Evolution of the Energy Efficient Window

Compass orientation, artificial shade, natural shade, and climate are all things to consider when selecting windows and glass doors. Until manufactured windows entered the mainstream, the essential criteria of orientation, shade, and climate were the only considerations.

It was the free market that brought low-energy windows into this world, but it was building codes that finally killed the old ones. And in 2019, that code got stricter. U-factors in Climate Zones 3 and 4 went from 0.35 to 0.32 and, in Climate Zones 5-8, from 0.32 to 0.30. In 1992, the first energy codes were introduced in the U.S. At the time, windows were not the focus of energy conservation, and the metric used to measure solar heat gain had not yet been devised. Along with U-factor, which measures the passive insulative value of glass windows and doors, the solar heat gain coefficient did not appear in the energy code until 2000. Luckily, low-energy windows were already common in residential construction, and window suppliers were well ahead of Uncle Sam’s curve. In fact, the Pella Corporation acquired a commercial window manufacturer (EFCO Corp.) the same year the first energy codes were instituted.

Coincidence? Guess again. Anticipating code changes can lead to a competitive advantage. Prior to the creation of energy codes, there wasn’t a choice of materials, either; nearly all windows in homes were installed with clear glass in single panes. Also, note the fact they were installed, not built. Windows are now manufactured as insulated glass units (IGU). Does your trimmer or framer know how to build a window sash or muntin? Possibly, but not likely anymore. Helping your home buyer understand the basic structure of a window or glass panel door and how their internal parts affect efficiency is important. It may guide them toward an upgrade or justify your window selection on a spec, perhaps even setting the house apart from another under consideration. Basically, the options include single-, double-, triple-, and quad-glazed windows, referring to the number of glass layers they contain. Between the layers of glass, gas or a combination of gasses are sealed into place, creating insulation inside of the window.

The component that separates and maintains space between the layers, referred to as the spacer, as well as the choice of frame material, also affect a window’s performance based on how much heat the material lets through. Every window manufacturer offers a similar but different window recipe, balancing efficiency and affordability with the desired aesthetic. The ENERGY STAR® label indicates a product is energy efficient, and National Fenestration Rating Council (NFRC) labels help window shoppers compare energy efficient products according to how they score on NFRC’s uniform national rating system for energy performance. NFRC rates U-factor, solar heat gain coefficient (how well a product can resist unwanted heat gain), and visible transmittance (how well a product is designed to effectively light your home with daylight). The lower the numbers, the better the NFRC energy performance rating.

insulated energy efficient windows

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.