In the U.S. market, there are literally thousands of window lines to choose from. Chris Mathis, principal of Mathis Consulting Co. (MC2) and the founding director of the National Fenestration Rating Council, underscores this fact by explaining that it doesn't take much to get into the window-manufacturing business and capture local markets with cut-rate pricing. "These are the companies," Mathis cautions, "that may not be around when the glass fails."
To pick a window that will last, start with the warranty, Mathis advises. Look for a non-prorated, transferable warranty good for 20 years on the glass and at least 10 years on non-glass parts. These solid warranties are available from most of the bigger window companies, but Mathis says it's surprising how many companies do not offer such a warranty at all, or do not offer it on all their window lines. This reflects a serious compromise in the quality of a large percentage of the windows sold. Nearly 30% of the replacement windows installed today will replace windows that are only seven years old, reports Mathis. "That's a sad commentary on the quality of many windows out there."
Common failures. The most common window failure is the breakdown of the seal on an insulated glass unit. If the seal blows, the window will fog up between the panes, and a lost seal causes a drastic reduction in window R-value. Even if the seal fails gradually, air and moisture slowly entering the windowpane tends to oxidize the low-E coating, turning it into a high-E coating that absorbs heat rather than reflects it away. This oxidation will look exactly like the salt spray that sticks to the glass, except that it can't be washed off. It looks like a permanent blur between the windowpanes.
While a 20-year glass warranty will adequately cover this most common window failure, a warranty on non-glass parts also becomes especially significant on the coast. Constant humidity, blowing rain, and salt attack will quickly degrade cheap finishes and hardware, and hinder sash movement.
Mathis, a former director of the National Fenestration Rating Council (NFRC) who now serves on the ASHRAE and ASTM code committees as well as on the International Energy Conservation Code Committee of the ICC, urges that choosing a manufacturer that will be around in 10 years to replace deteriorated parts may be the most important window selection criterion of all.
Warranty language. "If a window says ‘lifetime warranty,' it makes me nervous," says Mathis. "‘Lifetime' is usually written in big, bold letters, but you have to read the fine print to find out what this really means." Many warranties seem to cover a lot up front, but that impression quickly changes when all the details are spelled out.
Language to look for:
• "Non-prorated" warranties will cover the entire purchase price of the window for the term of the warranty. Unlike roofing materials, a well-made window shouldn't gradually degrade with exposure, so there is little justification for a prorated warranty.
• "Fully transferable" warranties are a sign that the window maker means business. For a homeowner selling a home, it can be a value-added feature that a builder or remodeler can make available when recommending a window.
• "Non-glass" components, particularly hardware, should carry a minimum 10-year warranty. A good window with bad hardware is a bad window, Mathis insists. If a lock breaks or the crank handle strips out, it will reflect poorly on the remodeler who installed the window. Think about the window manufacturer's capacity to stock replacement parts well into the future.
• Labor and installation. Unless a certified representative of the manufacturer installed the unit, few warranties will cover the cost of installation. Some may, but for the most part, this will fall to whoever installed the window. This is yet another reason for builders and remodelers to stick with trustworthy brands.
• Exclusions. This is key in coastal climates. Some warranties specifically exclude coverage for damage from environmental factors, such as high humidity or salt spray. The exclusion may apply to the glass as well as to the hardware and finishes.
• Finishes. Coverage on finishes is rare, but some warranties do cover exterior coatings and finishes on cladding. However, painting or refinishing the exterior to match the home may null this coverage. This is particularly true on aluminum-clad and vinyl units.
Mathis contends that if the warranty passes muster, you are probably dealing with a manufacturer willing to protect its reputation. Behind that reputation will be reliable performance specifications. So, in addition to the warranty, you want to look for an NFRC label that outlines the window's energy performance and an AAMA label that provides third-party assurances of basic structural performance and establishes a norm for air leakage and water penetration.
Much ado is made over low-E coatings — the invisible thin metallic coatings that block radiant heat flow — as well as other energy features, such as gas fills and low conductivity edge spacers. However, builders rarely have the option to choose these ala carte; they are usually sold in packages by the manufacturer with specific configurations. One useful tool is to compare the performance values for the various window configurations available is the Efficient Windows Collaborative's online Window Selection Tool (www.efficientwindows.org). To make sense of this tool, however, you need to first understand the baseline measurements of window performance provided on the NFRC label. See "Baseline Window Values" for a detailed look at thess important specs.
By now, impact-resistant windows using laminated glass are nothing new to coastal builders and remodelers. Many of the technical difficulties we reported ("Spec'ing Windows in High-Wind Zones" ) have been worked out, and manufacturers are offering a wider selection of sizes of windows in a much broader range of design pressures.
Impact resistance is tested by firing a 2x4 missile at the window and evaluating how well it then holds up to pressure cycling — a test that is intended to simulate the forces of wind-borne debris striking the window during a hurricane. An impact-resistant window typically shatters during an impact test but maintains its integrity if the interlayer and glass shards hold together. While keeping the home and its occupants out of danger, the window will leak significantly.
Opening protection. Installing impact-resistant windows is just one way to meet the structural requirements for wind-blown debris, but it is the most convenient method and, for vacation homes, at least, it may be the most practical. If windows without laminated glass are used, a house must be shuttered during a tropical storm.
However, though more convenient, impact-resistant windows do not necessarily offer the best protection. According to the Institute for Business and Home Safety (IBHS), steel storm panels that meet the Dade County standards for opening protection offer the highest level of protection possible. These products will prevent the windows behind them from breaking and keep water out better than impact-resistant windows or movable shutter systems. Plywood can be used for shutters, but according to the IBHS, to get near the protection offered by Dade County-approved storm panels requires 3/4-inch plywood, which makes for heavy shutters that are difficult to install. (IBHS recommends plywood over OSB, as it takes 30% thicker OSB to equal the impact resistance of plywood. See "Evaluating OSB for Coastal Roofs". Lighter 3/8-inch-thick plywood shutters offer only about half as much resistance to penetration as 3/4-inch plywood and provide few assurances that the building shell will not be breached during a major storm. Clear polycarbonate shutters offer nearly the same impact resistance as storm panels and are relatively light. Roll-down shutters, the most convenient option, often require putting up storm bars to offer adequate impact resistance.
The convenience offered by impact-resistant laminated glass may also mean the difference between adequate protection for the structure and its occupants or none at all. Post-hurricane damage assessments always turn up evidence of homes that were damaged because owners were unable to make arrangements to install shutters before a major storm struck. "Shutters," argues Dave Olsen, a code expert with Florida-based window maker PGT Industries, "demand that the homeowner is home, and able, to install them."
Cost. The most frequently cited shortcoming of impact-resistant glass is the cost. The added cost per unit varies with window size, shape, and design pressure (DP). Large windows, circle tops, and high-DP units add proportionally more to the price tag. Price quotes in the Florida and Virginia markets for double-hung units by several different manufacturers revealed costs for impact-resistant windows $150 to $350 higher than comparable units with conventional insulated glazing (for an added cost of about $13 to $30 per square foot of window area, with aluminum and vinyl windows at the lower end of the price range and clad wood at the upper end). This increase works out to $4,160 to $9,600 for 320 square feet of window area (or 20% window-to-floor area in a 1,600-square-foot home). In many southeastern markets, such an upcharge, which does not include markup or any added labor, might be a significant increase to the cost of a house, but in most housing markets along the mid-Atlantic and New England coast, it would likely be a relatively small proportion of the total house price.
Window manufactures cite a study commissioned by the North American Laminated Glass Information Center, which concludes that the cost for laminated glass is less than the installed cost of roll-down shutters — the most convenient but also most expensive shutter option available. The "Hurricane Shutter Guide" published by the South Florida Sun-Sentinel confirms this conclusion, estimating the installed cost of roll-down shutters to be $26 to $40 per square foot, or $8,320 to $12,800 for the equivalent window coverage used to compare window costs as noted above (320 square feet). The installed cost for the equivalent size and number of steel storm panels ranges from $1,280 to $2,240 — considerably less than the upcharge for laminated glass but considerably less convenient as well.
Added value. The benefits to using laminated glass go far beyond convenience in meeting the destructive force of a tropical cyclone. The clear advantages that should help justify the higher costs for impact-resistant windows include:
• Security. It's as difficult for a burglar to shatter a laminated windowpane as it is for a wind-blown roof tile to smash through it. Therefore, this option is especially attractive for vacation homes that are unoccupied for much of the year. Few beach communities allow homes to remain shuttered for more than a few hours before and after a storm (and few homeowners would be wise to do so, as it is an invitation to burglars, who will know nobody's home). Laminated glass also provides a measure of safety protection: Children are unlikely to fall through a laminated window or get hurt by a broken pane.
• Sound dampening. Glass is inherently brittle and readily transmits sound vibrations. Adding a flexible interlayer has the same effect as putting a finger on a drumhead: It deadens the vibration. An ordinary double-pane insulated glass window has a sound transfer coefficient (STC) of 28. When made with laminated glass, the window's STC jumps to 35 — about the same as an insulated 2x4 wall with 5/8-inch drywall on both sides.
• UV protection. The plastic interlayer blocks ultraviolet light, which might otherwise cause carpets and home fabrics to fade and vinyl flooring to yellow.
PGT's Olsen says that laminated glass has become a standard feature on a majority of windows sold in Europe, not for protecting homes from hurricanes but for the increased security, safety, and sound resistance afforded by these units. And he points to the fact that prices have fallen as manufacturers have become more familiar with the thicker glazing and have filled out their window lines.
Impact resistance and design pressure are two distinct structural requirements for windows used in hurricane zones: One resists failure from wind-blown debris, and the other resists failure caused by sustained wind pressure.
Wind pressures are simulated in the laboratory by applying a static pressure across a window specimen for a prescribed amount of time and measuring the maximum uniform load deflection. During this test, pressure is applied in both directions, simulating sustained positive and negative wind loads. Design pressure will vary with area, so the window dimensions are critical to choosing the correct design pressure.
To comply with the International Residential Code, all windows in high-wind zones must meet the "Components and Cladding" requirements, found in prescriptive tables listing minimum design pressures for windows in different positions on the house exterior. For example, windows placed near corners, where the wind pressures are highest, must have a higher design pressure. (For a detailed discussion of these window performance requirements, see again "Speccing Windows in High-Wind Zones," Winter 2005).
The design pressures used to meet the code's "Components and Cladding" requirements should not be confused with the pressure ratings that are central to the joint specification from the American Architectural Manufacturers Association (AAMA) and the National Wood Window and Door Association (NWWDA). Known as AAMA/NWWDA 101/I.S.2 (or shorthand as AAMA 101), this standard defines the baseline performance characteristics required for every window, regardless of the market. The primary specification is a "structural test pressure" rating, which is established using a test procedure developed by ASTM that evaluates the window after applying a pressure 50% higher than the design pressure. If no permanent deformation occurs to the window, it passes the test and is awarded a small gold label affixed to an inside surface of the frame (Figure 6). However, the minimum design pressure rating listed in AAMA 101 may be lower than what is required for "Components and Cladding." For example, the AAMA 101 baseline for residential-class windows starts at a minimum design pressure of 15 psf. While this pressure is equivalent to the pressure sustained by a 75-mph wind — the defining wind speed for a Category 1 hurricane — it would not pass muster in any coastal wind zone.
Currently, the AAMA 101 draws a distinction between several classes of windows, with residential (R) at the low end of the design pressure range, and heavy commercial (HC) and an unspecified "architectural" (AW) class at the upper end. Dean Lewis, manager of product certification for the AAMA, has suggested that these distinctions may become obsolete as more and more residential windows are being made to meet higher design pressures.
In addition to the structural performance ratings, AAMA 101 also establishes minimum air infiltration and water penetration values. The water penetration values are found using an ASTM test procedure that applies a negative pressure equal to 15% or 20% (depending on the window's performance class) of the design pressure to the inside of the test assembly, and then hoses the window exterior with a rack of water jets that is equivalent to a rainfall of 8 inches per hour (Figure 8). At best, a window meeting the AAMA 101 spec will leak in a 35-mph wind, which virtually guarantees it will leak in a hurricane.
The assessment of Joe Lstiburek of Building Science Corporation is even worse: In developing a report commissioned by the Florida Home Builders Association to evaluate the high incidence of water damage to homes during the 2004 hurricane season, Building Science Corp. found that most windows leak when no pressure is applied. For that report, Building Science Corp. tested 50 windows. "We had 100% failure," notes Lstiburek. "Okay, it was ‘only 50 windows,' you might say. ‘There are thousands of windows available.' But when every one of them leaked, we felt pretty safe drawing the general conclusion that windows leak."
The AAMA acknowledges the problem and issued the statement: "The lesson learned in 2004 is that water penetration from heavy wind-driven rains occurred in more cases than acceptable, leading to interior water-related damage that could have been prevented." And an AAMA Southeast Region Hurricane Standard Development Task Group is currently gathering data on real-world indoor/outdoor pressure differentials due to hurricane-force winds to find ways to better simulate actual wind gusting and effects of turbulent wind flow.
But despite his report, Lstiburek feels that the results should be kept in perspective. For starters, he contends, it's significant that the windows won't blow out, and the occupants can remain safe inside during a major wind event. In addition, the water leakage for windows is not just a shortcoming for the windows. "There are no standards for the walls, which will leak much more than the windows," said Lstiburek. "The point is, let's accept that walls and windows leak. But let's design them to first drain to the exterior, and second to dry out if they do get wet. That's the best way to avert widespread water damage." If the main criterion is reducing water leakage, Lstiburek recommends sealing the roof vents where the most water is likely to come into a home, and cause the most damage, during a hurricane.