Everything You Need to Know About Window Materials
In addition to choosing your window's style and type of operating mechanism, there are other practical decisions to make. You'll need to consider cost, insulation, frame material, and more.
In general you will pay more for a window that has greater insulating properties and a greater ability to hold up against the elements. It is usually most cost-effective in the long run to install the best window you can afford.
Window frames can be made of wood (which may be clad with aluminum or vinyl on the exterior portions), vinyl, fiberglass, or metal. Higher-quality (and generally costlier) windows have better weatherstripping to keep air from filtering in around the sashes. The window glass may be single-, double-, or even triple-paned, and it may be treated with a coating that keeps warmth inside during the winter and outside during the summer.
Nearly every window seals out wind when the temperature is comfortable. But when the mercury drops well below freezing, many types of weatherstripping shrink and stiffen, becoming brittle and eventually cracking. This compromises the window's seal. Only the highest-quality windows perform well when temperatures drop below 0 deg F.
We'll walk you through some of the performance values to consider, bus it's still valuable to consult with a knowledgeable salesperson at a home center or window supply source to choose the best window for your situation and budget.
Select and order your windows as early as possible. The least expensive windows come in standard sizes, which you may be able to simply pick up at a home center or window and door supply source. Custom windows cost more and may take several weeks to arrive. If the window is delivered with a flaw, is damaged in shipment, or is the wrong size (as can happen), you will have to reorder.
Window Factors to Watch For
Most windows bear a rating sticker that gives performance scores for at least some of the following factors:
R-value measures the window's ability to prevent heat transfer—how it keeps uncomfortable temperatures outside and comfortable temperatures inside. The higher the R-value, the better.
U-value (or U-factor) is essentially the inverse of the R-value; it measures the tendency to transfer heat. So the lower the U-value, the better.
Solar gain (also called solar heat gain coefficient, or SHGC) indicates how much the window will heat a room when the sun is shining. Solar gain is a good thing when the weather is cold, but it can definitely raise air-conditioning costs during the summer. The higher the number, the greater the heat gain.
Wind resistance, or air leakage, is measured in cubic feet per minute (cfm). There should be two numbers: one for 70 deg F and one for 0 deg F. The lower the numbers, the better the seal.
The Right Window for Your Region
In northern climates, sealing out cold is the primary concern; in southern areas, there is more concern for keeping out the heat. This map gives a general idea of the types of windows that are appropriate for each region; consult local window dealers for more specific recommendations. U-factor refers to rate of heat transfer; solar gain refers to the heat that penetrates the glass.
Types of Windows
This is usually the least expensive choice and is suitable for many applications. In lower-quality models, the weatherstripping (which is typically fuzzy) is not durable, and some of the plastic parts may break, especially when the window is tilted out for cleaning. Vinyl contracts and expands with changing temperatures, which reduces its ability to seal. It can also warp if exposed to very hot sunlight. Vinyl can be painted (it helps to apply alcohol-base primer first), but the paint may peel and need to be reapplied after a few years.
Wood generally costs more than vinyl, and it periodically needs to be sealed with paint or finish to prevent rotting and sun damage. However, wood has natural insulating properties, and most people prefer the way it looks. Some are made with stain-quality wood, but windows that use wood with obvious joints will not look good stained; plan to paint instead.
Many windows tilt out for easy cleaning from the inside. Beware, however, of cheap windows with this feature; the hardware could break while you tilt them out and snap them back in.
To make a wood window more durable, many manufacturers apply a cladding of aluminum, vinyl, or fiberglass to the exterior portions only. Aluminum cladding can be painted, as long as you first apply a primer. In most cases, tinted vinyl and fiberglass can be painted with no problem, but paint may have trouble sticking to white vinyl. You can also buy windows with hard-baked paint finishes.
In addition to the materials shown on these pages, some manufacturers make fiberglass windows. Fiberglass is stronger than vinyl, less prone to contracting and expanding, and less likely to warp. It needs to be protected by paint, but manufacturers apply a hard finish at the factory.
A double-hung window is the most common type, because of its versatility and ease of operation. It has a number of specialized parts. An exterior sill is slightly sloped so water can run off. The interior stool (often called the inside sill) is typically just wide enough for a small plant; if it were wider, people would bump into it. Stop moldings and parting stops must be precisely aligned so they can seal the window, yet allow sashes to slide up and down easily. The weight-and-pulley system shown here is typical of older wood windows; newer windows use friction or springs to keep the sashes in place when they are raised.
A well-made and tightly installed storm window can do wonders for an old window, greatly increasing its insulating properties by trapping several inches of air thickness between the window and the storm window. See instructions on choosing and installing storm windows.
Other Window Features
Flanged and Block Frames
A flanged window attaches to the house with a flange that is nailed or screwed to the exterior sheathing. A block-framed window has no flange and slides into an opening. It is the right choice when you want to install a replacement window in an existing frame.
Single-glazed windows, with a single pane of glass in each sash, are the most affordable type, but they allow plenty of heat transfer, making for hefty heating and air-conditioning costs. In an older window the pane is usually held in place with glazing compound (putty) on the outside. With newer windows snap-in molding pieces take the place of the putty.
A double-glazed pane, also called insulating glass (IG) or thermal glass, dramatically increases a window's energy efficiency. The two panes are sealed with an air space between them that creates the insulation. The thicker the air space, the greater the insulation.
Triple-glazed windows, with three panes and two air spaces, are also available. These are not common, because the extra insulation they offer is not generally considered worth the significant extra cost.
You can increase a double-pane's energy insulation by ordering it with argon or krypton gas between the panes rather than air. Gas-filling usually costs more and can add to delivery time. The gas will leach out, but very slowly; after 20 years the pane will retain 90 percent of its original gas.
A removable grid attaches over a single pane to provide the look of an old-fashioned window or door with muntins and many small panes of glass. The grid lifts off so cleaning the window is far easier.