This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages)(Learn how and when to remove this template message)
A replacement window is a window that is installed in an existing window opening as replacement of the existing window. Old weather beaten windows deteriorate and become loose and drafty. They need replacement not only to improve the appearance of the house but also to take advantage of modern energy efficient windows that bring about an overall improvement of the ambiance of the house at low recurring cost of heating and cooling.
Replacement windows are designed for a variety of installation situations and techniques.
In a full-frame installation, trim around the old window (interior and/or exterior) is removed and the old window frame is removed completely. The new replacement window is secured to the studs surrounding the window opening, and the trim is replaced.
Insert installations are sometimes used when replacing older wood windows with frames that are in good condition. In this case, the new replacement window is installed within the existing frame. This installation technique is simpler than a full-frame installation, but decreases the size of the window opening due to the nesting of the frames.
Another technique involves replacing the window sashes only, and re-using the existing frame.
New-construction windows of recent vintage typically have a "nailing fin" along the outer frame. This fin provides a surface so that the window can be nailed in from the outside of the home before the application of flashing, siding or brick and stone veneers. Most replacement windows are manufactured without this fin so that they can be installed with minimal disruption to the existing trim, siding, sheetrock or exterior veneer.
Replacement windows are available in several materials including wood, fiberglass, aluminum-clad wood, vinyl-clad wood, vinyl, glass blocks and other composite materials. The most common materials for new windows are PVC-u and wood.
Benefits of replacement windows
Wood windows were used from the early 1900s to the present but became less of a mainstay of the industry in the 1960s. They are prevalent in the Northern United States. Steel and aluminum casements and Steel Vertical Operators were used from the 1950s through the 1960s. Aluminum windows were used in the 1960s through the present. Vinyl windows were established in the 1970s through the present. The last decade has also seen the admission of composite materials such as fiberglass and vinyl-wood-polymer type products.
Wood "drop-in" replacement windows and vinyl windows are designed to sit in place of the existing sashes and are constructed at 3 1/4" thickness in most cases. These type windows sit in the opening where the top and bottom sash originally moved in their respective wooden "tracks" The stop between the two sashes must also be removed in this type of refurbishment or retrofit installation. It requires minimal movement of existing trims both inside and out.
The alternative is to replace the entire wood window including jambs. This requires the reworking of interior and exterior wood trim to accommodate the size of the modern wood window. Modern wood windows are available in with 4 9/16" jambs as a standard feature but can be equipped with "jamb extensions" to extend to 5 1/4" or 6 9/16". This is to accommodate the wall thickness as needed.
Modern windows have two or more layers of glass. This is known as double glazing or triple glazing. An argon gas has is usually held between these additional layers of glass which helps to make the windows more energy efficient and keep out outside noises. Triple glazed windows are more energy efficient than double glazed windows, but with their additional weight, they are not always available to work with every size of window frame. In the United States, the Energy Code sets certain standards for performance of products installed in homes. These codes now require Low-E Glass in all residential homes.
Low-E is a film that is several layers of metal poured microscopically thin over the surface of newly poured glass. This heat reflective film is transparent but can be darker or lighter depending on the type and manufacturer. This data is rated in Visible Light Transmission. Darker glass with heavier Low–E will have less VT. The NFRC rates most energy star rated window manufacturers.
Two main types of Low-Emissivity Glass are pyrolytic, or "hard coat", and spectrally selective, or "soft coat".
Pyrolitic glass is made mostly of tin oxides and is applied to "hot" float plate glass as it is cooling. Pyrolytic Low-e glass is extremely durable and gives glazing a lower u-value, or heat loss rating, than clear glass, making it ideal for northern Energy Star climate zones.
Spectrally selective glass is made of various metal oxides, mostly silver, and is applied to cool glass in an electro-magnetic vacuum sputter chamber. Spectrally selective low emissivity glass is very sensitive to oxygen and therefore has to be sealed in an insulated glass unit before it begins to oxidize. It scratches easily and is sensitive to pH, making it difficult to manufacture. It produces low u-values, both winter and night, and low summer daytime solar heat gain ratings, making it a preferred coating in mixed climate zones.
Introduced in the mid 2000s, newer "triple silver" low-e, also called High Performance low-e, are testing for even lower SHGC ratings, making the windows suitable for even the hottest southern climate (mostly cooling) zones. Also notable are new interior surface low-e coatings that provide very low u-values that are comparable to triple pane windows, often in the low 20s. Combining these two low-emissivity coatings can make a dual pane window exceed every Energy Star climate zone in the US.
Other options include triple-glazing (a third pane of glass), higher quality spacers between the panes, which reduce the failure rate and conduction that allows seal failure. This creates "fogging" or condensation to form between the panes. Modern windows also have optional gases between the panes that have higher insulative qualities than air, such as argon or krypton gases.
"Double-hung" windows are the most common traditional window. They have an upper sash and a lower sash, both of which slide up and down in the window opening. "Single-hung" windows operate the same as "double-hung" windows, but their upper sash is fixed in place. By virtue of being stationary and permanently secured, single-hungs are often more energy efficient that double-hung windows depending on the type and style.
Most vertical operators (single- and double-hungs) now feature "tilt-in" sashes for cleaning of the exterior surfaces. The industry moved towards this approach for service and replacement reasons as well as accessibility to the exterior from the inside of the home.
Sliding windows, or "sliders", are sometimes used in openings that are wider than they are tall.
Non-operable or "fixed" windows also called "picture windows" are common in larger openings.
Retrofit replacement windows are custom manufactured to fit finished openings in sizes down to 1/8" or 1/4" in most cases. Builders-grade windows are constructed in specific sizes depending on the manufacturer. Wood windows also have "Standard Sizes" that determine the installation and application. Custom-sized wood windows are a rarity but are the most expensive of modern window products.
In 2009, the United States Federal Government passed a stimulus package allowing a 30% tax credit, with a $1500 cap, on purchases up to $5000 for qualifying energy saving products purchased in 2009 and 2010. This includes insulation, radiant barrier, air conditioning upgrades and most energy-efficient replacement windows and doors.
 There are also additional programs through state governments and utility companies that offer low-interest loans and grants to replace your windows with energy-efficient ones.
Trade-offs of replacement windows
Due to the heavier weight and increased thickness of insulated glass, and the weakness of vinyl extrusions, window frames in replacement windows may be thicker in visual profile, thereby reducing glass area even in full-frame and sash replacement-only installations. This is not universally the case. Replacement window operation may not be identical to the windows that are replaced. For example, a typical existing older double hung window sash is capable of being opened nearly to the top of the window. Newer replacement window sashes typically can only be opened to approximately 4" from the top of the window - providing less open window area. On smaller bedroom windows that are required by building codes to allow egress in the event of a fire, the smaller opening area may not meet the code required minimum dimensions. And removing the sash, although it may be easy to do, is not allowed as a solution to this problem since the building codes specifically requires the window to be opened in the normal manner.