Windows are important not just to let light in and keep out the weather, but also as a defining feature of your homes’ style. Windows are categorized by several characteristics: material, number of panes of glass, method of opening, and any unique features. For example a wood, arch-top, 2/2, double-hung window has the following characteristics: (1) the sashes are constructed from wood, (2) the top sash is curved at the top, (3) each sash has two divided panes of glass, and (4) both sashes move up and down.

Repair and Replication
The key decision you will have to make is whether to repair or replace your windows. Historically speaking, repair is generally the preferred method. A tightly sealed, well-installed, high quality storm window installed over your repaired historic wood window will provide an acceptable barrier to the weather. Repairing a historic window may involve replicating deteriorated pieces of the frame or sill, reglazing the sashes, and rehanging the counterweights. Reglazing a window and rehanging the weights are skills that are easily learned by a novice. Replicating deteriorated wood pieces requires special tools and skills that may be better left to professionals or those with significant experience.

In the event that the sashes are beyond repair, some local mills will create replicas for much less than a full replacement window. They will also re-mill your existing sashes to accept insulated glass if you want to upgrade the glazing.

Window Replacement
If you decide to replace the windows, you will be faced with a myriad of decisions regarding the cost, style, construction, and efficiency of the replacement windows. Within the bounds of this overview, only some of these decisions will be discussed. The three major types of modern window construction are wood, aluminum, and vinyl. Almost all new single-hung and double-hung windows are friction mounted – very few use a counterweight system. Of the three types, wood is typically the most expensive but also the most historically accurate. Some aluminum windows can also replicate the general look of some historic windows, but the standard color selection is limited and arch-tops are not possibe. Vinyl windows are the least expensive, the least historically accurate, and will generally have a shorter lifecycle (regardless of what the salesperson claims).

Some rules to follow:

1. Square pegs don't fit into round holes - never replace an arch-top window with a square top window. They look ridiculous and diminish property values.
2. If your house is located in a local historic district, never replace windows without a CRO permit. They can make you remove a non-compliant window!
3. Never install an undersized replacement window just because you were able to get a "deal". The sizes and shapes of your windows are a key component to the historic character of your home. In some cases it may be desirable to raise the sill (do so in a sensitive manner) in order to install new cabinets or make some other architectural change.
4. Match the muntin patterns of your existing windows.
5. Install shutters only in historically appropriate locations (fasteners, hinges, and different brick color may indicate that there were previously shutters on your house). If you do, make sure they are appropriately sized.
6. Storm window size and shape should mirror the window behind. Never install two storm windows over a larger double hung window - it changes the character of the elevation.

If your home is located in a local historic district, the city’s Cultural Resources Office must approve any alterations or repairs to the front façade (specifically including windows). When you are rehabilitating a structure, this approval must be obtained prior to applying for your building permit. Usually the city will require that all windows visible from the street be retained or replicated – preserving the look and feel of the historic district. Vinyl windows are never allowed on a primary façade in a local historic district.

Energy Efficiency
A window’s energy efficiency is difficult to determine because the window is only one part of the entire “building envelope” system to keep out the elements. Window salespeople will, of course, try to sell you to a more efficient, and more expensive model. Keep in mind, however, that even the most efficient window will be ineffective if air can infiltrate through other openings. Look at your building as a system that is only as effective as its weakest component. Windows are one case where you have to balance the sometimes-conflicting goals of cost, efficiency, and style.