Window Installation

When the right window is installed incorrectly or worse yet, when wrong window or patio door is installed incorrectly, energy savings will be few, structure and furnishings may be damaged, and the occupants' safety may be jeopardized. The property owner and installers need to understand more than U-factors, solar heat gain coefficients, air infiltration, etc., when choosing and installing a window.

How to Install a Window “The RIGHT way”

Installing a window correctly takes a lot of careful attention. The exact procedure varies with the type of window being removed, how it is removed, and the type of replacement window. However, here are the basic steps for most window retrofits:


Measure the opening into which the retrofit window will be installed width, height, and depth. Size the replacement window slightly smaller than the opening, allowing clearances recommended by the manufacturer. Vinyl frames require more clearance than those made of wood, aluminum, fiberglass, and composite materials.

Removal of old window

Remove the old window. For wood vertical sliders (single- or double hung windows), remove stops, trim, sash cords, and pulleys. For finned windows, cut away siding to expose the fin so the entire unit can be removed, or cut back the frame as much as possible, leaving the old fins intact.


Prepare the opening. Remove all obstructions, such as loose materials and old caulk. If the opening is severely out of square, fill it in, leveling the bottom sill and plumbing the sides, before installing the new window. Try the replacement window to make sure it fits properly before applying sealant.


Restore the integrity of the moisture barrier and drip cap. If fins were exposed, seal breaches in the building paper. If the existing metal frame was collapsed to remove it, seal the entire perimeter gap liberally with silicon sealant. Reinstall or seal any damaged flashing. For box frame retrofits, make sure a drip cap is in place to drain away water.


Precaulk the stops, mounting surface, or fins with elastomeric sealant. When installing box frames against existing stops, caulk the surface of the stops against which the window will be placed. For finned windows, install a liberal and continuous bead of caulk either on the fin or on the mounting surface around the entire perimeter.

Bottom support

Install proper bottom support. Metal, wood, and fiberglass frames can be supported with shims spaced per manufacturer's instructions. Vinyl windows need solid, level support across the entire bottom (the sill jamb)


Install the new window level, plumb, and square. Use a level to plumb the unit. To make sure it is square, (a) adjust until both diagonal measurements are the same or (b) open the slider just past the edge of the sash channel and adjust until the gap is even along its entire length. Anchor the window securely per manufacturer's instructions. Use only corrosion-resistant nails and screws. Predrilling fins and frames is recommended. Anchors must penetrate solid material such as structural framing. Countersink anchors installed through wooden jambs.


Insulate gaps between the retrofit frame and the rough opening. Fill the cavity between the retrofit frame and the rough opening with foam backer rod or loosely packed insulation. Non-expanding foam is OK when installed carefully. Never use expanding foam. Weather seal the entire perimeter of the installation with elastomeric caulk. Butt joints and other gaps over 5/8 inch wide need backup material (such as foam backer rod). The seal must be continuous and should be tooled to fill gaps and to remove air pockets and excess material.


Install exterior and interior finishing trim, also called molding, and caulk It's a good idea to install prefinished weather protected trim using butt joints.

Common Window Installation Errors “The WRONG way”

Over the years, 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.

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 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.

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.

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)

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 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

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 from happening, 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.

Window and Door Egress

Installing retrofit windows and patio doors, especially when the existing window and patio frames are left in place, can significantly reduce the size of the clear opening by as much as 2 inches of height and 3 inches of width. This may prevent emergency egress. That is, occupants may not be able to get out, and rescuers may not be able to get in, if there's a fire.

Most model building codes specify egress requirements for sleeping rooms in the first three stories. They require at least one operable window or exterior door, approved for emergency escape or rescue, to open directly onto a public street, alley, yard, or court. If the emergency escape is a window, it must be operable from the inside without the use of tools. It must provide a minimum clear opening of 5.7 sq.ft. (with a minimum width of 20 inches and height of 24 inches), and the finished sill may be no more than 44 inches above the floor. Patio door glass must be tempered and if a window is less than 18 inches above the floor, its glass must be tempered.

Some jurisdictions have specific egress requirements for retrofit patio doors and windows. Others do not. Know what the local building department requires for egress before replacing a bedroom window.

Safety Glass

People often install standard glass because that's what was there before. This can be a serious mistake, since codes are constantly being upgraded. Installers who carelessly replace standard glass, like-for-like, could end up defending themselves in court if an occupant is injured by glass shards in a window that should have contained safety glass.

Safety glass crumbles into relatively safe little chunks resembling hailstones. Most codes require safety glass (almost always tempered glass) to be installed where a window may be subject to human impact. This includes all entrance doors; windows adjacent to a door when the glass is within a 24-inch arc of the closed door (increased from 12 inches in 1994) and the bottom of the window is less than 60 inches above the walking surface; and windows larger than 9 sq.ft. when the bottom is less than 18 inches above the floor, the top is more than 36 inches above the floor, and a walking surface is within 36 inches horizontally of the window.

Consider cost and timing well in advance, because tempered glass costs more and usually requires more lead time at the manufacturer, especially for custom-sized units. High-performance windows with spectrally selective coatings require even longer lead times.


Even a well-installed window with the best thermal characteristics can leak during a gusty rainstorm if the window is not the proper grade. Grade refers to the structural capability of the window, or its capacity to resist wind loads and to seal against air and water penetration (see "How to Select the Correct Grade of Window"). When the wrong grade of window is installed, the combination of wind pressure and driving rain can force water through the seals between the operable sash, the frame, and through weep holes.