Windows have come a long way since they were merely openings in walls that literally let in the wind. We still use them for ventilation, but today’s windows do a far better job at everything than their predecessors.
The Anatomy of a Window
The glass pane is the most basic part of a window. The sash is the glass and the frame that holds it. Containing the sash is the window frame. The sill is on the outside of a window, while the horizontal piece of interior trim at the window’s bottom is a stool. Extension jambs are added to extend the window frame to the face of the interior wall for trim when the wall is thicker than the window frame’s depth. Casing is the trim that surrounds a window on the inside.
Traditional panes were small because glass couldn’t be made in large sheets until the 19th century. The panes sit in rabbets in thin frames within the sash called muntins, held there with metal glazing points and sealed with glazing putty. Old windows have no insulating value to speak of, other than keeping most of the cold air out. Aluminum storm windows are commonly retrofitted to old windows to improve their energy efficiency.
Consider the Energy Efficiency of Windows
Modern windows are far more energy efficient than old ones. Newer sashes typically have two large panes of glass held about 1/2 inch apart by spacers and sealant. Contained between these panes is a gas, usually argon, that greatly increases the insulating value of the sash. Special coatings on the glass help in other ways. For cold climates, low-e coatings reduce how much radiant energy can exit through the glass. Other coatings reflect sunlight from the glass to reduce solar heat gain.
Particularly important in hot climates, the measure of this reflectance is called the solar heat gain coefficient. Larger numbers mean the window reflects more sunlight. The insulating value of a window is expressed as its U-factor, which is simply the mathematical reciprocal of the more commonly known R-value. Lower U-factors mean the window is a better insulator, but even the best windows aren’t great insulators. A good U-factor might be .31, and that converts to an R-value of 3.23. Compare that to the bare minimum code-required R-value for a wall of R-13.
The most common type of window is a double-hung, which has a top and a bottom sash that open by sliding up or down. Older double-hung sashes were made easier to raise and to leave in an open position by counterweights attached by rope or chain. These weights move up and down in pockets between the window frame and the wall stud. Modern double-hungs often use a spring balance the attaches to the sash with heavy-duty string.
Sliders are like double-hungs set on their sides. Casement windows crank out, hinged on the side like a door. Awning windows also crank out but with the hinge at the top. Awnings and casements tend to be more air-tight than double-hungs or sliders. Finally, there are fixed windows whose sashes don’t move at all.
Windows can be installed singly or mated into doubles or triples. Picture windows have a large, stationary center sash flanked by an operable window on either side. Bow and bay windows both protrude from the wall. Bow windows consist of several units configured as a segment of an arc, while bay windows are similar to picture windows, but with the flanking window units set at an angle.
Traditionally, windows are made from wood. While wood is still a common material, today it’s often clad (wrapped) in vinyl or aluminum for durability and low maintenance. In fact, window frames and sashes are frequently made entirely from vinyl or aluminum, and even fiberglass.
Windows are also divided into replacement and new construction types.
New construction windows are meant to be placed in openings in the framing before the house is sided. They typically have flanges on the outside that are nailed to the framing, and then integrated with flashing and adhesive tapes into the house’s weather resistive barrier to keep out water. Spray foam seals and insulates the small shim space between the window frame and the studs. Flashing may be the most important step in window installation. Done wrong, it can lead to rotten framing
Replacement windows utilize an existing window frame. The old sashes are removed, and the replacement unit fits snugly into the old frame. This is usually the most economical way to replace windows. However, if the old window frame is rotted out, you may need to replace the entire unit with a new construction window. This is generally a job for pros, and it can get expensive because the work requires removing siding and interior trim, setting the window, then replacing the siding and trim.
Window prices depend a lot on material and on the energy rating of the unit. Wood windows tend to be the most expensive, and vinyl the cheapest. And of course, the better the energy performance of a window, the more it’s likely to cost.
Test old windows for lead paint before working on them. If they test positive, it’s best to hire an RRP-certified pro.
Broken panes in old windows are common repairs, and a relatively simple matter of removing the broken glass, having a new pane cut to size, and installing it with glazing points and putty. You may want a pro for this if the window is high up. Another common issue with old windows is stuck sashes.
Often this is due to multiple coats of paint, and the sashes can be freed with judicious use of a knife and a scraper. Other times, the sash cords have broken and the counterweights don’t work anymore. In that case, jamb liners that hold the sashes with spring pressure can be used. Both of these repairs are in DIY territory.
Some rot on an old frame doesn’t necessarily mean it needs replacement. In many cases, you can repair the rot with structural epoxy putty, sand the repair flush, and repaint the window.