Common Window and Patio Door Installation Errors

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.

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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.

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.

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.

Nails and screws should be anchored into solid lumber at least 3/4 inch thick. When possible, attach the window to structural framing. Nails must penetrate the wood at least 3/4 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 1/2 inch thick and the space between it and the trimmer stud is 1/2 inch, for a total of 1 inch. The minimum anchor length is 1-3/4 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.

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 1/2 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 1/2 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.

Over packed insulation around windows and patio doors

When insulating the cavity surrounding the retrofit window, don't use expanding foam and don't pack mineral fiber in tightly. The expanded foam or over packed mineral fiber can apply excessive pressure to the window frame, distorting the frame and impeding smooth operation of the sash. When using a nonexpanding foam (such as one-part polyurethane), pay careful attention to how it affects the frame, and remove excess foam if you see any distortion (see "Urethane Foams and Air Leakage Control," Home Energy Magazine July/Aug '95)

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Improper flashing on windows and patio doors

When the nailing flange (fins or perimeter casing) is attached directly to the sheathing or framing, install flashing as well as sealant. Improper or nonexistent flashing can allow moisture to permeate the structure and ultimately result in water damage. Use 30 lb. felt, bituminous impregnated craft paper, or standard sheet-metal flashing (aluminum or galvanized steel). Twelve-inch width is recommended; nine-inch is the minimum.

Flashing techniques vary with the style of window or patio door and the type of opening; however, there are basic flashing principles that always apply. Start from the bottom and work to the top. Install the first strip (sill flashing) horizontally across the bottom of the rough opening and extend it beyond the edges of the opening by a distance exceeding the width of the material. Next, extend vertical strips (jamb flashing) from above the top of the rough opening to below the bottom of the sill flashing. Finally, after the window is secured, install the head flashing across the top of the window, extending beyond the jamb flashing on each side. The top edge of the flashing is attached to the structure, and the bottom edge is placed over the head fin.

Damaged Moisture Barrier around window and patio door frames

Sometimes installers cut out a strip of exterior siding (such as plywood or stucco) to expose the nailing fin and facilitate removal of the existing window or patio door. When this is done, the moisture barrier is usually cut in the process. Water may then penetrate the envelope and cause structural damage. This is also likely to occur when a finned metal frame window is forcibly pulled and collapsed (rather than exposing the fin and removing the anchors).

Wherever the existing building paper or flashing material is damaged, reestablish a weatherproof seal when the new window is installed. When possible, overlap the damage with additional material (such as 30-lb. felt). Apply elastomeric sealant liberally everywhere there is a potential for moisture penetration.

If a siding cutout extends 1 inch or more beyond the outer edge of the retrofit window fin, install flashing around the rough opening and seal around its outer perimeter (for example, by embedding it in a bead of caulk). If the cutout extends less than 1 inch beyond the outer edge of the retrofit window, a continuous layer of sealant can be spread over the exposed wall surface between the fin and the siding, instead of installing flashing. Whether or not the cut-out is wide enough to accommodate flashing, apply caulk liberally along the exposed edge of the siding (and force it behind the siding where possible) to keep moisture out of the wall.

Using the Wrong Caulk on windows and patio doors

Caulk creates the exterior seal, the first line of defense against infiltration of wind and water. The highest-quality window or patio door can be installed perfectly square, level, and plumb, and secured with utmost precision; but if the exterior seal fails or chemical damage occurs, the entire installation is compromised.

Using the wrong caulk can result in moisture penetration and water damage, or corrosive damage from a chemical reaction. In general, elastomeric joint sealants, including polysulfides, polyurethane, and silicone, adhere well to most materials while remaining flexible. Flexibility is especially important for caulk at the top of the retrofit unit, to prevent frame damage when the header flexes or sags.

Adverse chemical reactions with the window, mounting surface, and finished wall, can compromise the seal and damage materials. Entire vinyl window installations have been ruined when the caulk stained the fins, or worse yet, literally dissolved them. Always check factory specifications, especially when installing vinyl windows and clad products. To be absolutely safe, test a small amount of the sealant in an inconspicuous part of the frame or fin before proceeding.

Also, be careful about installing a non-paintable caulk (typically silicone) where painting must be done. Adjacent visible surfaces must be primed, painted, and allowed to dry before the caulk is applied, or the color of the caulk must be compatible with surrounding materials.

Improper Installation of Caulk Around Windows or Patio Doors

Installing caulk improperly (even when it is the correct product) is another common mistake. Follow manufacturer's instructions regarding surface preparation, application temperature, joint type and size, bead quality and size, and tooling the bead.Caulk will not adhere properly, and the seal will not be weather tight, unless the surfaces are free of loose or cracked caulk, dirt, debris, oily substances, and moisture (unless allowed by the caulk manufacturer). Surfaces should also be primed when called for in the instructions.

Install caulk when the outdoor temperature is within the range specified by the manufacturer. Joints are subject to expansion and contraction with changing temperatures. If surfaces are hot during installation and the joint and bead are small, the sealant can tear when substrates contract and the bead stretches in winter. A large bead applied in cold weather may become over compressed by substrate expansion during summer.

Install the caulk to the proper depth, in a continuous bead that is free of voids. Generally, caulk gaps up to 1/2 inch wide with elastomeric sealants. However, for deep joints over 5/8 inch wide, use backup material, such as foam backer rod, to partially fill the cavity. Backer rod controls the depth of the bead and prevents the caulk from sticking to the bottom of the joint (which could cause the seal to tear when the joint expands). The bead should be about half as deep as it is wide, and the depth should never exceed the width. Don't caulk gaps wider than 7/8 inch; make them narrower with solid fill strips.

Tool the bead to create an "hourglass" profile (slightly concave surface) and to force out bubbles and fill gaps. Where caulk is visible, tooling helps remove excess material and enhances the finished appearance.

Not caulking the Windows and Patio Doors in the right places

If you don't put caulk in all the right places the window will leak. On a finned window, for example, use enough caulk that some squeezes out around the entire fin perimeter. After the window or patio door is secured in place, the caulk, which has been squeezed out, should be tooled to seal the edge of the fin. Remove or redistribute any excess. Anywhere caulk did not squeeze out, apply more and tool as needed to create a continuous seal.

Caulk the corners of mechanically joined frames and any other joints where leakage could occur. If there is a gap (potential water leak) between the edge of the siding and the rough opening, caulk that also. Never caulk the weep holes in the sill jamb, though, because they provide condensate drainage.

Caulk any wood strips installed to fill the gap left where siding was removed. Finishing trim (also known as casing) installed over shim strips does not need to be caulked; however, a neat bead around the edges usually creates a more finished, professional appearance. For box frame windows, caulk the gap between the window frame and the opening with a continuous bead free of bubbles and voids.

Dissimilar materials touching

Placing dissimilar metals, such as aluminum and steel, into direct contact with each other can cause accelerated corrosion and premature failure of the weather seal. To prevent this, separate incompatible materials with an inert medium, such as non-absorptive plastic or elastomeric tapes or gaskets. Foam glazing tape is commonly available and easy to use. A thick coating of good quality caulk is also acceptable.

Unfinished or improper casing

Most bare wood left untreated and exposed to the elements will warp and deteriorate, so use a good-quality primer and exterior paint. Cedar and redwood may be treated with a clear sealer when paint is not desired; however, when they are to be painted, a special primer must be used to prevent oils in the wood from bleeding through the finish.

To avoid damaging new and unpainted wood windows or patio doors, follow manufacturer's recommendations on acceptable protective coatings and application techniques. Open or remove sashes while painting the frame. When painting the sash, lap the finish coat 1/16 inch onto the glass unless instructed otherwise. Take care not to apply paint to weather-stripping, vinyl, fiber glass, or other non-wood parts.