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Common Window and Patio Door Installation Errors
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Install exterior and interior window trim finishes also called molding, and caulk It's a good idea to install prefinished weather protected trim using butt joints.
During 30 years of building science experience, which includes training contractors who retrofit high performance windows in utility sponsored programs, we've observed several mistakes that contractors often make when installing retrofit windows.
Retrofit window or patio door too large for the rough opening
When a retrofit window or patio door is installed without adequate space between the retrofit frame and the rough opening or old frame, seasonal expansion and contraction can cause distortion of the window frame and failure of the weather seal, resulting in water leakage into the wall and window cavities. In addition, the windows or patio doors may operate poorly, the glass can break, and the insulated glass edge seals can fail, allowing the inert gas to escape and water to condense between the panes. Top clearance is of greatest concern, because the largest structural movement usually occurs in the header.
Window or patio door not level, square, and plumb
If the window is not squared or level, the sash will not close properly, and the weather stripping may not be able to provide an adequate weather seal.
 After exterior painting, canvas awning, roofing, and interlocking pavement |
First, make sure the sill jamb is level. If it is not, squaring the unit will be virtually impossible. Then, with the window centered in the opening and the bottom secured, open the sash just past the edge of the frame and check to see if the gap is uniform all along the edge. If it is not, adjust the jambs until the gap is even, and secure them near the top. As midpoint anchors are installed in the jambs, check for vertical alignment again to be sure the jambs have not bowed in or out.
Improper frame support for Retrofit Windows
Failing to support the retrofit window frame properly, particularly across the bottom, can cause the frame to contort and allow the sill to sag. This will result in such problems as rough sash operation, incomplete closure, leakage of air and water around the sash, and, in the worst cases, failure of the perimeter caulk and damage to the insulated glass seal.
Support the frame as instructed by the manufacturer and follow general good workmanship practices. Permanent shims should be doubled (one wedge placed upon another, oriented in opposite directions) so pressure is evenly distributed across the entire width of the frame. Driving in a single wedge can exert excessive force on one edge of the frame, causing it to twist.
 Vapor Barrier |
Proper frame support is especially important along the sill jamb, because the bottom of the window must be level and stable. For windows other than vinyl, install a minimum of two bottom shims at quarter points. Support vinyl windows along the entire bottom surface (the full length and width of the sill) with solid stock, such as slat shims. If you use spaced shims, the vinyl sill may sag between supports which will probably cause the sash to operate roughly in horizontal sliders.
Inadequate or improper attachment of Windows and Patio Doors
All anchors (screws and nails) must be non-corrosive to resist rust stains, oxidation, and deterioration. Nails can be aluminum or galvanized steel. Screws should be plated with cadmium, zinc, nickel, or chrome. In all cases, fasteners should conform to the window manufacturer's specifications.
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Nails and screws should be anchored into solid lumber at least ¾ inch thick. When possible, attach the window to structural framing. Nails must penetrate the wood at least ¾ inch, and screws at least 5/8 inch. These depths are what determine minimum anchor length. For example, assume that the retrofit window jamb is ½ inch thick and the space between it and the trimmer stud is ½ inch, for a total of 1 inch. The minimum anchor length is 1-¾ inches for a nail and 1 5/8 inches for a screw.
Common errors include using anchors that are too small or too short and either not installing them deep enough or driving them into the wrong substrate. Often the result is a window frame that sags in place, contorts, pulls loose in the wind, and leaks in the rain. Secure finned windows with the equivalent of 6d or 8d nails or with #8 sheet metal screws. Roofing nails work well, because the wide, flat head will nicely cover larger anchoring slots in nailing fins. Use casing nails when attaching through perimeter casing.
 Alumawood lattice cover |
For box frame windows, which have no mounting flange, the anchors usually penetrate the jambs inside the sash or screen pocket. Wood windows are usually attached with casing or finish nails or with #8 wood screws installed through predrilled holes. Anchor box frame vinyl and aluminum windows with #8 screws, not nails. Screws provide greater control and allow for adjustments to prevent the frame from bending.
Window or patio door top anchored to the header
The structural header expands and contracts with the seasons, and can sag. Anchoring the top of the retrofit window (the fin or head jamb) to the header will lead to a distorted window frame, failed seals, and hampered window operation.
For finned windows, install support nails ½ inch above the top fin and bend them down over the fin; or install a roofing nail (snug but not tight) into a slotted hole in the fin; or use oblong fender washers lapping over the fin and anchored ½ inch or so above the fin with screws or nails. A box frame wooden window can safely be secured to the header with finish nails countersunk into the head jamb. If the header were to sag, the small heads would allow the nails to move in the jamb without distorting the frame. At worst, the nails might need to be reset or the holes respackled.
