Building a Future for Distributors and Dealers of Building Products

March  2005                                Volume 44,  Issue 2

Jobsite Advice
Avoid Making Mistakes in Window Installation
by Abe Gaskins

Our company—MGM Industries—learned how to install windows the hard way: by being called to the construction site and remedying the fenestration product that was poorly or inadequately installed. We learned from it, but we still see the same mistakes being made by others in the installation process.

Time after time, our service people go to a jobsite to solve an installation problem only to hear, “I’ve been installing windows for years and these windows are bad. Here, I’ll show you. Let me slap this framing square into the corner of this window, it will prove beyond a shadow of a doubt that these windows are inferior.”

We’re certainly not alone in trying to eliminate poor installation practices. The industry as a whole has worked to address this issue. In 2002, ASTM International approved the ASTM E2112 standard designed to outline proper installation practices.

In coordination with that effort, InstallationMasters™ Training and Certification Program educates builders, contractors and installers about the proper procedures that need to be followed.

This article isn’t designed to replace any of that information. Instead, it focuses on some of the common mistakes we see out in the field and offers some simple guidelines on how to avoid them. In particular, it looks at some of the mistakes commonly made when installing vinyl windows. One of the biggest reasons we see mistakes in the field is the fact that many installers have worked with wood or other types of windows for years and don’t understand the unique characteristics of vinyl products.

Relying on the Jambliner
Sashes that tilt-in for easy cleaning are a feature promoted heavily by the vinyl window business, and now are a “must” in most current and future new construction window designs. This feature has brought an added convenience to the homeowner, but has added complexity to the installation process.

Let’s describe the typical window installation. The installer removes the sash and positions the window frame into the rough opening. In this way, one person can do the complete installation without the use of a ladder from the inside of the house. With the sash removed, the installer takes a framing square and squares the window in the opening. The installation is nearing completion as the window is nailed in place. Lastly, the installer “drops” the top and bottom sash into the window. “Voila!” The window installation is complete in less than 20 minutes.

This is a tried-and-true technique for installing wood windows, but it will not work when installing vinyl windows. It does not work, for that matter, for any window that does not incorporate compression-style jambliners. Windows featuring such jambliners typically have a 1/2-inch of compressible foam affixed to the backside of the vinyl jambliner. When the window is installed, the jambliner makes up for any problems created as a result of the manufacturing process or the process of installation.

This is a great answer for all installers as it makes installation quick and easy. It also results in thinking that no specialized skill is required to install windows. It creates significant problems for vinyl windows, however. It can even affect the function of wood windows with jambliners. Often, sashes are nearly impossible to tilt-in because the jambliner locks them into the non-tilt configuration.

Conceptual Change
When installing vinyl windows the installer needs to make a conceptual change to install the window correctly. First, forget about the framing square: instead focus on the “sightlines” of the window. By this, I mean all the interfaces between the sash and the main frame. All gaps between the sash and frame should be consistent and even. The interface between the bottom sash rail and the mainframe sill should be parallel and both members should not be bowed in any way. Likewise, the interface between the top sash header and the mainframe header should be parallel and not bowed. The meeting rails should not be skewed. This is easily determined by raising the lower sash slightly and verifying that the rail of the lower sash is parallel to the bottom rail of the upper sash. Finally, the mainframe jamb should not be bowed and the sill should be straight. This description is what we refer to generically as “sightlines.” 

Allow me to introduce an engineering term called “creep.” All materials experience this phenomenon as a result of the external forces applied to them. It is as simple as the moniker implies. It means that material will creep or move opposite to the direction of an external force in order to lessen the internal stresses developed as a result of that external force. As an example of creep, did you know that glass would sag in a window as a result of gravity pulling the individual glass molecules down?

In the case of glass, it might take two or three lifetimes before the sagging would be noticed. In the case of PVC, the sagging or creep can develop very quickly as a result of the forces applied externally. Typically, it will take two or three weeks before vinyl will creep and relieve these forces. File this concept away for when we describe how this can be a bane to the homeowner, the contractor, the distributor and the manufacturer.

Now, with these ideas in mind, let’s go through the various stages of a window installation and look at some of the more common errors made along the way and steps that can be taken to eliminate them.

Before Beginning
First, the rough opening needs to be checked. It needs to be at least 1/2-inch bigger in width and height than the window unit that is being installed. Our company recommends 1/2-inch, but if the opening is crafted carefully to 5/8 inch, it is actually a better allowance on the width. Be sure you know what your window manufacturer recommends. 

Secondly, inspect the window for squareness and workmanship before the window is earmarked for installation. Check that the window is free from defects. Verify that the factory’s workmanship is up to par. If the unit is mulled, check to see if the unit is caulked correctly at all joints. 

In order to make the window manageable, sashes are often removed prior to installation. Be careful because you can damage the balance system if you are not familiar with the type of the window you are installing. Note very carefully how the sash tilt-pins engage the balance shoe. Spend a few minutes learning how the sash/tilt-pin is configured and learn how to “drop” or install the sash correctly back into the window. Do this before attempting to install the window.

Sighting the Frame
Prior to installing the main frame into the opening, we recommend a bead of silicone on the header nail fin. Other manufacturers may offer their own specific instructions regarding application of sealant to the nail fin before installing the window frame. Following the manufacturer’s installation instructions here is a key step in preventing leaks. 

We occasionally see windows that have been shimmed in the center, causing the sill to crown immediately or two or three weeks later as a result of creep. To avoid this, shim the window underneath the mainframe jambs of the window. If the rough opening sill plate is crowned, the window will have to be shimmed up enough that the window itself does not crown as a result of the rough opening sill plate. 

At this point, the installer should begin to “sight” the window. Too often, we see this process started after the brick or drywall is installed. If the window sightlines are determined to be wrong that late in the game, you will have no choice but to rip the window out of the opening and re-install the window.

It is much easier and more cost-effective to do the sighting and adjusting early. Use your eye and verify that there is not a crown in the sill. If you see a crown, shim the window more. Shim until the crown is removed from the window. We recommend that the window be screwed into the opening temporarily. Put a screw in the top left corner, the top right corner and in either of the left or right bottom corner of the window. Now go inside the home and install the sash into the window. Take particular care to install the sash tilt-pins correctly into the balance shoe. If the pins are installed correctly, this sets the correct gap between the sash and the mainframe to insure ease of window operation. Now you can sight the meeting rail and the sash/sill and sash/header interfaces. All interfaces should be parallel. 

Think about how an exterior or interior door is installed. You are looking at sightlines. You are not looking for squareness. Look to see that the sightlines are correct. 

Adjustments may have to be made because of sightlines both inside and out. If the sightlines are wrong you must unscrew the lower screw and move the sill left or right until the meeting rails and interfaces are parallel. A 1/4-inch movement will make a huge difference in the sightlines. Sometimes the sill may have to be moved laterally 1/2-inch. If you do not have enough room in the rough opening you may have to remove all screws and re-position the window in the rough opening in order to have enough room to adjust the window correctly. 

The need to move the sill left or right (or re-position the entire window) is precisely the reason to make a final check of the window before the brick or drywall is installed. It is easy to adjust the window at this point. 

Now re-sight the sill for crowning. After this, you can sight sash and main frame interfaces. Again you are looking for consistent gaps. The gaps should be equal to the preset gap at the balance shoe/tilt-pin interface. Check to see that the main frame jamb is not bowed. If it is, it will be necessary to temporarily shim the jamb to eliminate the bowing. 

Nailing the Window
After all sightlines are correct, it’s time to nail the window in place. Our company recommends that nails should be on 12-inch centers or every other nail fin slot that is pre-punched in the window nail fin (each company has its own recommendations). One error we see sometimes is that nails on the jambs are driven “home” before temporary shims are removed. Temporary shims need to be removed, because they can apply pressure to vinyl (or other jamb materials) that may creep. Nails should be driven only partially first.

Temporary shims should then be removed. Finally, if the sightlines are good, the nails can be driven “home.” 

Another mistake installers make is attempting to toe-nail nails or drive the nails in at an angle. Sometimes installers will toe-nail windows because the rough opening is too big. Toe-nailing will cause the window jambs to creep and will result in bad sightlines two or three weeks later. Toe-nailing can cause such a stress in the window that the first time the sash is removed, the jamb will move away from the sash. This can result in large gaps, and, in the worst case, a sash that can not be reinstalled in the balance shoe. For the record, if the rough opening is too big, do not attempt to install the window. You will be sorry. 

Following this stage of the window installation, we recommend removing the sash completely and re-installing it. If the sash can be re-installed easily and the sightlines remain good, you have a good installation. We advise that the job superintendent do this final check. If the superintendent signs off, then it is reasonable to assume that the installation is good.

Flashing and Sealing
The next stage in the process is filling and insulating the area between the window and the rough opening. We recommend fiberglass for this application, but note that a number of suppliers offer low-expansion foams specifically formulated to fill the cavity between the window and the rough opening. 

Next the window should be flashed correctly. At our company, we have found that 90 percent of all service calls that are not installation problems are the result of improper flashing. Butyl tape or some other engineered tape should be applied to the nail fin and vapor barrier to guard against leaks. If a vapor barrier is not required by code, then flash the window with plastic all the way to the soffit of the house. Extend the plastic three feet on both sides of the window. Lap the plastic over the nail fin and not under it. Brick and siding is porous and will leak. Water will run down the sheeting of the house and leak between the nail fin and the sheeting if not flashed correctly. 

One final note for buildings that are going to be bricked, leave a 1/8- to 1/4-inch gap between the window frame and the brick, then seal it with caulk. Keep in mind that settling will occur. The gap sealed with caulk will give the window mainframe enough room to follow the wood without causing problems with window operation. 

If you are sure you’ve done everything correctly and there continues to be problems with the window’s function, get in touch with your distributor immediately and do not allow the building to be bricked or dry walled. If the house is bricked with a poorly installed window, I hope you realize by now that it will be very expensive to correct the window 

It’s also important to remember that vinyl has to be finessed into the opening, not just slammed in and nailed into place. Vinyl windows will provide the end user with easy operation, but the builder or contractor must give more consideration to the installation and the manufacturer, distributor and dealer need to emphasize that proper methods be used. y

Abe Gaskins is president of MGM Industries of Hendersonville, Tenn. His company has produced vinyl windows for the replacement and new construction markets since 1985.

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