MAINTENANCE AND RESOURCE MANUAL: WINDOWS

Windows serve two important purposes: they are designed to create a weather envelope to prevent the exchange of air and water between the inside and the outside of the building and serve as important character-defining features for older buildings.

The original windows and many later replacement windows in the survey area are made out of wood. Later replacement windows are made out of a combination of plastics, such as vinyl. Wood windows are repairable while plastic windows are not. When a plastic window breaks, the solution is usually total replacement. In contrast, wood windows are almost always repairable. Therefore this section of the maintenance manual will focus on the repair and maintenance of wood windows.

There are many kinds of wood windows from casement designs to the extremely common double-hung window. With few exceptions, nearly all of the wood windows in the survey are traditional, double-hung wood windows.

A double-hung window consists of two nearly identical sashes. Each sash is "hung" or suspended on a balanced counterweight system, one on top of the other. The top sash is suspended approximately two to three inches closer to the outside than the bottom sash. In a properly operating double-hung window system, the top sash can be pulled down, and the bottom sash can be pulled upward.

Each sash can be made of a single pane of glass enclosed in a frame of wood. The horizontal wood pieces are called rails and the vertical pieces are called stiles. Often, a sash can be divided into multiple panes or lights, each divided by a several mullions. It is common to refer to double hung sash windows by the number of divided panes in each sash. For instance, a double-hung window with six lights in the top sash and one light in the bottom sash is referred to as a "six over one window" which can be abbreviated to "1/1 window."

The sashes are mounted in a window frame and slide in two channels. The inner sash is held into the frame by a staff bead and a parting bead. The outer sash is held into a channel by the parting bead and a projecting portion of the window frame.

Each pane of glass is held into the sash by a series of points, small flattened and pointed pieces of metal, that are tapped into the rails, stiles, and mullions. The pane of glass rests in a special groove and is surrounded by glazing putty, a caulk-like compound traditionally made from linseed oil and white lead or chalk.

The brick mold is a special piece of wood trim that covers any space between the window frame and the rough opening in the wall. A rectangular piece of stone, concrete, or wood is placed above the window opening in order to distribute the load from the wall to either side of the window. This piece is called a lintel. The entire window assembly rests on the sill which can be made of stone, concrete or wood.

Common Problems

An original window that is maintained can last for hundreds of years without any major repairs. Wood windows are far from indestructible, however: decay is their primary enemy. Decay sets in when wood is allowed to get wet and dry in repeated cycles, thereby creating the perfect environment for fungal growth. The structural integrity of wood is compromised when brown and white rot, which are different types of fungi, use the wood as food. The same decay process that turns fallen trees into soil will, if given the chance, do the same to wood windows.

The key to the longevity of wood windows lies in keeping wood dry. Dry wood will not decay. An intact layer of paint and glazing will keep water from contacting and soaking into wood. An angled window sill will drain water away from the wood window. Lastly, a properly designed window sill will have a “drip edge” to prevent water from rolling underneath the window.

Anything that interferes with these systems will cause the wood in the window to decay. Typically, these deleterious factors occur when regular maintenance, such as painting and glazing are not kept up. Breaks in paint and cracks in glazing allow water to be trapped and seep into wood. Exterior storm windows will cause condensation problems and trap water on the window sill.

Sometimes the mechanical operation of the window is compromised. Excessive paint will cause windows to stick; cotton sash cords will fray and break. And of course, window panes break.

Maintenance and Repair

At least once a year, windows should be inspected for paint and glazing compound failure, wood decay, and proper operation. This inspection should be done every spring or fall. Problems should be corrected as soon as possible. If you have exterior storm windows, every couple of months check to make sure that water properly drains from the window sill past the storm window. Promptly clear any accumulated debris from drain holes.

Expect to repaint windows every five to fifteen years. Windows that have been previously stripped to bare wood, primed, and then painted should perform at the upper range of this cycle. Glazing compound should remain in good condition for ten or twenty years or more.

Note that local environmental conditions, the direction a window faces, and the configuration of a building will have significant effects on the length of time between painting and reglazing of windows. Regular inspections are critical to catch small problems before they become larger ones.

Painting: Wood windows need to be painted on a regular basis—usually on an interval of every five to fifteen years depending on the quality of the substrate, the quality of the paint, and the level of continuing moisture infiltration. A properly functioning paint layer helps keep the wood dry and shields the wood from the damaging effects of ultraviolet radiation. Latex (i.e., plastic, acrylic-based) paints may have an advantage over oil paints in that they allow a great amount of water vapor to migrate out of the wood substrate. However, oil paints have been used successfully on wood windows for hundreds of years (the modern version is known as alkyd paint).

Signs of paint failure appear as cracks, alligatoring, and detachment of paint from the underlying substrate. All paint will fail, but if water is being absorbed into the wood of the window—especially from any end grain—the migration of water as vapor will cause early paint failure. It is important to ascertain that the water shedding mechanisms of the window and its frame are functioning properly and these problems rectified before the window is repainted. (Please see the section on FINISHES for issues dealing with lead paint.)

Since a window has many friction surfaces, all paint on the window frame and sashes should be removed to bare wood. Paint removal is normally performed in a window restoration because paint buildup interferes with the normal operation of the window. Removal by sanding should be avoided—especially dry sanding which will substantially increase the amount of lead dust within the room. Some sources recommend wet sanding and scraping, but in general chemical stripping leads to the minimal dispersal of lead dust. Methylene chloride based paint removers are better than caustic, highly alkaline strippers. A high pH stripper (i.e., very alkaline) forces existing paint into the pores of the wood, making complete lead paint removal extremely difficult. Removal with heat guns and torches can be extremely dangerous as lead vapors will be liberated and can be directly absorbed into the blood through the lungs. Heat guns and torches also represent a very real fire hazard. A new infrared based paint removal tool has been recently introduced on the market that promises to use much lower temperatures, thereby mitigating problems with lead vapor generation and fire hazards.

Although it is legal for property owners to perform their own abatement, it is best to leave lead paint removal to professionals. Property owners who attempt abatement must be familiar with the hazards of lead paint removal and wear appropriate respirators and clothing to protect against lead dust, lead vapor, and chemical solvents. Lead paint is considered hazardous waste and must be disposed of properly. Under no circumstances should pregnant women or children perform lead paint abatement.

Glazing putty: Glazing putty is a special caulk-like compound that is applied to the interface between the window stiles and rails and the glass, usually at an angle. Historically, it was primarily made of a mixture of linseed oil and white lead or chalk. When initially applied, the putty is plastic and will create a good seal between the window glass and the rails and stiles. Air and water migration is stopped by a Over time, the glazing compound will crack due to the oxidation of the linseed oil. These cracks will allow air and water into the window and cause wood decay issues. If the glazing compound does not form a continuous, unbroken seal around the window, it should be renewed. This process is known as reglazing. Reglazing will also be necessary if a window pane is cracked or broken and needs to be replaced.

Replacement of sash cord or chain: Sash cords were originally made from cotton fiber. Over time, the fiber decays and will eventually break. Two sash cords, attached on either side of the top of each sash, connects to a heavy weight inside of a pocket on each side of the window. These pockets are an integral part of the window frame. This system creates a balance with the sash on one side and the weight on the other.

Broken sash cords are easy to replace. Nylon and metal cords are now available that will give greater performance than original cotton cords.

Window sill: The window sill is the weakest part of the system. If not properly maintained, wooden window sills frequently decay, especially if water can gain access to the end grain of the wood. Make sure that the window sill slopes away from the window and offers a smooth surface that will repel water.

If the wood of the sill is in generally good condition, but it is checked or cracked, the sill can be repaired with epoxy or wood filler and then painted to recreate a smooth, water shedding surface.

Make sure that the brick molds are properly attached to the sides of the window and that there are no large gaps that may admit water. Caulk can be used to fill some holes, but it has a tendency to trap water and exacerbate decay. It is best to avoid caulk on the exterior of windows if at all possible. Instead, use caulk in the inside in order to stop air infiltration. Under no circumstances apply caulk to the underside of a window sill or around the entire brick mold. Doing so will inevitably trap water, accelerate the entry of water, and cause the decay of the sill.

External storm windows have a tendency to trap water at the window sill and cause problems with wood decay. For this reason, internal storm windows are preferable.