Even the most energy-efficient window must be properly installed to ensure that its energy performance is achieved and that it does not contribute to a home's moisture problems. Therefore, it's best to have a professional install your windows.
Window installation varies depending on the types of the following materials:
Windows should be installed following manufacturing recommendations, along with the following additional guidelines (primarily for windows with fin mounting systems).
The window opening must be flashed and integrated into the home's weather-resistive barrier so that any potential water leaks do not cause damage. A weather-resistive barrier is a drainage plane that allows water that has penetrated past the siding to drain away from the wall system. Details will vary with siding, window type, the installation sequence for the window, trim, and weather-resistive barrier. It is advisable to install window head and sill flashing, whether it is metal, plastic, or a self-sticking, elastomeric membrane. Avoid relying on tapes or sealants to provide waterproofing, as these products may fail over time.
It's common practice and recommended by some manufacturers to cut an "X" in housewrap placed over window openings, pull the material inside, and secure it by stapling. Other manufacturers require alternative methods, such as the modified "I"-cut, depending on the overall flashing approach. The "I"-cut allows the vertical leg of the head flashing to be placed under the weather-resistive barrier and then taped or sealed.
It's best to divert drainage onto the face of the weather-resistive barrier. Do not tape down or seal behind the bottom nailing flange of the window, as doing so could accidentally trap in water.
Windows must also be properly air sealed during installation to perform correctly. To air seal the window, caulk the backsides of the window mounting flanges (top and sides only) to the weather-resistive barrier during installation. The mounting flange (nailing fin) is an integral part of most window frames that laps over the conventional stud construction. Nails are driven through it to secure the frame in place. Also, from inside the house, seal the gap between the window frame and rough opening using backer rod and caulk or non-expanding latex-based spray foams that will not pinch jambs or void window warranties. Backer rod is a closed-cell foam or rope caulk that is pressed into cracks or gaps with a screwdriver or putty knife. Insulation stuffed into this crack does not stop air flow.