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Installation Contradictions

These kinds of inconsistencies carry over into the installation instructions provided by various manufacturers. Here, for example, are some of the most striking contradictions we encountered when asking about basic installation guidelines: Should the sill be shimmed? Several manufacturers’ instructions recommend shimming between the rough sill and the window frame in two or three locations, just as with wood windows. Others recommend that their windows be installed on a continuous, level rough sill. "There is no guarantee that [a vinyl window] won’t sag in the center if you shim it in two spots," says Doug McDougall, a window certification manager at CertainTeed. "You’re better off providing full support under the sill." One manufacturer, Simonton, introduces a third option. It instructs the installer to use shims as with a wood window, then to remove the shims after nailing fins are fastened – leaving the window hanging by the nailing fins. Should the fasteners be driven home or left proud? The instructions from Willmar Windows recommend leaving the fasteners proud: "Do not over-tighten screws or set nails too tight. This will restrict the window from expanding and contracting." Installers of Jeld-Wen Windows, on the other hand, are instructed to use roofing nails driven all the way in. When we posed this question to other manufacturers’ technical experts, four recommended driving the fasteners home, while four others recommended leaving the fasteners a little loose. Should the nailing fin at the head of the window be left unfastened? Some manufacturers recommend fastening the nailing fin at the head of the window, while others warn against the practice (see, Figure 4). Some installers avoid the problem by driving a nail above the fin, then bending it down to pinch the fin. This procedure is also part of some companies’ installation instructions. Superior’s instructions, for example, say, "Do not nail through the nail fin at the head – put the nails 1/2 inch above the fin and bend them over the fin." But the nail head in this case can tear the building paper and interfere with other flashing and siding.


Figure 4. Comparing installation instructions from different vinyl window manufacturers reveals a jumble of contradictions. For example, some recommend attaching the fin at the top of the window, while others warn against it. The conflicting advice causes many installers – including the one who installed this window – to risk voiding the warranty by ignoring the manufacturer’s instructions.

Among the variety of reasons given for not fastening the head fin is one that might be called "the myth of the settling header." For example, CertainTeed’s McDougall says, "You don’t want the rough header to settle down and distort the window." A variation of this myth comes from Scott Clauss, product development engineer at Insulate Windows, who says, "If the house settles, it will pull the sill down, but the head will stay." Given a properly sized and installed header, this concern seems misplaced. While headers may shrink a little, this movement is far less than that of the vinyl window itself due to thermal expansion. Should a vinyl window be fastened at the corners? This question, too, produces no consensus. In some instructions, including those from Vinylcrest, the installer is told to fasten the nailing fins at the corners. Other manufacturers warn against nailing near the corners. "When the corners are fastened tight," explains Jeld-Wen’s Jeff Ward, "that’s where the window can crack, because it won’t have room to expand."

Installing a Vinyl Window

All of this conflicting advice from window experts is confusing for installers. It can also be costly, since the warranty will be void if you fail to follow the manufacturer’s written instructions. This is a particular problem for installers who work with several brands of windows, since it is easier for them to confuse instructions or fail to notice subtle differences in recommended procedures. Finally, some of the recommended procedures may run counter to reliable methods that you have developed over the years. If the manufacturer’s written instructions seem ill-conceived, your best recourse may be to buy windows from a manufacturer whose instructions you can live with. The installation procedures listed below are consistent with those of many, but not all, vinyl window manufacturers. These procedures also incorporate best practices for avoiding leaks when installing flanged windows of any kind ( Avoid cold-weather installation, when a hammer can shatter the brittle vinyl nailing fins. If you must install a window when the temperature is below 20ûF, use flat-head screws instead of nails. Size the rough opening according to the manufacturer’s instructions. Most manufacturers specify an opening 1/2 inch larger than the window frame. The window should then be centered, left to right, leaving a 1/4-inch gap on either side. Be careful: If the rough opening is even a little bit larger, the nailing fins might not catch the framing. Don’t shim the sill – instead, install the window on a continuous, level base. Some installers prefer to double up the rough sill, using stacked 2x4s or 2x6s. This permits easy shimming between the two sills, and also gives wide blocking for stucco installers to fasten their wire lath without puncturing the window’s nailing fins. Install flexible flashing around the perimeter of the window opening. Flexible flashing, which generally comes in 9-inch wide rolls, is made from a variety of materials, including polyethylene and rubberized asphalt (see, Figure 5). Flashing suppliers are listed at the end of the article.


Figure 5. Flexible flashing at the side of the rough opening should extend over the flashing at the bottom. Here, the side flashing was cut too short, and the installer incorrectly failed to lap the filler piece over the bottom flashing. When housewrap or felt paper is later installed, it should be tucked under the horizontal flashing at the bottom of the window.

Lap the flashing in the proper sequence to ensure that the flashing conveys moisture away from the sheathing. Fasten the nailing fins according to the manufacturer’s instructions. Fasteners should be installed tight, unless the manufacturer directs you to leave them proud (see, Figure 6).

Don’t puncture the nailing fin with extra holes when installing housewrap, exterior trim, wire lath, or siding. In all cases, fasteners should be held back from the nailing fin. Leave a 1/4-inch gap between the siding and the window to allow for expansion. Otherwise, wood or fiber-cement siding may restrict window expansion, causing the window to deform; in the case of stucco, window expansion may cause cracking. You may need to adjust the gap depending on the temperature at the time the siding is installed. In cold weather, for example, the window will be at its smallest dimension, so the gap between the window and the siding may need to be larger. The reverse is true during very hot weather. Don’t use spray foam to fill the gap between a vinyl window and the rough opening. Vinyl windows frames, being less rigid than wood or aluminum frames, are particularly susceptible to being deformed by expanding foam.

Repair and Maintenance

Vinyl is a relatively soft material that scratches easily. Fortunately, shallow scratches can be rubbed out with light steel wool, fine emery cloth, or Soft Scrub cleaner (an abrasive cleaner available in grocery stores). Scratches can also be removed with acetone, a solvent – but be careful, because too much acetone can dissolve the vinyl. For deeper gouges, some manufacturers recommend a liquid vinyl product called Stelmax Gap Filler (available for about $22 from G-U Hardware; 800/927-1097). Before repairing a window, contact the manufacturer so as not to void the warranty. How long will vinyl windows last? Eventually, any window will need to be replaced, and vinyl windows are no exception. "Some vinyl window manufacturers are now replacing the first vinyl windows from 20 years ago," says Jon Hills, sales manager for Dayton Technologies, a vinyl extruder. "But the formulation of vinyl has evolved since then. Will today’s vinyl windows last longer than 20 years? To know that, we’ll have to wait and see."

Martin Holladay

is an assistant editor at the Journal of Light Construction