Thinking Outside the Energy Box
Today’s array of window features and performance claims emphasize energy efficiency. While energy performance is obviously of major importance, windows must serve purposes beyond saving energy. The best window for a given application and site is a virtually customized mix of features that represents an ideal balance of specific performance attributes.
As we enter the fall season–marked by hurricanes in the Southeast, wildfires out West and impending cold weather that forces Northerners indoors–it is instructive to revisit some of these other performance attributes.
An Eye to Hurricanes
For windows and doors located in hurricane-prone coastal areas, there are three key considerations: structural resistance to high wind pressures. resistance to impact from wind-borne debris and water penetration from wind-driven torrential rains.
Improved structural resistance to the force of high winds starts with the basic performance requirements for windows and doors in AAMA/WDMA/CSA 101/I.S.2/A440-08, the North American Fenestration Standard. NAFS-08 and its applicable predecessors delineate different Performance Classes of windows and doors that meet increasingly stringent structural requirements, based on Design Pressures derived from the maximum wind velocity likely to be experienced at a given location.
But standing up to high winds is just the beginning. A significant majority of window breakage in major storms is caused by impacts from flying debris. AAMA 506-08, Voluntary Specifications for Impact and Cycle Testing of Fenestration Products, provides a system for rating the ability of fenestration products to withstand impact and pressure cycling generally associated with hurricane conditions. Impact-resistant windows usually feature laminated glass with a heavy interlayer, commonly polyvinyl butyral, sandwiched between two pieces of double-strength glass. This lite can then be integrated with a double-strength tempered glass lite to form an insulating glass unit–which serves double duty by reducing energy loss.
Finally, the penetration of wind-driven rain can lead to extensive restoration expenses. Under NAFS-08, the water penetration resistance capability of a window is linked to its structural integrity through the Design Pressure. Certification testing subjects the exterior surfaces to a water application rate roughly equivalent to eight inches of rain per hour – at prescribed pressures up to the equivalent of that exerted by a Category One (or greater) hurricane.
Taking this a step further, AAMA 520-09, Voluntary Specification for Rating the Severe Wind-Driven Rain Resistance of Windows, Doors and Unit Skylights, released just this past June, provides an optional rating of the ability of fenestration products to resist severe wind-driven rain. The specification, championed by the AAMA Southeast Region organization, describes the rapidly pulsed pressure differential test method used (which better simulate actual storm conditions than do static pressure tests).
Windows are considered to be one of the most vulnerable components of a structure when it is exposed to brush and forest fires of the type that strike our Western states from time to time, particularly in the dry late summer and early fall.
The 2006 International Urban-Wildland Interface Code requires windows and doors used in high-risk areas to have a fire protection rating of at least 20 minutes, which means the product can withstand a 20-minute or longer exposure to fire as simulated in test conditions. Such performance typically requires windows made of tempered glass, multilayered glazed panels or glass block, and doors made of noncombustible material or of solid core wood not less than 1¾ inches thick.
As winter approaches and people spend more time indoors, they are more likely to be affected by intrusive noise, especially if they live in urban areas near freeways, airports or rail lines. But more is at stake than mere annoyance. The health effects of lower level but persistent “background noise” are well documented.
Fortunately, the features of windows designed to reduce noise transmission–e.g., insulating and laminated glass–also promote energy efficiency and impact resistance. AAMA 1801, Voluntary Specification for the Acoustical Rating of Windows, Doors and Glazed Wall Sections, describes how to measure the sound transmission loss for windows and doors.
Focusing on and optimizing one parameter of window performance can overlook or even adversely affect another. We are best served by looking at the big picture. That means mixing and matching features and design options to best address the mix of local conditions, climate and economic reality in which we live.