Engineering developments have produced windows that are much more resistant to air leakage and thermal transmission than ordinary single glazed units. Consider this: Double glazing (two layers of glass separated by an airspace, often called insulating glass) generally offers twice the R-value (resistance to heat transfer) as single glazing. Double glazed windows with low-E glass (windows treated with a low emissive coating, a microscopically thin metallic layer that permits the passage of light while reflecting heat) generally offer twice the R-value of ordinary insulating glass units. Low-E windows in which the airspace is filled with an inert gas such as Argon or Krypton offer approximately twice the R-value of ordinary low-E units. The sash and frame materials also affect resistance to heat flow; wood and vinyl offer the best energy performance, while aluminum and steel windows fall at the lower efficiency end of the spectrum. To set the range of choices in perspective, the most energy efficient windows available have ratings in the neighborhood of R-8;conventional single glazed units are rated R-1.
A new guideline developed by the National Fenestration Rating Council (NFRC) provides a standard for evaluating and comparing the performance of all windows. Most manufacturers now list the U-value of their windows on a label attached to the window glass. U-value measures the total heat flow through the entire window, taking into account frame, window dividers and glazing edges. The lower the U-value, the better the windows performance.
Wood frames still rank as one of the best thermal performers. A wood window’s quality, however, depends largely on the lumber used in its construction. Quality wood windows contain only clear knot-free lumber. Well maintained, they will last for decades. They must be repainted every few years unless they are clad with vinyl or an aluminum shell. Wood has natural insulating qualities and a very low coefficient of thermal expansion, so it can adjust to rapid shifts in temperatures without degrading.
Vinyl window frames consist of hollow extrusion of polyvinyl chloride (PVC), sometimes filled with polyurethane foam. The thermal performance of foam-filled vinyl windows often exceeds that of wood windows, PVC, however, has an extremely high coefficient of thermal expansion. Vinyl windows subjected to extreme temperature cycles may crack and lose their integrity over time.
Aluminum or steel framed windows must include thermal breaks to minimize heat gain or loss. In all but the mildest climates, units without breaks suffer from unacceptable thermal bridging, a phenomenon in which heat moves rapidly between the inside and outside of the home.
Other less known but promising new window frames materials include fiberglass and PVC-wood composites. Both materials offer performance similar to wood, coupling minimal thermal expansion and contraction with low maintenance.
High performance windows and doors are considerably more expensive than standard types, and in making your specifications, you must consider whether the climate really requires a high degree of energy efficiency. In extreme climates, you may be able to make up the extra initial cost for premium windows and doors through lower energy bills, but in mild or moderate climates, it may not make financial sense to buy super efficient units.

Window Installation Details
By the time you reach the framing stage, most questions of solar orientation and preferred views have been resolved. When you see the actual hole in the wall, it may give you second thoughts. You do have the right to change your mind. But be aware that the larger and more complex the window(s) in question, the more expensive the change. If practical, consider exchanging the objectionable window for a larger or smaller unit rather restructuring the entire opening.
The process of setting windows closely parallels that of hanging doors. The builder creates a rough opening, covers it with house-wrap, drops in the window, shims it plumb and level, then secures it to the frame. Unlike doors, however, banks of windows often employ more than one different operating system. For example, double-hung units that slide up and down frequently share wall space with hinged, casement-style windows that swing open like doors. On the same elevation, awning windows may tilt out on a horizontal axis.
The margin for error allowed when setting windows with moving parts is generally only about ¼ inch. Improper shimming or an un-level sill may cause sliding windows to stick in their frames and cause casements to swing awkwardly or show uneven gaps.
If you take time to open all movable windows shortly after installation, flaws will become apparent at once. Your builder normally can fix them, and should be happy to do so by adding another shim or two.

Final Inspection
Before you shut the door on this phase of construction, take a final look at your windows and doors. This time, think about some potential problems you may not have considered during planning stages:
The Big Stretch Windows that are too difficult to reach often become overpriced fixed glass. A custom builder in Maine, for example, put several transom windows near the top of a 16-ft. window wall, hoping to ventilate excessive heat. After the owner moved in, he found the 16-ft. poles designed to operate the windows impossible to use. He has since stopped trying.
The Narrow Escape Although most building codes require a bedroom window egress, a hole big enough to allow escape from a fire, they may not specify the type of window. In an older person’s bedroom, for example, a double-hung unit may prove difficult to operate, and a sill set at 44 inches from the floor might create an imposing obstacle.

Overexposure Prolonged exposure to unfiltered ultraviolet light can damage rugs, furniture, wall-coverings, and hardwood floors. It’s worth including UV-filtering glass in windows or patio doors that admit several hours of sunlight each day.
Voyeurs Delight Although many people agonize for hours about the views from inside their home; they forget that neighbors may have intimate perspective on their bedroom or master bath. Draperies and blinds can solve the problem easily, but if a window will be covered most of the time, does it serve the purpose you intended?
Restricted Passages If you have an idea how air should flow through your home when you open the windows, keep an eye out for windows (particularly casements) that, when open, restrict this natural flow. The builder may be able to reverse them.

Anticipating Intangibles
With careful planning, you can solve most of the problems likely to arise from mistakes in window or door placement. If you discover later that glare from that circle window renders the television useless during the day, for example, you can always add a shade. A snap-in grille can make that plain patio door into an architectural statement.
If every window and door opens and closes properly, brightens the home’s living areas and keeps out the elements, you’re most of the way home.