Windows 101: "What to look for when choosing a window for your home" (by Brooks)
Proper Installation: Proper installation includes proper measurement for custom sizing. Windows should also be insulated well, with the exterior trimmed and sealed well.
Strong Frame: Use first-generation vinyl only (wood and aluminum are proven not to be energy efficient). The frame must be fusion welded, and not mechanically fastened or screwed together. The design pressure should be a minimum of 30 pounds per square foot.
Seals: Windows must seal out air and water. They should have plenty of weather stripping, which must be channeled into the frame and sash, and never stapled or glued on. Once the window is closed, there should be little flex between the frame and panels. The air-infiltration rating should be 25CFM/FT2 or less.
Glass Spacing: Proper spacing between glass is crucial. Spacing should be at least three-quarters of an inch. Anything less greatly reduces energy efficiency as heat has less distance to travel through the glass.
Spacer System: There should be little or no metal in the spacer system. Too much metal in the space between the glass panes results in: more heat conduction through the glass, more condensation, and thermal expansion and contraction stresses on the glass pack.
Movement System: Having a movement system that will last is vital. Make sure it has a lifetime warranty, as inferior movement systems are often the first to go on windows and patio doors. A manufacturer will not give a lifetime warranty to inferior parts that they most likely will have to replace.
Energy Efficiency: The glass must be the proper type low E for your climate. The whole unit must be energy efficient, not just the glass. The U factor is a measure of how heat energy will travel out through the window, and the SHGC factor (Solar Heat Gain Coefficient) is a measure of how much of the sun?s heat energy will come in through the window. The lower these factors are, the more energy efficient the window is. For Wisconsin's climate a window or patio door must have a U factor of .30 or less and a SHGC of .30 or less.
Non-Prorated Lifetime Warranty: With some so-called lifetime warranties, you could be paying for parts in as few as five years, so make sure that the warranty does not offer less coverage as time goes on. Your best warranties will also offer lifetime glass breakage coverage and will be fully transferable.