Another key category used to determine the efficiency of fenestration is air leakage. While NFRC also rates air leakage, this energy gremlin is unique in that it cannot be entirely anticipated in a lab and cannot be remedied by a tint, better supplier, or an extra glaze. The real rate of air leakage is, ultimately, the responsibility of the person who frames the hole and installs the window. I live in a mid-rise building completed in 2015. In one of my rooms, there is a double-pane IGU with an air fill. Its frame is steel and hollow to reduce manufacturing cost and shipping weight, and it leaks air and moisture. Upon inspection, I noticed an error in installation: A single screw tip permeates the air-filled cavity between the panes. If shipped as a single unit, this could be a manufacturing defect. However, it is a large window, so I believe it was assembled on-site. Because of this, the installer is at fault, but it is advisable to inspect any IGUs for defects prior to installation.
The screw’s permeation of the cavity does not inherently reduce thermal qualities because the breach is plugged by the screw itself, but if it were a vacuum-fill IGU, it would be catastrophic damage. The increase in air leakage and permeation of moisture, then, must be a result of the power tool that drove the screw; the torque warped the soft, steel window sash and created a gap between the window frame and sash not visible to the naked eye but large enough to cause permeation. Moreover, the steel is painted during manufacturing to prevent oxidation, but when permeation of the IGU takes place, the moisture doesn’t go conveniently to the painted exterior of the sash — it floods the hollow steel cavity. Thus, my window is rusting from the inside and will need to be replaced at less than 20 percent of its intended life. If rainwater ever saturates the desiccant below the fill cavity, the window will also fog over.
A savvy builder with a keen eye can anticipate these issues. There are many other considerations when selecting windows and doors. These choices affect the way your project looks, the cost of upkeep, and the initial cost to build. But your role in building an energy-efficient home doesn’t end with choosing the product best-suited for the job. Without superior quality of workmanship from your installers and close inspection by your superintendent, even a window earning top ratings is not going to perform successfully.