R-5 Triples or R-5 Doubles?
September 9, 2010
FEATURE ARTICLE | Design & Performance, Energy Efficiency, Materials & Components
It is certainly evident that the North American window industry is on a charted path toward improved energy efficiency. In fact, it appears to be moving toward common standards for both new construction and remodel and replacement markets. These markets are converging near a future recommended R-5 rating–or U-values of 0.22 for operable windows and 0.20 for non-operable windows. This convergence is driving manufacturers to embrace high-performance technologies that provide more efficient windows for consumers.
The growing demand for R-5 windows has initiated a surge in triple-pane window designs coming to market. However, contrary to popular thought, producers are able to meet R-5 demands without investing in the additional raw materials and production reconfigurations required to add triples to their manufacturing operations. Instead, they can achieve R-5 values in double-pane windows by using a combination of low-emissivity coatings and thermally efficient components.
Therein lies a dilemma between which R-5 design to produce. This article will examine the logistical inputs manufacturers need to contemplate when making that decision.
Drivers for Higher-Performance Windows
Several government initiatives are ratcheting up demand for higher-performance windows, some of which include:
► Energy Star Phase I, which went into effect April 1, 2010, implemented stricter U-value qualification criteria to 0.30 (R-3). Phase II – tentatively slated for March 2013 – is expected to further reduce U-value criteria in most or all climate zones, including potentially as low as 0.20 (R-5) in the northern zone.
► The NAHB Research Center Green Approved Products program sets U-value targets as low as 0.25 (R-4).
► The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) Version 3 places U-value targets as low as 0.23 (R-4).
► The U.S. Department of Energy’s Highly-Insulating R-5 Windows and Low-E Storm Windows Volume Purchase Program is expanding the market for affordable R-5 windows. Phase II, anticipated for spring 2011, will consider U-value specification changes.
► Proposed Home Star legislation currently working its way through Congress would offer homeowners rebates up to $3,000 for completing energy-efficient upgrades, such as replacing existing windows with those featuring minimum U-values of 0.30.
In addition, utility companies are offering incentives for reducing energy use, and homeowners are looking for ways to counter their rising energy costs. Both trends are increasing the need for more efficient window options.
With new and shifting standards pushing window performance ratings higher (see sidebar at right), insulating glass fabricators and window manufacturers need to determine how to best meet market demands. That process may involve rethinking their selection of window components to include more advanced technologies that offer higher thermal performance. When making component and design decisions, these producers ultimately have one goal in mind: to build units in the most efficient manner possible to achieve a desired performance rating, such as R-5.
That’s a tall order, considering the wide array of component options–spacers, glass, framing systems, gas filling and more–and how they interact to affect total window performance. Manufacturers can produce R-5 triples using a mixture of performance-rated components, scaling back the efficiency of certain components in favor of others that provide maximum U-value improvements. Alternatively, they may employ a combination of high-performance components to achieve R-5 ratings in a double. Doing so may be more economical in the long run compared to retooling traditional manufacturing operations to accommodate triples.
However, manufacturers also need to consider the likelihood that future energy efficiency standards and consumer demand will go beyond R-5, including ratings that are harder to achieve economically in a double, if at all. Therefore, manufacturers opting to meet today’s requirements in a double may be delaying an inevitable transition to triples production in the future.
Let’s examine what goes into each design to achieve R-5 ratings:
Triples–Reaching the R-5 target for triples is relatively easy. The extra airspace alone creates an additional barrier to heat transfer and enables improved thermal performance. It also gives manufacturers another option for gas filling. In addition, the extra glass lite provides additional surfaces for low-emissivity coatings.
By specifying low-conductivity spacers, gas filling, thermally enhanced frames and two lites of very low-E glass, manufacturers can produce triples with U-values as low as 0.15 (R-7). Each component adds cost, but choosing the optimal combination can reduce the total manufactured cost.
For example, a triple with high-performance spacer and frame systems can meet the 0.22 U-value target without using krypton, as long as producers adhere to the 3-millimeter grid proximity rule. Triples should have a minimum air space of 3 mm between internal muntin bars and the glass to minimize heat transfer from one lite through the grid to the opposite lite. Because of frame width limitations, manufacturers may need to employ an offset triple design in which one glazing cavity is wider than the other. Neglecting the 3 mm grid proximity rule may force manufacturers to use krypton to achieve R-5 values, which significantly increases costs.
Doubles–Even when using krypton, manufacturers are not able to realize R-5 values in a traditional double. Figure 1 shows that the best possible center-of-glass (COG) U-value is just above 0.20 in a double featuring a 0.018 low-E coating on glass surface 2, krypton gas filling and high-performance spacer and frame systems. That COG U-value results in a total window U-value of 0.23 or R-4.
However, by applying low-E coatings to two of the glass surfaces in a double (the same 0.018 low-E coating on surface 2 and a pyrolytic low-e coating on Surface 4), manufacturers are able to achieve R-5 values using lesser expensive argon gas (see Figure 2). The design reduces COG U-values to 0.20, resulting in an R-5 (0.22 U-value) window. Applying this same concept to argon-filled triples–with 0.018 low-e coatings on Surfaces 2 and 4 and a pyrolytic coating on Surface 6–drops COG U-values to 0.10, yielding an R-7 (0.15 U-value) window. Naturally, krypton provides further COG U-value reductions, but with added cost.
One potential tradeoff in this design relates to surface temperature. The coating on Surface 4 reflects heat back into the home, minimizing the amount of radiant heat passing through to warm the interior glass surface (Surface 3). The cooler interior surface may increase the potential for condensation inside the home.
The anatomy differences between R-5 triples and doubles introduce a variety of factors manufacturers must consider when determining which design will work best for their operations. They need to compare the cumulative costs of added material and processing requirements between the two designs, some of which include:
- Glass. Obviously, triples require an extra lite of glass compared to doubles. That means extra raw materials, storage, handling, washing and processing, each of which represents potential cost increases. Manufacturing R-5 doubles also adds cost in terms of coatings. Therefore, a manufacturer needs to compare the cost differential between the two options. Another consideration with triples is that the extra lite gives manufacturers two additional coating surfaces, which enables room for further improved thermal efficiency–although, at an added expense.
- Spacers. With triples, manufacturers essentially double their spacer costs compared to double-pane windows. In addition, the extra spacer profile in a triple creates an extra moisture vapor transmission path, which is an additional point of potential failure in each window. Whether producing doubles or triples, manufacturers should consider the thermal efficiency advantages of using spacer systems that do not contain metal. Moving from the least efficient metal spacer to the most efficient spacer in a double can yield about a 0.04 U-value improvement.
- Gas Filling. Filling airspaces with gas is a necessity in a double to push it over the R-5 threshold, but not necessarily in a triple. However, the low cost-to-thermal-benefit ratio of argon makes it a desirable addition to most production lines. Argon may enable a 0.04 U-value improvement in a double for as little as 1 percent of the total raw material cost of a window. Krypton and xenon offer even greater U-value improvements, but their high cost makes them harder to justify. An advantage of gas filling in a triple is that manufacturers can fill both airspaces for optimal thermal performance.
- Framing. High-performance vinyl framing systems, including those with integral insulated air-cell cores or foam filling, offer potential U-value savings up to 0.04 at a price premium. However, producing a double with a high-performance vinyl frame may be less expensive than manufacturing a triple hollow-framed window due to potential retooling costs associated with redesigning the frame glazing pocket to accommodate a thicker IG unit. Triples may also require additional frame material and reinforcements to support the extra weight of the heavier IG unit. In wood frames, hybrid technologies that combine wood with higher-performing foam-filled PVC or pultruded fiberglass help to improve thermal performance. However, it remains to be seen if R-5 values can be achieved in a wood double.
- Hardware. Because of their added weight, triples often require heavier-grade hardware compared to doubles. For example, manufacturers may need to specify heavier-duty spring balances for double-hung triples and stronger arms and hinges for triple casement windows.
- Transportation. Manufacturers must also consider any extra packaging and shipping costs associated with the heavier weight of triples. Heavier units may require more robust packaging, and producers may endure higher fuel costs due to the added weight and the potential to fit fewer units on a truck.
- Production. Finally, manufacturers may need to reconfigure their operations to accommodate triples production as the added raw materials create new staging and workflow challenges. For example, the extra glass inventory required at the beginning of the line and the reduction in the number of IGUs that fit on a cart may require additional carts and storage space throughout a facility.
It may take years for R-5 windows to become the standard for energy-efficient windows. However, their growing acceptance in the marketplace is driving more manufacturers to add these high-performance designs to their product lines. Doing so creates a point of differentiation that could potentially increase one’s success rate and market share.
Today’s homeowners are realizing that high-performance windows are a worthwhile investment to ensure energy savings and reduced utility bills. Some may have the perception that a triple is necessary to achieve high efficiency, and manufacturers and retailers may be able to use that opinion to their advantage when developing marketing programs. However, sellers should primarily focus on a window’s performance rating–those details found on the National Fenestration Rating Council label.
How producers arrive at the best performance ratings in the most efficient manner possible is up for debate. Both R-5 triples and R-5 doubles are viable options. It’s up to producers to determine which design–or both–is right for their operations.