that to plungers and tension springs, which are considered a commodity hardware item. The same plungers and the same springs may be applied to many different window designs and be equally effective in those various applications. Whereas plungers and springs are utilized specifically in the edges of the screen, clips can be installed in the edges of screens as well as on the face of them for retaining purposes. This adds to the versatility of the clip as a screen hardware device. Clips are frequently made of plastic, aluminum or steel, and are often used to retain the screen by penetrating a kerf in the window.

Those three types of hardware comprise the majority of screen applications being used today. There are certainly additional kinds of hardware, as well as other window designs in which the aforementioned hardware devices can be used. Whenever possible and practical, a superior screen application provides for screen retention on all four sides of the window. The appropriate use of hardware can help accomplish this level of retention.

Screen Vs. Window Sizes
Now let’s take a quick look at screen sizes in comparison to window sizes. When determining the proper screen size to fit a window, it is extremely important to keep tolerances in mind. This applies to both the window and the screen tolerance. Invariably, some windows will be made slightly larger than the specified size, and some slightly smaller. The same theory pertains to the window screens themselves. Most windows are made to within a +/- 1/8-inch tolerance, and screens usually are held to an even tighter tolerance of +/- 1/16 inches. So, by applying some simple arithmetic to the worst-case scenario, it is possible to have a combination of a window that is 1/8 inch larger than the specified size, and a screen that is 1/16 inch smaller than the specified size. Both components would be considered acceptable and within their allowable tolerances, but we now have a situation in which the screen-to-window-size relationship is different by a total of 3/16 inch from the nominal size. And yet, the window and screen must work in tandem to have a viable application. In this case, the screen is much smaller than the window, which can lead to a screen fit that is too loose. This situation may cause gaps between the screen and the window that allow insects to enter. It may also cause the screen to rattle during windy conditions. Or even worse, it may cause the screen to become dislodged from the window.

Conversely, the same principle can be applied in the opposite manner, in which we have a window that is 1/8 inch smaller than the specified size in combination with a screen that is 1/16 inch larger than the specified size. Again, both the window and the screen are within their individual tolerances, and, the larger screen must still fit into the smaller window properly. In this case, the screen is much larger than the window. The result is a screen that is difficult to install and remove due to the tighter fit. This is probably the biggest mistake that is made when determining the correct screen sizes to fit into window—that is, specifying screens which fit too tightly initially. 
Of course, window manufacturers and screen manufacturers can agree that tighter tolerances be held for their products. Reducing the allowable range of acceptable finished sizes for both the screen and the window may help to minimize the problems associated with proper screen fit. It may even allow for the use of additional hardware options. However, reducing tolerances can have negative effects on production and the bottom line. Generally speaking, as the tolerance becomes tighter, more units have to be rejected for exceeding the acceptable guidelines. This increase in defects can become costly. 

Unfortunately there is no real rule of thumb that can be applied when determining the ideal screen size to fit a particular window. After all, when the factors of actual window design, screen hardware chosen and tolerances are considered, every screen to window application should be handled individually for best results. The goal is to specify a nominal screen size that will fit adequately when the screen is its largest and the window its smallest, and vice-versa. It is wise to plan for these worst-case scenarios in screen-to-window-size relationships, because inevitably, they will occur. 

Points to Remember
In summary, the ideal approach to take when considering a screen application is to first determine the best type of hardware to use to retain the screen. After the hardware has been specified, determine the optimum screen size to fit the window based on tolerances. The best screen to window applications are those in which both the tolerance of the window and the screen are considered when determining screen size. Understanding the effects of these tolerances can help ensure that screens rarely fit too loosely or too tightly with the windows into which they are applied. Achieving this balance can become a formidable challenge to the window designer. But, exceptional window design takes into account the application of the screen when the window is first being conceived, rather than as an afterthought. To this end, most screen suppliers are willing to assist the window manufacturer in developing an appropriate screen in the early window design stages. This assistance should include hardware recommendations in addition to tolerance considerations in order to attain an acceptable screen fit.

Ultimately, it is the marriage of hardware choice with the proper determination of screen size to 
window size that combines for a successful screen application. 

So, the next time you’re bugged take a close look at your screens. If they fit well and the hardware retains them, you probably shouldn’t blame your screens for the sudden influx of creepy crawlers. Just make sure to go in and out of that front door a little more quickly. 

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