Over packed insulation

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

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Improper flashing

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 and 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

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. When this is done, the moisture barrier can be 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 where ever   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

Caulk creates the exterior seal, the first line of defense against infiltration of wind and water. The highest-quality window 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.

Installing Caulk Poorly

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 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 is secured in place, the caulk squeezed out should be tooled to seal the edge of the fin and to 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 unpainted wood windows, 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.