Modern windows: Energy black holes or not? Part 1
Windows are among the most complex building components in a home, and also the most expensive. Look at what we ask windows to do. We want a visual connection to the outdoors that lets in the most daylight. We expect windows to provide fresh air when open, and are completely airtight when closed. We also want them to pass the sun's heat in winter and shade the sun's heat in summer. We need windows to be durable; resistant to condensation, wind and driving rain; and we want them to integrate with the rest of the building envelope; and given they are a big investment, they should last for several decades.
In addition to the important architectural contribution, windows have far-reaching energy consequences. Their performance, total area, and orientation can make or break the energy efficiency of a high-performance home.
The National Fenestration Rating Council (NFRC) rates windows on three criteria: U-factor, Solar Heat Gain Coefficient (SHGC), and visual transmittance (VT). NFRC ratings are whole window ratings that takes into account the different U-factors of the window's frame, sash, edge of glass, and center of glass and not glass-only ratings. Glass-only ratings can be 10 percent to 40 percent better than the whole product value, so look for the NFRC label on rated windows, and be suspicious of those without one.
U-factor measures how much heat is transmitted through the glass due to conduction, convection and radiation. The lower the U-factor, the more efficiently the window blocks heat transfer. A 2015 Prescott energy code window has a maximum U-factor of 0.35, the equivalent of an R-2.8 insulated wall.
SHGC indicates how much of the sun's radiant energy striking the window is transmitted through the window as heat. It is the fraction of solar radiation, expressed as a number between 0 and 1. Low SHGC means less heat is transmitted through the glass. A 2015 Prescott energy code window must have a maximum SHGC of 0.4.
Visual transmittance (VT) is the fraction of visible light energy that makes it through the window glass. Visible light is made up of those wavelengths detectable by the human eye. Visible light contains about 47 percent of the energy in sunlight. The higher the fraction, the more visible light will reach into the room.
Maximizing VT while getting the right combination of U-factor and SHGC can be challenging. All three properties must be considered and balanced to evaluate overall window performance. You may also find that you need different window styles and performance levels throughout your home depending on design preferences, the direction that your windows face and your local climate.
There are many different window designs, with the most popular being fixed or picture windows that cannot be opened; and used for light or visibility alone. Sliding windows that open horizontally can feature two or more sashes and offer a clear opening for good ventilation. Single-hung sash windows open vertically (usually the bottom sash) and the other is fixed. Double hung windows open both sashes vertically, and screens can be installed outside the window frame.
Casement windows are hinged at the sides and open outward, with screens on the inside; they may be left-handed, right-handed, or both and are usually opened using a crank. An awning window is a casement window that is hung horizontally, and hinged on top. Casement windows are the dominant type now found in modern buildings.
A bay window is a multi-panel window, with at least three panels set at different angles to create a protrusion from the wall line. Eyebrow windows are a curved top window in a wall or in an eyebrow dormer. Bifold windows have two or more panels folding onto themselves, and louver windows have a series of blades that tilt to open.
Next time we will look at the various window design considerations, materials and performance enhancements.
For more information, contact Paul Scrivens at www.greenhomeenergyadvisors.com